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Influence of Slag Chemistry on the Hydration of Alkali-Activated Blast-Furnace Slag — Part II: Effect of Al2O3
Abstract
The hydration and microstructural evolution of three alkali activated slags (AAS) with Al2O3 contents between 7 and 17% wt.% have been investigated. The slags were hydrated in the presence of two different alkaline activators, NaOH and Na2SiO3·5H2O. The formation of C(A)–S–H and hydrotalcite was observed in all samples by X-ray diffraction, thermal analysis and scanning electron microscopy. Higher Al2O3 content of the slag decreased the Mg/Al ratio of hydrotalcite, increased the Al incorporation in the C(A)-S-H and led to the formation of strätlingite. Increasing Al2O3 content of the slag slowed down the early hydration and a lower compressive strength during the first days was observed. At 28 days and longer, no significant effects of slag Al2O3 content on the degree of hydration, the volume of the hydrates, the coarse porosity or on the compressive strengths were observed.
... The ionic bonds between Ca 2+ , Mg 2+ ions and oxygen atoms are much weaker than Si-O and Al-O bonds, representing the weak interaction during dissolution; therefore, a higher CaO content of glassy phase in GGBFS favours an increased rate of reactivity over fly ash [2]. The hydration product precipitated from dissolved ions in the pore solution is determined by the minimisation of the Gibbs energy [18]; hence, thermodynamics models can be used to analyse supersaturation towards the hydration product and predict the amount of hydration products [1,19,20]. ...
... The regions containing OP and fully reacted small OPC or slag particles show a moderate grey level in BSE-images, which is the main binder phase in the pastes. These regions occupy most of the volume and determine most of the performances such as the strength and durability of the blended pastes [19,20]. ...
... The distributions of the atomic fractions after normalisation on the scanning line at 7, 14, 28, 56 and 180d are shown in Figs. 17,18,19,20 and 21, respectively. The scanning lines are marked with white arrows. ...
The nano-structural evolution of the inner product of ground granulated blast furnace slag in blended cement at 7, 14, 28 56 and 180d was revealed. Three distinct layered inner hydration product region were observed because of the spatial zonation of hydrotalcite-like phase, C-A-S-H gel and Ca-Al LDH phase. The zonation of inner hydration product explains the presence of the multi-rims features of the slag grains with a relatively high reaction degree at very late ages. Diffusion of dissolved ion from slag grains and the effect of pore space on the reorganization of hydration products play a key role in forming the spatial zonation of inner hydration product. The findings of the zonation of inner region argue that the lack of pore space in the inner region makes the continued dissolution of slag hard to complete.
... Furthermore, different experimental methods have been used in the cement literature for direct or indirect estimation of the overall DOR for AAMs, blended OPC systems and other types of cements. These include deconvolution of 29 Si and/or 27 Al nuclear magnetic resonance (NMR) spectra [10,[14][15][16], image analysis based on backscattered scanning electron microscopy (SEM) [8,[17][18][19], selective acid or alkaline dissolution [8,9,[19][20][21], differential scanning calorimetry (DSC) [8], non-evaporable water or portlandite content from thermogravimetric analysis (TGA) [9,10,18,22,23], isothermal conduction calorimetry (ICC) [24][25][26], and chemical shrinkage [8], with their relative strengths and shortcomings discussed in refs. [8,19,24,27]. ...
... Furthermore, different experimental methods have been used in the cement literature for direct or indirect estimation of the overall DOR for AAMs, blended OPC systems and other types of cements. These include deconvolution of 29 Si and/or 27 Al nuclear magnetic resonance (NMR) spectra [10,[14][15][16], image analysis based on backscattered scanning electron microscopy (SEM) [8,[17][18][19], selective acid or alkaline dissolution [8,9,[19][20][21], differential scanning calorimetry (DSC) [8], non-evaporable water or portlandite content from thermogravimetric analysis (TGA) [9,10,18,22,23], isothermal conduction calorimetry (ICC) [24][25][26], and chemical shrinkage [8], with their relative strengths and shortcomings discussed in refs. [8,19,24,27]. ...
... In this investigation, we utilize in situ synchrotron-based XRD and subsequent X-ray PDF analysis to track the phase transformations and associated evolution of the local atomic structure occurring during sodium hydroxide activation of amorphous GGBS. According to previous investigations [5,13,17,18,[37][38][39], the main reaction product in sodium hydroxide-activated GGBS is a sodiumcontaining calcium-alumino-silicate-hydrate (C-(N)-A-S-H) gel with a structure resembling that of poorly ordered C-S-H (I) [40]. In addition to the main C-(N)-A-S-H binder gel, secondary layered double hydroxide (LDH) phases are often observed in alkali-activated GGBSs [37,38,[41][42][43], along with the presence of a substantial amount of unreacted GGBS even at an advanced age of curing (e.g., 6-12 months) [14,15,17,18]. ...
Here, an approach to quantify the amorphous-to-disordered/crystalline transformation occurring in NaOH-activated ground granulated blast-furnace slag (GGBS) is outlined that combines atomistic modeling with in situ pair distribution function (PDF) analysis. Firstly, by using force-field molecular dynamics (MD) simulations, a detailed structural representation is generated for the amorphous GGBS that is in agreement with experimental X-ray scattering data. Use of this structural representation along with literature-derived structures for the reaction products allows for real space X-ray PDF refinement of the alkaline activation of GGBS, resulting in the quantification of all phases and the degree of reaction (DOR) as a function of reaction time. All phases and the DOR are seen to approximately follow a logarithmic-type time-dependent behavior up to 5 months, while at early age (up to 11 hours) the DOR is accurately captured by a modified pseudo-single step first-order reaction model. Lastly, the evolution of DOR is found to agree with several other complementary in situ data containing quantitative reaction information, including isothermal conduction calorimetry, Fourier transform infrared spectroscopy, and quasi-elastic neutron scattering.
... Based on studies on the activation of slag with the conventional activators (i.e. NS and/or NH), it has been reported that the surface chemistry of slag plays a role in the alkali activation (Ben Haha et al., 2011;Sakulich et al., 2010;Ansari rad et al., 2020;Ben Haha et al., 2012). However, the degree of influence of the slag chemistry is dependent on the type of activator used. ...
... Ben Haha et al. (2012) also investigated the effect of slag with different Al 2 O 3 (7% to 17%) on the hydration of slag activated with NS and NH. Similar to the observation made by Sakulich et al. (2010), the presence of high Al 2 O 3 resulted in a delay in the early hydration of the slag and a corresponding lower compressive strength at early ages when NS was used as the activator (Ben Haha et al., 2012). ...
... Ben Haha et al. (2012) also investigated the effect of slag with different Al 2 O 3 (7% to 17%) on the hydration of slag activated with NS and NH. Similar to the observation made by Sakulich et al. (2010), the presence of high Al 2 O 3 resulted in a delay in the early hydration of the slag and a corresponding lower compressive strength at early ages when NS was used as the activator (Ben Haha et al., 2012). How ever, there was no significant influence of Al 2 O 3 content on the compressive strength when NH was used as the activator. ...
The production of Portland cement (PC) which is the main binder in conventional cementitious materials contributes about 7% to the world’s anthropogenic carbon dioxide emission. As the demand for PC is expected to increase significantly in the coming years, it is imperative to find other environmentally friendly alternatives. Alkali-activated binders (AABs) obtained by alkali activation of aluminosilicate precursors are viable alternatives for PC as they possess lower embodied carbon and energy compared to PC. However, the conventional activators (i.e. sodium silicate and sodium hydroxide) are corrosive, expensive and have a high environmental footprint. These limitations have resulted in impractical and expensive applications of such binders on a large scale. On the other hand, sodium carbonate, which is less corrosive, cheaper and available naturally in the environment can be used as a sustainable alternative to sodium silicate and sodium hydroxide. In a quest to propel the application of sustainable binders, this study was undertaken to explore the properties of slag activated with sodium carbonate at ambient temperature. The effect of sodium carbonate dosage on the fresh, mechanical and durability properties are discussed. A simplified sustainability assessment of concrete made with different binders was also carried out and the results showed that the sodium carbonate activated slag concretes are a sustainable alternative to PC concrete to achieve a cleaner environment.
... The chemical composition, minerology and particle size of these precursor materials and SCMs can vary considerably depending on their type, source location and processing parameters. Even for GGBSs, which have relatively small chemical variability compared to fly ash, their main oxide components do vary, consisting of CaO (30-50 wt.%), SiO2 (28-38 wt.%), Al2O3 (8-24 wt.%) and MgO (1-18 wt.%) along with the presence of other trace elements (e.g., S, Ti, Na, K, Mn and Fe) as well as crystalline impurities (e.g., merwinite, gehlenite, åkermanite, calcite and quartz) [4,[7][8][9][10][11]. These inherent variabilities can have a dramatic impact on precursor/SCM reactivity in both AAM and blended Portland cement systems, as well as the resulting pore structure and engineering properties of the final cementitious product [1,3,7,8,[11][12][13]. ...
... Even for GGBSs, which have relatively small chemical variability compared to fly ash, their main oxide components do vary, consisting of CaO (30-50 wt.%), SiO2 (28-38 wt.%), Al2O3 (8-24 wt.%) and MgO (1-18 wt.%) along with the presence of other trace elements (e.g., S, Ti, Na, K, Mn and Fe) as well as crystalline impurities (e.g., merwinite, gehlenite, åkermanite, calcite and quartz) [4,[7][8][9][10][11]. These inherent variabilities can have a dramatic impact on precursor/SCM reactivity in both AAM and blended Portland cement systems, as well as the resulting pore structure and engineering properties of the final cementitious product [1,3,7,8,[11][12][13]. The impact of Ca content is * Experimental density values from ref. [15]. ...
... The impact of alumina content on the reactivity of amorphous aluminosilicates and engineering properties of the resulting AAM and blended Portland cements has also been investigated [11,15]. ...
In this investigation, force field-based molecular dynamics (MD) simulations have been employed to generate detailed structural representations for a range of amorphous quaternary CaO-MgO-Al2O3-SiO2 (CMAS) and ternary CaO-Al2O3-SiO2 (CAS) glasses. Comparison of the simulation results with select experimental X-ray and neutron total scattering and literature data reveals that the MD-generated structures have captured the key structural features of these CMAS and CAS glasses. Based on the MD-generated structural representations, we have developed two structural descriptors, specifically (i) average metal oxide dissociation energy (AMODE) and (ii) average self-diffusion coefficient (ASDC) of all the atoms at melting. Both structural descriptors are seen to more accurately predict the relative glass reactivity than the commonly used degree of depolymerization parameter, especially for the eight synthetic CAS glasses that span a wide compositional range. Hence these descriptors hold great promise for predicting CMAS and CAS glass reactivity in alkaline environments from compositional information.
... Even when no water treatment was performed for the raw CFBC ash, some of the free lime had already hydrated to portlandite, and this phenomenon has been reported in previous studies [41,42,45]. It was hard to locate the gypsum (CaSO 4 ·2H 2 O) and hemihydrate (CaSO 4 ·1/ 2H 2 O) peaks in the XRD pattern of raw ash (Fig. 2), and similar results were observed in the semi-quantitative calculations of the crystals ( Table 2). ...
... Escalante-Garcia et al. [35] measured the degree of the reaction (i.e., the reacted fraction of the slag) of the sodium silicate-based AAS to be less than 30% after 120 d of curing. Ben Haha et al. [6] and Ben Haha et al. [41] measured the degree of hydration of the sodium silicate-activated slag mixture cured under sealed conditions, and observed that it was less than 50% even after 180 d. Note that the mix proportions in the studies [35,41] were almost similar to our study. ...
... Ben Haha et al. [6] and Ben Haha et al. [41] measured the degree of hydration of the sodium silicate-activated slag mixture cured under sealed conditions, and observed that it was less than 50% even after 180 d. Note that the mix proportions in the studies [35,41] were almost similar to our study. On the other hand, even when additional water was not supplied from outside, the OPC grains reacted with the mixed water and showed a higher degree of hydration compared to the AAS, which led to less fluctuations in the development of strength under moist conditions during curing [42]. ...
Circulating fluidized bed combustion (CFBC) ash was applied in sodium silicate-activated slag system as a secondary activator and/or for the substitution of commercialized expanding admixtures. To understand the effects of this ash on such alkali-activated slag (AAS) system, a series of experiments relating to hydration kinetics, hydration products and microstructure, and engineering properties were performed. It was found that the introduction of the ash resulted in different types of hydration products in AAS, leading to increases in both the size and volume of the pores. Although the setting time was retarded, the autogenous and dry shrinkages of the AAS decreased by the increase in the ash content. The shrinkage and strength of AAS affected by the external moisture conditions during curing could be controlled by substituting the slag with the ash.
... It was shown that in different modern day GGBS, 2 day compressive strength in mortars with 75 % GGBS varied by a factor ×4. 8 Main source of variation of the reactivity of GGBS in various cementitious systems was their chemical composition and intensive research has been carried out on the influence of major element composition. [8][9][10][11][12][13][14][15] Differences in reactivity are likely due to differences in chemical durability of the slag glass, as for strength development GGBS needs to dissolve and reprecipitate as cementitious phase. 6 Chemically, GGBS are glasses (>99%) of the CaO-Al 2 O 3 -SiO 2 compositional system, also containing significant amounts of Mg and Ti, and a wide variety of other minor and trace elements. ...
... This is the case for example for Al 2 O 3 , which by the FSI would be considered detrimental for GGBS reactivity but actually is beneficial. 8,9,13,14 This is likely due to modification of secondary phase assemblage due to the presence of Al 2 O 3 and not increased glass corrosion. Nevertheless, the FSI gives a handy tool to estimate the effect of variations of minor element content on GGBS reactivity in blended cements. ...
Ground granulated blast furnace slags (GGBS) are glasses (>99%) of the CaO‐Al2O3‐SiO2 compositional system and are widely used as supplementary cementitious materials. Differences in reactivity of GGBS were screened by modifying the content of 11 minor elements (namely Ba, Ce, Cs, Cr, Mn, P, Sn, Sr, Ti, V, Zr). SEM observations showed that most elements entered the silicate glass matrix, only Sn was reduced to its metallic form and P accumulated in minor minerals. Mortar strength tests showed that 2d‐compressive strength was reduced by > 50% for a TiO2 content of 2.5 wt.% in the slag. At 28 days the loss in compressive strength was still > 40%. Calorimetric tests on other element additions showed that network modifiers (Ba, Cs and Sr) and GGBS reactivity are positively correlated, whereas Ce, Cr, V and Zr significantly decreased reactivity. It is shown that these effects can be well estimated by the weighted field strength of the added element.
... Despite a large number of existing studies on the effects of the chemistry of precursors and/or activators on the performances of alkali-activated materials [15,16,19,[23][24][25][26][27][28], it is still very difficult to establish a general composition-property correlation for AAMs in a single model. This is because the reaction kinetics and microstructure developments in AAMs are extremely complex and vary when different precursors are been used [15,[29][30][31]. ...
... Alkali-activation is one of the most promising and efficient chemical routes that can drastically improve the resource efficiency by utilisation of versatile aluminosilicate resources, including natural minerals [9][10][11], waste and recycled materials [12][13][14]. The reaction kinetics, phase evolution, mechanical performances, and durability of alkali-activated materials are primarily controlled by the chemistry of the aluminosilicate precursors and the dosage and type of activators [2,15,16]. A wide range of aluminosilicate rich precursors can be used to prepare AAM, from high calcium content precursors (e.g. ...
Alkali-activation is one of the most promising routes for utilisation of versatile aluminosilicate resources. However, the variations of chemical compositions in these resources have increased the challenge of designing alkali-activated materials (AAMs) with multiple sources, posing the demand for establishing composition-property correlations that can represent a wide range of AAMs. This study proposes a data-driven approach to develop such composition-property correlations combining machine learning with global sensitivity analysis and thermodynamic modelling. The strength performance of alkali-activated concretes was investigated for a benchmark study (196 data inputs). The impact of the five key chemical compositions, CaO–SiO2–Al2O3–MgO–Na2O, has been assessed. The results show that despite the use of different aluminosilicate precursors, there appear to be coherent connections between bulk binder chemical compositions, phase assemblages, and the performance of AAMs. The composition-property correlations established via machine learning can be used to facilitate the on-demand design of AAMs utilising varying aluminosilicate resources.
... The purity of all samples was 97+ wt%, mostly validated by X-ray diffraction. The purity of the slags [55,56], xonotlite [57], and tobermorites [21] was confirmed in previous studies. ...
... Note that, distinguishing these phases in complex cementitious systems using bulk XANES techniques with peak deconvolutions is still challenging. The Si K-edge of slags is generally at lower energies compared to the nanocrystalline C-S-H, which could be explained by the lower Q n species of silicate in slags (mainly Q 1 with a broad range of chemical shifts as Q 0 -Q 2 in 29 Si NMR [56]) than in the nanocrystalline C-S-H (majorly Q 1 and Q 2 ) [83]. In addition, the weaker shoulder peak C of nanocrystalline C-S-H implicates its lower Ca/Mn-Si connectivity compared to slags. ...
Understanding the silicate polymerization of calcium silicate hydrate (C-S-H) gel and its crystalline polymorphs is important in cement science. NMR can determine Si environments, but the measurement can be time-consuming and provides no spatial information. X-ray absorption near-edge structure (XANES) spectroscopy is a fast tool for probing Si coordination, possibly with spatial information. However, there lacks an understanding of Si K-edge XANES spectra of cement-related silicate phases. Here, a Si K-edge XANES spectral database of nanocrystalline C-S-H, C-S-H minerals, blast-furnace slags, and metakaolin is provided. Si K-edge of C-S-H minerals shifts to higher energies with higher polymerized Si and lower CaSi connectivity in the Si second nearest neighbor shell. Si K-edge energy shows weak correlations with Ca/Si ratio, average SiO bond length, and SiO4 distortion due to the structural complexity of silicates. The substitution of Al for Si shifts the Si K-edge of tobermorite and slags to higher energies.
... The DTG curves at the different curing ages are represented in Figure 7. On the DTG curves, three mass loss intervals were selected: (a) the first interval was below 300 • C and showed two peaks. Peak 1 can be attributed to the loss of the combined water of CSH gel and peak 2 to the loss of the combined water of the CASH gel [19,33,34]. The main peak was the CSH gel; (b) the second was 300-500 • C, in which peak 3 developed (attributed to the decomposition of the hydrotalcite phase [19,33]); (c) the last interval was between 500 and 1000 • C (it contained peaks 4 and 5) and was attributed to carbonate phases like calcium and sodium carbonates or carboaluminates [19]. ...
... Peak 1 can be attributed to the loss of the combined water of CSH gel and peak 2 to the loss of the combined water of the CASH gel [19,33,34]. The main peak was the CSH gel; (b) the second was 300-500 • C, in which peak 3 developed (attributed to the decomposition of the hydrotalcite phase [19,33]); (c) the last interval was between 500 and 1000 • C (it contained peaks 4 and 5) and was attributed to carbonate phases like calcium and sodium carbonates or carboaluminates [19]. ...
Worldwide cement production is around 4.2 billion tons, and the fabrication of one ton of ordinary Portland cement emits around 900 kg of CO2. Blast furnace slag (BFS) is a byproduct used to produce alkali-activated materials (AAM). BFS production was estimated at about 350 million tons in 2018, and the BFS reuse rate in construction materials of developing countries is low. AAM can reduce CO2 emissions in relation to Portland cement materials: Its use in construction would be a golden opportunity for developing countries in forthcoming decades. The present research aims to formulate AAM destined for future applications in developing countries. Two activators were used: NaOH, Na2CO3, and a mixture of both. The results showed that compressive strengths within the 42–56 MPa range after 28 curing days were obtained for the Na2CO3-activated mortars. The characterization analysis confirmed the presence of hydrotalcite, carbonated phases, CSH and CASH. The economic study showed that Na2CO3 was the cheapest activator in terms of the relative cost per ton and MPa of manufactured mortars. Finally, the environmental benefits of mortars based on this reagent were evidenced, and, in terms of kgCO2 emissions per ton and MPa, the mortars with Na2CO3 yielded 50% lower values than with NaOH.
... That is the reason why the structural formation is very high at the earliest time, while the further increasing rate of structural build-up is significantly low since it is limited by the diffusion rate of the activator solution into the GGBFS grain. With the time elapsed, the diffusion rate of ions played a more significant role in the microstructural development at the early time due to the availability of the ions in the pore solution and their ability to penetrate the inner product shell have a direct influence on the formation of reaction products [59,60]. The microstructural evolution of NaOH activated mixture was from grain surface to inside unreacted GGBFS, which is similar to the shrinking core model generally used for OPC hydration [59,61]. ...
... With the time elapsed, the diffusion rate of ions played a more significant role in the microstructural development at the early time due to the availability of the ions in the pore solution and their ability to penetrate the inner product shell have a direct influence on the formation of reaction products [59,60]. The microstructural evolution of NaOH activated mixture was from grain surface to inside unreacted GGBFS, which is similar to the shrinking core model generally used for OPC hydration [59,61]. (ii) For water glass (SS) activated mixture (N5M s 1.6), SS activators promote the dissolution of GGBFS for a considerable amount of time [62], which led to a longer time for microstructural development in the early ages. ...
The effects of ground granulated blast furnace slag/fly ash (GGBFS/FA) ratio, mass ratio of SiO2 to Na2O (Ms) of activator solution and sodium silicate dosage on structural build-up, flow properties and setting characteristics of alkali-activated cement (AAC) mixtures were investigated. The solid-like behavior becomes more dominant with increasing GGBFS/FA ratio. Ms value had significant effect on the structural build-up. Significantly higher initial storage modulus with a low increasing rate was observed for the Ms values lower than 0.8. However, for higher Ms values, a sudden increase in storage modulus was observed after negligible initial structuration. An increase in sodium silicate dosage caused a considerable delay in the abrupt increase in the structural formation. It was observed that flow curves of AAC fit the Herschel-Bulkley model with shear-thickening behavior. ICP-OES tests revealed the lower release of aluminum and calcium into the pore solution of GGBFS/FA mixtures with low Ms
values.
... Les courbes thermogravimétriques des formulations activées en présence de silicates à 28 jours, ainsi que le suivi des gaz dégagés en fonction de la température sont présentés dans la Ces résultats sont en accord avec ceux d'une étude antérieure [33], où moins d'eau liée a été observé dans les produits de réaction avec NaOH que dans le GGBFS activé par en présence de silicates [184]. Ces conclusions montrent encore une fois que, quelle que soit la teneur en eau dans le système, la quantité d'eau liée est plus importante pour les formulations sans sédiments. ...
... Ceci permet une diffusion organisée des produits de réaction dans l'espace interstitiel entre les grains de laitier et un meilleur développement des propriétés mécaniques à un âge avancé[27]. La porosité à 28 jours des formulations activées par le Géosil est alors plus faible (8,4 %) que celle dans les systèmes activés par de la soude (28,3 %)[1,184]. L'ajout de sédiments entraine une diminution des résistances à la compression des mélanges ainsi qu'une augmentation de la porosité(16,8 %). Cela peut être expliqué par un effet de dilution du GGBFS, le précurseur principal de l'activation alcaline dans le milieu suite à l'incorporation des sédiments. ...
Dans un monde qui s’oriente de plus en plus vers le développement durable, les matériaux alcali-activés (AAM) à base de co-produits industriels comme le laitier granulé de hauts fourneaux moulu (GGBFS), sont apparus comme étant de nouveaux types de liants pour diverses applications dans le domaine du Génie Civil. Les AAM sont d’une grande importance puisqu’ils présentent une résistance mécanique élevée et une bonne résistance aux attaques chimiques.D’autre part, les sédiments marins dragués peuvent également être utilisés comme précurseurs de tels liants. En Europe, de grands volumes de sédiments sont dragués annuellement dans les ports mais l’évolution de la réglementation va tendre à restreindre leur immersion en mer.La valorisation d’une partie de ces sédiments en tant que matières premières entrant dans la composition de liants contribuerait à limiter leur stockage et leur donnerait de la valeur ajoutée.Notre étude est orientée alors vers la réalisation d’un liant minéral alcali-activé, basé sur l'activation du GGBFS, incorporant la fraction fine de sédiments marins de dragage non calcinés.Le premier objectif de ce travail est d’étudier l’effet de l’incorporation des sédiments sur l’activation alcaline et le second est de déterminer les constituants des sédiments qui sont à la base de la perturbation de la cinétique d'activation alcaline. Une approche par analyses multi-échelles macro et microstructurales a été réalisée pour caractériser les matériaux bruts et formulés et mettre en lumière les facteurs d'influence.
... Activation of slag in alkaline solutions with dissolved silica produces higher strength. In the presence of soluble silica, more Si is incorporated in the reaction product, reducing the Ca=Si ratio in the calcium silicate hydrate (C-S-H) (Haha et al. 2012). The strength varies inversely with the Ca=Si ratio; reaction product with a lower Ca=Si ratio gives a higher strength (Haha et al. 2011a). ...
... For clarity, the heat flow recorded within the first 1 h after mixing is shown in the inset in Fig. 3(a). The trends in the heat of hydration are consistent with observations reported previously (Deir et al. 2014;Haha et al. 2012;Shi and Day 1995). An initial small exothermic peak corresponding to wetting and dissolution of slag is observed within 15-30 min after mixing. ...
The reaction of blast furnace slag in sodium hydroxide (NaOH) solutions of different molarities is evaluated. The compressive strength of the activated slag does not scale with the molarity of NaOH. The primary reaction product in the activated slag is identified with calcium aluminosilicate hydrate [C(A)SH]. While the early reactivity of slag is enhanced at higher alkalinity, and the dissolution of slag increases with the molarity of NaOH, the quantity of C(A)SH in the hydrating system does not scale with the molarity of NaOH in the activated slag. From X-ray diffraction (XRD) analysis, an additional water-soluble, sodium-based amorphous product is identified in the reaction products. The water-soluble product, which does not contribute to strength, increases proportionately with the initial Na content in the solution. At higher molarity, there is a larger proportion of the water-soluble product relative to C(A)SH in the reaction products. The Ca/Si ratio and Al/Si ratios in the C(A)SH gel are relatively invariant of the NaOH molarity in the activating solution. The compressive strength gain in the alkali-activated slag is determined by the quantities of C(A)SH and the intrinsic sodium-filled water-soluble product.
... Cationic species could also prevent the precipitation of calcium aluminate hydrate gel on the grain surface that could hinder further dissolution or ionic diffusion [27]. Previous studies showed that slag's hydration largely depends on its chemical composition, on its particle size distribution, and on the nature of the activation mix [3,9,[28][29][30][10][11][12][13][14][15][16]26]. A detailed study of the activation of one particular slag should then be made in order to characterize the hydrated products formed depending on activation conditions. ...
The use of cement substitute, such as alkali-activated slag, is of great interest in the field of earth-based construction materials to preserve their environmental benefit. Nonetheless, the cationic activity of clayey particles of earth-based materials induces new issues by affecting the alkaline conditions in which slag is activated. This work aims to present how alkaline conditions modify the nature and the amount of hydrates formed, the kinetics for reaction and the resulting mechanical properties. A minimum of 5 wt% of activator with respect to the slag was needed to trigger consolidation of the samples within the first 24 h, while an activator content of 15% led to an excess of sodium, especially remarkable at early age. Besides, hydration kinetics were followed in situ during the first 3 days by infrared spectroscopy. The formation of portlandite and hydrotalcite-like hydrates were only observable from 10% of activator and the presence of hydrotalcite is correlated with an increase in mechanical properties. Finally, these results are of interest for the formulation of earth-based materials by predicting hydrated phases formed during slag activation depending on alkaline conditions.
... On the other hand, as mentioned by Sakulich et al. [9] and Huang and Han [10], changing the Si/Al ratio of the paste by adding Al 2 O 3 could obtain a suitable setting time, and the problem of low compressive strength also existed. e addition of Al 2 O 3 generated precipitates wrapped on the surface of slag in the early phase and the hydration reaction was inhibited that caused a long setting time [11]. e low concentration of OH − ions in the late phase of hydration reaction was because of the increased of Al 3+ ions which consumed a large number of OH − ions in the paste and led to insufficient hydration reaction power and then hard to generate highly cross-linked gel which was the main reason of low growth ratio of AAS compressive strength. ...
Extensive research into alkali-activated slag as a green gel material to substitute for cement has been done because of the advantages of low-carbon dioxide emissions and recycling of industrial solid waste. Alkali-activated slag usually has good mechanical properties, but the too fast setting time restricted its application and promotion. Changing the composition of alkaline activator could optimize setting time, usually making it by adding sodium carbonate or sodium sulfate but this would cause insufficient hydration reaction power and hinder compressive strength growth. In this paper, the effect of sodium aluminate dosage as an alkaline activator on the setting time, fluidity, compressive strength, hydration products, and microstructures was studied through experiments. It is fair to say that an appropriate amount of sodium aluminate could obtain a suitable setting time and better compressive strength. Sodium aluminate provided enough hydroxyl ions for the paste to promote the hydration reaction process that ensured obtaining high compressive strength and soluble aluminium formed precipitate wrapped on the surface of slag to inhibit the hydration reaction process in the early phase that prolonged setting time. The hydration mechanism research found that sodium aluminate played a key role in the formation of higher cross-linked gel hydration products in the late phase of the process. Preparing an alkali-activated slag with excellent mechanical properties and suitable setting time will significantly contribute to its application and promotion.
1. Introduction
Cement as one of the most common commodities has been widely used all over the world [1]. The production of cement consumed a large number of mineral resources and electricity, emitting big quantities of carbon dioxide that account for 5–8% of the total global carbon dioxide emissions, which caused serious damage to the ecological environment [2]. Therefore, it is urgent to find a low-carbon environment-friendly gel material to substitute cement. Alkali-activated slag (AAS) used industrial solid waste slag as raw material, formed to gel material under strong alkali excitation; in this case, it was extreme to be the green ecofriendly substitution of cement [3,4].
AAS had the characteristics of low-carbon dioxide emissions and recycling of industrial solid waste, and the advantages of its good early-phase mechanical properties, high-temperature resistance, and strong corrosion resistance had been proved by many studies [5]. However, the disadvantages of AAS were the too fast setting time, cumbersome preparation process, and alkali corrosive damage; these points restricted the AAS application and promotion. Scholars spent lots of effort in solving the AAS too fast setting time and reported that the composition and content of alkaline activator were the effect factors of AAS setting time. Jiao et al. [6] demonstrated that to add Na2CO3 to NaOH as an alkaline activator could extend the setting time obviously, but the compressive strength was seriously lost. Cengiz Duran Atişt et al. [7] found that to use Na2CO3 as an alkaline activator could obtain a longer setting time than NaSiO3 and NaOH by experiment. Longer setting time led to uncompleted hydration reaction which resulted in low compressive strength when Na2CO3 was used as an alkaline activator. Furthermore, the calcite was generated that Na2CO3 was used as an alkaline activator, which was harmful to compressive strength growth. Rashad et al. [8] believed that choosing Na2SO4 as an alkaline activator could earn long setting time and low compressive strength played out at a slow rate. Previous research found that changing the alkaline activator composition by adding Na2CO3 and Na2SO4 could delay setting time but result in low compressive strength which was a key property for AAS. On the other hand, as mentioned by Sakulich et al. [9] and Huang and Han [10], changing the Si/Al ratio of the paste by adding Al2O3 could obtain a suitable setting time, and the problem of low compressive strength also existed. The addition of Al2O3 generated precipitates wrapped on the surface of slag in the early phase and the hydration reaction was inhibited that caused a long setting time [11]. The low concentration of OH⁻ ions in the late phase of hydration reaction was because of the increased of Al³⁺ ions which consumed a large number of OH⁻ ions in the paste and led to insufficient hydration reaction power and then hard to generate highly cross-linked gel which was the main reason of low growth ratio of AAS compressive strength.
Many experts already noted that Na2SiO3 provided more Si⁴⁺ and OH⁻ ions for the paste which promoted the hydration reaction progress and obtained better mechanical properties, but the too fast setting time happened against the actual operation. Although adding Na2CO3, Na2SO4, and Al2O3 could avoid too fast setting time, the big consumption or lack of much OH⁻ ions resulted in insufficient hydration power which led to the slow growth ratio of AAS compressive strength. As we all know, the hydration power of AAS mainly relied on the number of OH⁻ ions carried by the alkaline activator. Therefore, we here proposed using NaAlO2 as an alkaline activator and studied the effects on the setting time, fluidity, and mechanical properties of AAS. A large number of OH⁻ ions and more soluble Al³⁺ ions were provided by the addition of NaAlO2 which ensured better mechanical properties and delayed the hydration reaction rate, respectively. This paper studied the NaAlO2 dosage effects on properties of fresh and hardened Na2SiO3 based AAS paste. Considering the cumbersome preparation process of liquid alkaline activator and the alkali corrosion damage during the preparation and curing, this study proposed to used solid NaAlO2 and Na2SiO3 as a composite alkaline activator. The benefit was that the slag and the solid alkaline activator were dry mixture initially and stored. The way of using was the same as cement by adding water and mixing them directly, which improved the operation convenience and reduced alkali corrosion damage.
In this article, the effect of five different NaAlO2 dosages and five different Na2O contents on the setting time, fluidity, and compressive strength of AAS was studied. In addition, XRD, SEM-EDS, and FT-IR were used to analyze the hydration products, microstructures, and the vibration of chemical bonds in order to deeply explore the influence of NaAlO2 on the hydration mechanism.
2. Materials and Test Methods
2.1. Materials
The chemical components and the photograph of the slag are shown in Table 1 and Figure 1, respectively. Blast furnace slag was used with a specific gravity of 2.9 g/cm³ and a specific surface of 435 m²/kg obtained from Minmetals Yingkou Medium Plate (Yingkou, China). According to the GB/T 18046-2017 [12], the slag was classified as S95. The composite solid alkaline activator was made by the mixture of solid NaAlO2 and solid Na2SiO3. NaAlO2 solid (analytical pure) was provided by Dalu Chemical Reagent Factory (Tianjin, China). Na2SiO3 solid (Na2SiO3·9H2O; ratio of Na2O to SiO2 content was 1.03 ± 0.03) was provided by Xilong Scientific Chemical Reagent Factory (Shantou, China). PO 42.5 cement (control group) was provided by Dalian Tianrui Cement Limited Company (Dalian, China).
SiO2
Al2O3
CaO
MgO
MnO
TiO2
S
FeO
S95
32.93
14.98
40.92
8.01
0.55
0.93
0.89
0.79
... The uptake of aluminum by C-S-H is considered with Al/Si ratio to be 0.06 in C-A-S-H as proposed by Lothenbach et al. [46]. The uptake of K + and Na + by C-A-S-H is considered with phases of ((KOH) 2 [56][57][58]. ...
The hydration processes of binders determine two principal factors that controls the ions transport during external corrosion: porosity and chemical composition (solids and pore solutions) of the cementitious system. In this study, hydration modelling is introduced to reactive transporting model, from which a self-sufficient framework for reactive transport in saturated cement-based materials from hydration to corrosion is successfully developed. The proposed model gives information on transformation of both phase assemblage and pore solutions compositions. The numerical implementation implies that the developed framework can be applied to successfully predict reactive transport in saturated cement-based materials. Furthermore, mechanism and controlling factors of sulfate attack are clarified based on simulation on sulfate ingress.
... This results in an incongruent dissolution of slags, due to a preferential release of Al into solution and the development of a silica-rich surface layer [48]. During glass dissolution, it is expected that the [4] Al to [6] Al coordination change may slow down the early hydration rate, though this influence vanishes after 28 days [49]. ...
This study presents a description of the structure of amorphous slags, with particular attention to the Ca local environment and the medium range arrangement of Ca-sites. Using Ca K-edge X -ray absorption spectroscopy (XANES and EXAFS) and pair distribution function (PDF) as well as numerical modeling combining Molecular Dynamics and Reverse Monte-Carlo, we demonstrate that Ca occurs in a complex environment with a distribution between 6- and 7-coordinated sites. Ca atoms are not distributed randomly within the glass structure: the Ca sites are linked by edges and form a complex percolating sub-network of network-modifying cations. The aluminosilicate network in slags is highly depolymerized with, on average, Q2.06 arrangements for Si, indicating the predominance of chain-like structural units and Q2.67 arrangements for Al in more polymerized units cross-linking the Ca-domains. The Ca percolating sub-network forms clusters that may enhance the early dissolution rate of slags in water.
... Compared with OPC, alkali-activated slag cement exhibits similar or even better mechanical properties and durability performance [8][9][10]. All of these excellent properties are closely related to the reaction process of alkali-activated slag, because the reaction process governs the reaction products formation, microstructure development and thus microstructure-related-physical-properties, including mechanical properties and durability performance [11][12][13]. ...
Many calorimetric studies have been carried out to investigate the reaction process of alkali-activated slag paste. However, the origin of the induction period and action mechanism of soluble Si in the dissolution of slag are still not clear. Moreover, the mechanisms behind different reaction periods are not well described. In this study, the reaction kinetics of alkali-activated slag paste was monitored by isothermal calorimetry and the effect of soluble Si was investigated through a dissolution test. The results showed that occurrence of the induction period in hydration of alkali-activated slag paste depended on the presence of soluble Si in alkaline activator and the soluble Si slowed down the dissolution of slag. A dissolution theory-based mechanism was introduced and applied to the dissolution of slag, showing good interpretation of the action mechanism of soluble Si. With this dissolution theory-based mechanism, origin of the induction period in hydration of alkali-activated slag was explicitly interpreted.
... AAS is obtained by using an alkali to dissolve the aluminate and silicate monomers to produce a binder similar to that of PC. AAS has been found to possess outstanding mechanical and durability performance similar or higher compared to that of PC [8][9][10][11]. The advantage of AAS compared to other alkali-activated binders is due to the ability to cure it at ambient temperature coupled with a huge amount of different slags produced as waste materials all over the world [12]. ...
The use of sodium carbonate as an alkali activator for slag to produce alkali-activated slag is promising due to its sustainable, economic and user-friendly properties. However, the lower early age performance of composites made with such binder has limited its use especially in applications where higher early age is required. Hence, in order to propel the application of this sustainable binder, it is imperative to find ways in which the early age performance can be enhanced without having a detrimental effect on later age performance. One of the effective and sustainable ways to enhance the early age strength of sodium carbonate activated slag is by incorporation of various additives as partial replacement of sodium carbonate on/and slag. In order to propel more application of sodium carbonate slag for various applications, this current study was undertaken. In this paper, an overview of the types of various additives that can be used to enhance the early age compressive strength of sodium carbonate activated slag composites was discussed. The mechanism and dosage of each of the additives were briefly discussed alongside the limitation and advantages of the additives. Findings from this overview showed that the early age compressive strength of sodium carbonate activated slag can be enhanced with the use of additives such as calcium oxide, calcium hydroxide, Portland cement, sodium hydroxide and sodium silicate.
... This is possibly associated with the hydration of calcium-carrying compounds in slag within the first few days of water curing, leading to the formation of an intermix of hydration and activation products and ultimately, densifying the matrix. It is also possible that hydrotalcite formed as a reaction product, causing an increase in the volume of hydrates with a higher space-filling ability [62,63]. Further details are reported later in the microstructure analysis sections. ...
The use of alkali-activated materials to produce concrete is a promising technology. However, information on the optimum curing regime required to maximize the performance of such concrete is lacking. This paper examines the fresh and hardened properties and microstructure characteristics of alkali-activated slag-fly ash blended concrete subjected to various curing regimes. Results showed that the optimum curing regime for alkali-activated blended concrete mixtures made with 0 and 25% fly ash was a combination of water and subsequent air curing, while that for mixes made with 50% fly ash was continuous water curing. Among these mixes, that incorporating 25% fly ash presented superior density, bulk resistivity, water absorption, compressive strength, and modulus of elasticity. Microstructure analysis revealed that the reaction products were an intermix of calcium aluminosilicate hydrate and sodium aluminosilicate hydrate. Analytical regression models were also developed to correlate the hardened properties of alkali-activated slag-fly ash blended concrete.
... The XRD patterns of 70FA30SN after curing and efflorescence are shown in Fig. 7e, illustrating increases in peak intensities at 2h = 29.5°showing the formation of a highly-crystalline C-A-S-H gel [36]. The increased slag content favoured the formation of C- A-S-H gel and suppressed the formation of Na-P1 zeolitic products. ...
Efflorescence in alkali-activated materials is a strong function of precursor and activator composition, which dictates their engineering properties and durability. In this study, the efflorescence of naturally cured NaOH/Na2SiO3 alkali-activated fly ash and alkali-activated fly ash-slag blended binder mixes was assessed with alkali concentration of 9 wt% Na2O, and 10 to 30 wt% of slag, and compared with binder mixes with 9 wt% Na2O, and 10 to 30 wt% of slag along with 2 wt% Na2CO3. The effects of efflorescence were assessed using visual and leaching inspections, and the compressive and split tensile strengths were determined post activation. Atomic absorption spectrometry was used to quantify free alkalis in the leachate, and X-ray diffraction, and Fourier transform infrared spectroscopy, magic-angle-spinning nuclear magnetic resonance and thermo-gravimetric analysis were performed to analyse the phase changes in binder pastes after efflorescence. The increased slag content facilitated the formation of C-A-S-H gel and enhanced both chemical and mechanical properties of binder pastes. Furthermore, the inclusion of slag content also led to the reduction of the open porosity and efflorescence formation. Subsequent exposure of binder specimens to efflorescence conditions aided the formation of carbonate products, degradation of N-A-S-H and N-(C)-A-S-H gel, and a decrease in split tensile strength in the binder paste specimens.
... The main source of variation in GGBS reactivity in various cementitious systems are differences in their chemical composition [11,12,[14][15][16][17][18][19]. During the hydration reaction of blended cements, the slag glass dissolves in an alkaline aqueous medium (pH ≈12-13, due to the presence of cement or an alkaline activator). ...
... The reason is that Al, similar to Mg, stays in the rims of the GBFS particles (see Figure 16) rather than in the outer gel. The measured Al/Si ratios (0.25-0.27) of CaNaAlSi gel in sealed cured samples were consistent with literature [31][32][33]. The Al/Si ratio increases over time in sealed conditions until 28 days, after which no changes are observed until 1 year. ...
Understanding the role of curing conditions on the microstructure and phase chemistry of alkali-activated materials (AAMs) is essential for the evaluation of the long-term performance as well as the optimization of the processing methods for achieving more durable AAMs-based concretes. However, this information cannot be obtained with the common material characterization techniques as they often deliver limited information on the chemical domains and proportions of reaction products. This paper presents the use of PhAse Recognition and Characterization (PARC) software to overcome this obstacle for the first time. A single precursor (ground granulated blast-furnace slag (GBFS)) and a binary precursor (50% GBFS–50% fly ash) alkali-activated paste are investigated. The pastes are prepared and then cured in sealed and unsealed conditions for up to one year. The development of the microstructure and phase chemistry are investigated with PARC, and the obtained results are compared with independent bulk analytical techniques X-ray Powder Fluorescence and X-ray Powder Diffraction. PARC allowed the determination of the type of reaction products and GBFS and FA’s spatial distribution and degree of reaction at different curing ages and conditions. The results showed that the pastes react at different rates with the dominant reaction products of Mg-rich gel around GBFS particles, i.e., Ca-Mg-Na-Al-Si, and with Ca-Na-Al-Si gel, in the bulk paste. The microstructure evolution was significantly affected in the unsealed curing conditions due to the Na+ loss. The effect of the curing conditions was more pronounced in the binary system.
... For DW-3d specimen, the mass loss located at around 200°C can be attributed to the existence of hydrotalcite-like phases [7,51,52], corresponding well with the XRD results (Fig. 12). The broad peaks centered at 546°C and 653°C are possibly associated with the bound water loss from C-A-S-H [53]. For SW-3d specimen, however, Clhydrocalumite was detected as evidenced by the mass loss at about 154 and 330°C [54], in accordance with the XRD patterns. ...
A series of experiments were conducted to investigate the effect of seawater on the properties of alkali-activated slag (AAS) binders to seek further potentiality of these binders in marine environments. The experimental results show that on the one hand, seawater may lead to increased setting time, reduced compressive strength after 3 days and a less compact microstructure. On the other hand, however, it could also slightly improve the early age strength, flexural bending strength (after 28 and 56 days) and fractural toughness, and shows almost no impact on the fluidity of AAS binders. Moreover, with the formation of some new hydration products due to seawater mixing, the water absorption and volume of permeable voids had small decreases while the capillary sorptivity obtained a small increase. The concomitant of the acceleration effect due to the presence of salts and hindrance caused by the interaction between the seawater-based alkaline activator and slag make the AAS binding system more complex compared to the distilled water mixed counterparts. The findings of this study reveal the possibility of seawater used as the mixing water for AAS binders in marine or coastal areas.
... The slightly higher calcium oxide content of the DPFA compared to that of the aluminate is expected to aid with the early strength gain when DPFA is utilized in the production of AABs (Nath 2018;Gülşan et al. 2019). Also, additives can be incorporated alongside DPFA in order to increase the aluminate content (Haha et al. 2012). The viability of using DPFA as the precursor in the production of AABs would result in more reduction in the cost and carbon footprint of cementitious materials. ...
Environmental threats posed by the cement manufacturing industry and agro-industrial waste discharge have shifted the direction of research towards building sustainable construction without compromising the technical merits of the developed binders. Date palm trees are one of the highest numbers of trees in the world whose generated wastes can be beneficially recycled and reused by the concrete industry. In this study, ordinary Portland cement (OPC) and date palm frond ash (DPFA)-based binders were synthesized by varying ratio of DPFA/(OPC + DPFA) between the range of 0 to 0.3 at an interval of 0.1. Both base materials were characterized by physical, chemical, and thermal techniques. The developed binders were assessed by flow, setting time, and compressive strength up to 360 days of curing. Scanning electron microscopy (SEM) was performed to complement the strength results. It is postulated that the DPFA/(OPC + DPFA) ratio of up to 0.2 outperforms the DPFA-free binder in terms of the overall performance. The properties of binders were negatively affected by the total precursor composition ratio of CaO/SiO2 and Al2O3/SiO2 below 2.06 and 0.18, respectively. The optimum synergy of OPC–DPFA resulted in superior microstructural density attributed to the uniform skeletal framework of gel products. Strengths, weaknesses, opportunities, and threats analysis of the use of DPFA in cementitious materials showed that there is a high potential for its use in terms of sustainability and economic benefits. However, various weaknesses and threats associated with the use of DPFA as a cementitious material need to be resolved.
... The presence of hydrotalcite in C-AAS was corroborated by EDS analyses plotted in Figure 3.17(c) and and decreased to 1.67 after attack. Ben Haha et al. [190] showed that alkali-activated slags presented hydrotalcite with Mg/Al atomic ratios between 2.06 and 1.29. The authors showed that high amounts of aluminum reduced the Mg/Al atomic ratios of hydrotalcite. ...
Excavation operations produce several tons of soil generally contaminated by the presence of pollutants. Excavated soil is considered as waste and it can be either sent to landfill or destined for reuse depending on the level of pollution. In any case, soil should be properly treated in order to: (i) decrease the release of pollutants into the environment, and (ii) minimize the problems involved in civil engineering applications due to the reactions between cementitious phases and pollutants. In the context of this thesis, we focused on sulfates and molybdenum (Mo). Concerning sulfates, we considered two main issues: (i) external sulfate attack of concrete structures, which are in direct contact with sulfate-rich soil (e.g. dams, foundations), and (ii) the release of sulfates into solution in addition to the swelling and mechanical strength loss in sulfate-rich soil intended for valorization (e.g. reuse in road construction). In the case of Mo, its release into solution is also a serious concern as it can lead to significant risks for the environment. Therefore, in this thesis, we investigated the reaction of concrete in contact with sulfates, and the stabilization of sulfates by using alternative binders for pollution reduction and for reuse of soil. Additionally, we studied the interaction of Mo with alternative binders and their capacity to stabilize Mo. First, we studied the capacity of seven different concretes to resist external sulfate attack under similar experimental conditions. It was found that ordinary Portland cement had high expansions (>0.1%) due to the formation of ettringite in excess caused by the reaction between aluminates and sulfates. Portland cement without C_3A presented lower expansions but gypsum was found to be responsible of cracking at later ages. Meanwhile, alternative binders had low expansions in the range of 0.01-0.03% explained by the absence of C3A and portlandite, in addition to the formation of ettringite during hydration (case of ettringite binders) and the absence of calcium (case of the geopolymer-based metakaolin). Second, we compared the capacity of four different binders to stabilize sulfates in a sulfate-spiked soil. Binders having high C_3A content led to high volume expansions (>5%) caused by the formation of ettringite in excess. These binders also released heavy metals into solution due to their high clinker content. In contrast, binders containing ground granulated blast furnace slag (GGBS) led to low expansions (<2%), sulfate retention was about 89% and lower heavy metals contents were detected in solution. Sulfate solubility was controlled by ettringite, which did not lead to expansion probably due to the low kinetics of precipitation in addition to the absence of portlandite, which is often related to expansive ettringite.[...]
... Positive influence of magnesium is the formation of hydrotalcite which is known for its effect on strength development [18,19]. C-(-A)-S-H and hydrotalcite are co-present with the amount of each of them would be defined by the quantity of magnesium available for the reaction [20]. Akermanite mineral is typically found in dolomite/limestone and common ingredient in the magnesium containing cement precursors such as slags from different sources [15,[21][22][23]. ...
Mining activities are inevitable for the growing industrialization and urbanization in the world. Disposal of mine tailings creates a severe impact to the environment and ultimately to the society. Identifying the applications of mine tailings as potential secondary raw materials would help the mining industries in achieving circular economy. Alkali activation of tailings for their utilization as building materials or backfill in mining sites is one such popular technique. In this paper, mechanically treated silicate tailings rich in magnesium/aluminum content were chosen, to be used as aluminosilicate precursors. The effect of different alkaline sources was accessed by using sodium silicate (Na2SiO3), sodium sulphate (Na2SO4) and sodium carbonate (Na2CO3) as activators. Alkali activated tailings pastes were studied by examining the compressive strength and microstructural properties. High magnesium tailings show good strength results with sodium silicate activator whereas sulphate-based activator performed well in high alumina tailings. Sodium carbonate seems to be efficient in early phases but does not display improvement in later ages. This behavior of alkali activated tailings with different alkali sources were tried to be correlated with the mineralogy of the tailings and its reactivity using FTIR, XRD and TGA.
... It is difficult to come to an exact conclusion. However, bearing in mind the complicated mineral composition of the slag and the numerous and inconclusive reaction mechanisms and chemical reactions of AAS [12][13][14][15], hydration heat is still used to characterize and calculate the reaction kinetics. However, data entirely extracted from the monitoring of the exothermic process cannot be used to calculate the hydration kinetics because the reaction between slag and alkaline solutions is initially rapid and intense, making it difficult to observe the first exothermic peak (initial period), as it lasts for a few minutes and is caused by the partial dissolution of the slag. ...
In this study, we explore a new method based on color variation data to derive the kinetics of the entire process of the hydration of alkali-activated slag (AAS). Using this image analysis technique, we can monitor the induction period that cannot be observed using conventional microcalorimetry techniques. Color variation was recorded across a sequence of 9999 images, which were processed via MATLAB software package. Further, an average pixel value (APV) was determined to represent the color in each image. Reaction parameters, such as color variation velocity v(t), reaction speed ε(t), and hydration degree α(t), that govern the entire hydration process were determined. On the basis of the reaction parameters and a Krstulovic–Dabic kinetic model, integral and differential equations were derived to simulate the three basic processes of AAS hydration. Equations describing the reaction kinetics of AAS with solutions of three different concentrations of NaOH were extracted using this method.
... This calcium hydroxide could come from the saturated calcium hydroxide solution used in the experiments. A small peak at approximately 360 °C in the bulk paste is observed that can be attributed to the decomposition of hydrotalcite [224]. The peak between 550 °C and 750 °C, observed in the DTG curves of the healing products, can possibly be due to the decomposition of calcium carbonate [225]. ...
Mainly due to its excellent performance and low cost compared to other engineering materials, cement concrete is known to be the most widely used engineering material all around the world. Nevertheless, crack formation still remains an ongoing challenge in concrete. Due to the relatively low tensile strength, cracking in concrete commonly occurs. When cracks form, a pathway is generated allowing water and gas to ingress in the cementitious matrix. The water may contain deleterious substances, such as sulfates and chlorides, etc., which can lead to the corrosion of the steel reinforcement compromising the durability of concrete. Superabsorbent hydrogels have emerged as an innovative material to impart beneficial properties into cementitious materials. They have been shown to mitigate autogenous shrinkage and induce self-healing in cementitious materials.
In this research the interaction between hydrogels and cementitious materials is examined and the effect of hydrogels on the properties and self-healing of cementitious materials is investigated.
... Hence, changing any of these parameters related to the activator(s) and/or precursor(s) would have a significant influence on the alkali activation and the resulting performance of the composites. For example, the alkali activation of the precursor such as SL would result in the formation of C-(A)-S-H gel, ettringite and other products such as zeolite and phases with double-layered hydroxides [46][47][48][49][50][51]. Nevertheless, other factors such as curing conditions, presence of fillers, water to solid ratio, etc., also have a significant influence on the resulting properties of AACs [52,53]. ...
Alkali-activated composites are promising sustainable alternatives that can be utilized as a replacement for Portland cement materials. Alkali-activated composites are made with a binder obtained from the alkali activation of aluminosilicate precursors. However, the types of conventional activators used posed a huge challenge due to their high embodied energy and carbon. On the other hand, sodium sulphate which exists naturally in the environment and does not require complicated production systems can be utilized as a sustainable alternative activator in the production of alkali-activated composites.
Compared to the use of conventional activators in the production of alkali-activated composites, the use of sodium sulphate is limited. Hence, in order to gear more research, developments, and applications of alkali-activated composites produced with sodium sulphate as an activator; this review was carried out. In this overview, the performance of sodium sulphate-activated composites in terms of its fresh and hardened properties is discussed.
The waste generated during the mineral wool production makes up to 30 % of the finished product mass. These wastes can be used for producing building materials, in particular as raw materials for the production of geopolymers (alkali-activated binders). The research aim was to determine the influence of the chemical composition of mineral wool production wastes (MWPW) on the phase composition, structure, and physico-mechanical properties of geopolymers. Five types of MWPW with various chemical compositions and specific surfaces were hydrated in the presence of NaOH (from 2 to 4 wt. %). The experimental results were obtained using the methods of X ray differential (XRD), differential thermal (DTA) and thermogravimetric (DTG) analyses. Moreover, scanning electron microscopy (SEM) and physical and mechanical tests were used. The main hydration product of MWPW in the NaOH presence is determined to be calcium hydrosilicates of the C–A–S–H fiber texture type. The largest amount of C–A–S–H was detected in geopolymer samples made of wastes with an acidity modulus between 1.4 and 1.6. The compressive strength of the obtained materials reaches 80 MPa. They are also characterized by high water resistance. The Al2O3 content in the waste should be about 10 % in order to obtain geopolymers with stable properties. The obtained results made it possible to define the correlation between the structure, composition, and physic-mechanical properties of geopolymers made of MWPW. The practical effect of the research results lies in the possibility of obtaining higher strength classes concrete.
It is important to understand the high temperature response of a newly developed construction material in order to assess the effect of fire exposures. The effects of different percentages of ground ferronickel slag with fly ash and different temperature exposures on geopolymer mortars were evaluated by the changes in mass, cracking behaviour, compressive strength, microstructure and ultrasonic pulse velocity after exposure to temperatures up to 1000 °C. Higher residual strength and lower voids and cracks were found in geopolymer mortars containing 50% ground ferronickel slag. The improvement is attributed to the production of sodium magnesium aluminosilicate hydrate (N-M-A-S-H), which was confirmed by the EDS and XRD analysis. The residual compressive strengths of geopolymer mortar with 50% ground ferronickel slag were 76, 65, 51, 30 and 29 MPa after 2 h of exposure at 200, 400, 600, 800 and 1000 °C, respectively. The XRD results showed a significant increase in the number of crystalline peaks and a decrease of amorphous content at 1000 °C. The amorphous content of the geopolymer with 50% ground ferronickel slag decreased from 79.6% at ambient temperature to 75.5%, 73.7% and 53.1% at 200, 600 and 1000 °C, respectively. The differential variation in higher and lower frequency components of the ultrasonic signal was associated with the residual strength of these specimens. They revealed that geopolymer mortar with 50% ground ferronickel slag had the least amount of pores and cracks. Overall, geopolymer mortar containing 50% ground ferronickel slag showed better performance against high temperature exposure compared to the other mixes.
This paper describes research into the effectiveness of using various nanomaterials, admixtures, and fibers as concrete additives to increase the compressive strength of non-dispersible underwater concrete (NUC). The suitable content of nanomaterials, admixtures, and fibers was determined based on slump tests, cement mass loss, and compressive strength tests for specimens. X-ray diffraction (XRD), mercury intrusion porosimetry (MIP), and scanning electron microscopy (SEM) were used to identify the crystalline composition and microstructure characteristics of the modified NUCs. The results showed that the addition of nano-SiO2 or nanometakaolin significantly increased the compressive strength of the NUC. The compressive strength of the NUC containing 3% nano-SiO2 or 5% nano-metakaolin increased by 33.5% or 26.4% at 3 days, respectively, and by 52.1 % or 32.6% at 28 days, respectively. XRD, MIP and SEM showed that the nano-SiO2 and nanometakaolin filled internal pores and initiated a pozzolanic effect, making the NUC more complex and increasing its density. The nanomaterials altered the internal structure of the NUC and thus increased its compressive strength. The recommended additives to NUC will make NUC better able to resist a damaging seawater environment, and they have usage potential for real projects.
Alkali-activated materials are an emerging technology that can serve as an alternative solution to ordinary Portland cement. Due to their alkaline nature, these materials are inherently more electrically conductive than ordinary Portland cement, and have therefore seen numerous applications as sensors and self-sensing materials. This review outlines the current state-of-the-art in strain, temperature and moisture sensors that have been developed using alkali activated materials. Sensor fabrication methods, electrical conductivity mechanisms, and comparisons with self-sensing ordinary Portland cements are all outlined to highlight best practice and propose future directions for research.
The current standardized methods used to investigate the carbonation performance of concrete are based on the direct determination of the pH variation on the surface of a concrete specimen exposed to ambient or higher CO2 concentration. These methods are either time-consuming (natural carbonation) or of a questionable accuracy (accelerated carbonation). The carbonation physicochemical process involves two major mechanisms: gaseous CO2 diffusion into the cementitious material’s porous network and its dissolution and reaction with CaO of the hardened cement paste. Most carbonation depth prediction models require the CO2-effective diffusion coefficient and the amount of carbonatable products as input parameters. Hence the aim of this work is to develop two simple and reliable test methods to determine these two properties in a reliable and cost-effective manner.First we developed and validated a test method to determine the oxygen-effective diffusion coefficient (De,O2) of nine different hardened cement pastes preconditioned at different relative humidity levels, and 44 concrete mixtures. The influence of the hydration duration, water-per-binder ratio, accelerated carbonation, and binder type on the oxygen diffusivity was investigated. The dependence of the De,O2 on the tested concrete specimen thickness was investigated at the dry state and after conditioning at 93%RH. The De,O2 was determined before and after full carbonation of six concrete mixtures previously conditioned at different RH. A correlation between oxygen permeability and diffusivity is investigated on 44 concrete mixtures.A second test method is developed to determine the instantaneous CO2 binding rate and the amount of carbonatable products of powdered hydrated cement pastes and synthetic anhydrous and hydrates. The samples were carbonated in open systems at ambient CO2 concentration and controlled relative humidity, and then the system switches into a closed configuration while the measurement of the CO2-uptake is performed over a short period of time. The test method allows for the measurement of the carbonation reaction rate and capacity; and their evolution as function of time under different RH. The developed method shows advantages for being nondestructive, allowing the samples to carbonate at controlled CO2 concentration and humidity, and providing measurements with low cost equipment. A good agreement between the test method results and thermogravimetric analysis was observed, which highlights the reliability and accuracy of the developed test method.The results obtained from the gaseous diffusion coefficient and carbonatable products test methods were used as inputs for carbonation depth prediction models. A correlation was investigated between the measured carbonation depth on different concrete and hydrated cement pastes mixtures by means of phenolphthalein solution under both natural and accelerated exposure. The results were compared with the calculated carbonation depth using our experimental results.
Slag is widely used as mineral admixtures in cement-based materials by its potential hydration activity. It has the advantage of saving resources and energy, reducing carbon emission, improving the performance of concrete, and plays an increasingly important role in the building materials industry. But the early strength of slag is low, and the industrialization of useful hydration products also need to be activated, so the utilization rate of slag in high grade cement is restricted. The hydration activity of slag depends not only on the content of vitreous in slag, but also on the structure of vitreous slag. To explore slag glass micro composition and structure of its active role, The slag micro-structure was analyzed from the structure levels, and then the factors affecting the activity of slag was evaluated. The potential advantages and disadvantages of some different methods to active slag were discussed such as physical ways, chemical activation method and compound activation way. The existing problems and development direction of improving the activity of slag were summarized , which could provide a valuable reference for the efficient use of slag.
PCEs are well known to improve the initial fluidity of CSA. However, their dispersion efficiency drops quickly over time. This issue can be solved by incorporating retarders. In this context, this paper deals with the influence of citric acid, used as a retarder, PCE and their combination on the hydration and workability of CSA. Isothermal calorimetry, XRD and TG analysis were used to describe the hydration process, while workability was characterized with the mini-cone test. Adsorption behavior was investigated using total organic carbon analyzer coupled with ion chromatography. Results show that the introduction of citric acid retained the dispersion efficiency of PCE over time. However, the initial dispersion efficiency of PCE was decreased by citric acid as the latter tend to adsorb first on the surface of cement grains, inhibiting the adsorption of PCE. A dispersion model was proposed to describe the acting mechanism of these admixtures on CSA.
Slag-containing pastes and concretes were analysed by element-specific synchrotron-based techniques to determine the speciation of iron on crushed materials through spatially resolved micro-spectroscopic studies. The investigated cement samples were hydrated either in the laboratory, or exposed to river or sea water. Metallic iron, along with minor proportions of iron sulphide and magnetite was detected in the laboratory sample. Iron sulphide, goethite, and siliceous hydrogarnet were discovered in the blended slag cements hydrated in contact with river water for up to 7 years. In contrast, no Fe(0) was observed in blended concretes exposed to sea water. Instead, iron sulphide, iron(II)-hydroxide and -oxide, hematite, magnetite, siliceous hydrogarnet, and goethite were detected as well as ilmenite (FeTiO3) in the aggregates. The strong acceleration of Fe oxidation in samples exposed to sea water and the long-term passivation observed in the other samples indicate comparable processes as those occurring on steel bars.
Quaternary CaO−MgO−Al2O3−SiO2 (CMAS) glasses are important constituents of the Earth's lower crust and mantle, and they also have important industrial applications such as in metallurgical processes, concrete production, and emerging low-CO2 cement technologies. In particular, these applications rely heavily on the composition-structure-reactivity relationships for CMAS glasses, which are not yet well established. In this study, we combined force-field molecular dynamics (MD) simulations and density functional theory (DFT) calculations to generate detailed structural representations for a CMAS glass. The generated structures are not only thermodynamically favorable (according to DFT calculations) but also agree with experiments (including our x-ray and neutron total scattering data as well as literature data). Detailed analysis of the final structure (including partial pair distribution functions, coordination number, and oxygen environment) enabled existing discrepancies in the literature to be reconciled and has revealed important structural information on the CMAS glass, specifically (i) the unambiguous assignment of medium-range atomic ordering, (ii) the preferential role of Ca atoms as charge compensators and Mg atoms as network modifiers, (iii) the proximity of Mg atoms to free oxygen sites, and (iv) clustering of Mg atoms. Electronic property calculations suggest higher reactivity for Ca atoms as compared with Mg atoms, and that the reactivity of oxygen atoms varies considerably depending on their local bonding environment. Overall, this information may enhance our mechanistic understanding on CMAS glass dissolution behavior in the future, including dissolution-related mechanisms occurring during the formation of low-CO2 cements.
Activator type significantly affects the properties of alkali-activated slag (AAS) cement. Reactive MgO may have different effects on AAS cement activated with different activators. Four activators, sodium hydroxide (NaOH), water glass (WG), sodium carbonate (Na2CO3) and sodium sulfate (Na2SO4) were used in the present paper. The effect of reactive MgO on the fluidity, setting time of AAS paste and the compressive strength of AAS mortar was studied. The hydration process was assessed with pH and hydration heat. Microstructure and hydrates were also characterized by XRD, SEM and EDS. The results show that the addition of reactive MgO decreases the fluidity and setting time of AAS paste regardless of activator type. In NaOH activated slag, the compressive strength decreases with the increase of reactive MgO. While in the other three activators, there exists an optimal reactive MgO content of about 4%–6%. Now the compressive strength is higher. Reactive MgO increases the pH, heat evolution rate and cumulative hydration heat of AAS cement. Hydrotalcite and C-(A)-S-H gel are the main hydration products. More hydration products are promoted by reactive MgO, which results in the densification of AAS matrix. The above changes due to reactive MgO are especially remarkable for Na2CO3 and Na2SO4 activated AAS cement. AFt is more likely to be formed than hydrotalcite in Na2SO4 activated slag and reactive MgO enhances this trend. Reactive MgO is especially suitable for Na2CO3 and Na2SO4 activated AAS cement. It can accelerate the hydration, adjust the fluidity and setting time, and improve the mechanical properties to a reasonable degree.
Resistance to carbonation is one important attribute that low-CO2 cement alternatives must possess, and is particularly crucial for cement alternatives subjected to aggressive CO2 concentrations such as those used in construction of oil wells and wells for below ground carbon sequestration. Here, a parametric study of alkali-activated slag (AAS) carbonation in aggressive environments has been conducted to examine (i) calcium carbonate polymorphism using X-ray diffraction (XRD) and Fourier-transform infrared (FTIR) spectroscopy, and (ii) the extent of calcium carbonate formation and CO2 adsorption using thermogravimetric analysis (TGA). A range of AASs have been studied by varying the magnesium content of the slag, the activator type (sodium hydroxide and sodium silicate), the activator concentration, and the curing time prior to carbonation. It was uncovered that both (i) magnesium from the slag and (ii) silica from the activating solution are needed to reduce the propensity for the sodium-containing calcium-alumino-silicate-hydrate gel to undergo decalcification.
This paper investigates the atomic structure and phase assemblages in new sodium-stabilized magnesium aluminosilicate hydrate (M-(N)-A-S-H) cementitious binders. Results indicate that in the absence of Ca²⁺, Mg²⁺ promotes a binder atomic structure of Si-Al tetrahedral sheets and octahedral Mg sheets with hydrated Na⁺ cations likely in the interlayer sites similar to trioctahedral micas (phyllosilicates). NMR studies verify the incorporation of Al in tetrahedral silicate sheets. XRD demonstrates the ability of these regions to nucleate and form zeolites (i.e., sodalite) as well as the formation of Mg-Al layered double hydroxide (LDH) phases (i.e., meixnerite), which is expected due to high concentrations of Mg and Al. TGA results indicate that M-(N)-A-S-H possesses chemically bound water and hydroxyl units similar to other Mg binders. These results evince the critical role of Mg to form unique atomic structures and durability-linked phases in low-calcium alkali-activated materials.
During the last few years, the valorization of marine sediment has been marked by the incorporation of this material in cementitious matrices. Some studies have depicted, more particularly, the use of the sediment in alkaline activation of Ground Granulated Blast Furnace Slag (GGBFS) for Civil Engineering applications. The objective of this work is to understand the effect of clear sediment incorporation on the reactivity of GGBFS alkali-activation. Two alkaline activators are used, a sodium hydroxide solution (NaOH) and a mixture of sodium hydroxide and sodium silicate solution (NaOH + silicate). The influence of sediment incorporation on the reactivity of Alkali-Activated GGBFS (AAS) is determined through X-ray Diffraction XRD, thermogravimetry TG and Solid State Nuclear Magnetic Resonance (NMR) analyses. Results show that the presence of sediment does not modify the structure of the hydration products formed, neither the proportion of Aluminum atoms incorporated into hydrate chains. The substitution of slag by sediment causes a delay in the initiation of alkaline activation particularly in the short term, but this effect diminishes at 28 days.
The effects of magnesium content in the precursors on the phase formation and microstructural variation of alkali-activated binders under hydrothermal conditions are investigated. One-step hydrothermal treatment method was applied on all samples synthesized from industrial by-products (e.g., fly ash and slag), MgO powders, and alkali activator, and they were characterized by compressive strength tests, XRD, SEM, N2 sorption, and FTIR analyses. The samples containing zeolite Na-P1 and hydrotalcite crystals were obtained until the Mg/(Al + Si) molar ratio was higher than 0.58 with a 20 wt.% slag content in the precursor, however, the samples with 40 wt.% slag content possessed both of these phases even at lower Mg/(Al + Si) molar ratio of 0.39. Specifically, increasing the Mg/(Al + Si) ratio significantly reduced the formation of zeolite Na-P1, while promoted the formation of hydrotalcite. Furthermore, the extended hydrothermal treatment promoted the formation of zeolite Na-P1, but simultaneously reduced the formation of hydrotalcite. All samples exhibited mesoporous characteristics having major sorption behaviors of multilayer physisorption and capillary condensation.
The high autogenous shrinkage of alkali-activated materials made from slag and fly ash is recognised as a major drawback with regard to the use as construction materials. In this study, metakaolin was introduced into the alkali-activated slag-fly ash (AASF) paste to mitigate the autogenous shrinkage. The shrinkage mitigation mechanism of metakaolin was explained by studying the influences of metakaolin on the microstructure, shrinkage related properties, and mechanical properties of AASF paste. It was found that adding metakaolin could significantly reduce the chemical and autogenous shrinkage of AASF paste. This shrinkage mitigation is accompanied by a decrease in the alkalinity of AASF paste pore solution, a reduced drop in internal relative humidity, and an increase in porosity of AASF paste. Moreover, the incorporation of metakaolin does not change the type of the reaction products, but greatly delays the formation of the reaction products of AASF paste. The addition of metakaolin, above 5% of the binder, results in lower 28-day compressive and flexural strength of AASF paste.
This is an experiment on the effect of mixing time for alkali-activated cement (AAC) using a binder mixed with ground granulated blast furnace slag (slag) and fly ash (FA) in a ratio of 1:1 on the mechanical properties. The mixing method of ASTM C305 was used as the basic mixing method, and the following mixing method was changed. Simply adding the same mixing time and procedure, the difference in the order of mixing slag and FA, and controlling the amount of activator and mixed water were considered. As a result of the experiment, the addition of the same mixing time and procedure, pre-injection of slag, and high-alkali mixed water in which half of the activator and mixing water were mixed showed the highest mechanical properties and a dense pore structure. As a result, the design of a blending method that can promote the activation action of slag rather than FA at room temperature was effective in improving the mechanical properties of AAC. In addition, these blending factors showed a clearer effect as the concentration of the activator increased. Through the results of this experiment, it was shown that high-temperature curing, high fineness of the binder, or even changing the setting of the mixing method without the use of excessive activators can lead to an improvement of mechanical properties.
This research reports on the microstructure and nano-mechanical performance of different hardened pastes exposed to simulated coastal underground environments. The structure and density of the calcium (aluminium) silicate hydrate [C-(A)-S-H] phases in the hardened pastes were explored by nuclear magnetic resonance (NMR), backscatter electron (BSE) detector and nano-indentation. The results indicate that the average main chain length and C-A-S-H ratio of C-(A)-S-H in reactive powder hardened paste under 60℃ corrosion conditions for 420 days (RPHP-SE2) reached 7.19 and 47.39%, respectively. The two values were much higher than those of high-performance hardened paste (HPHP) under equivalent conditions. Meanwhile, the C-(A)-S-H in RPHP-SE2 had the densest gel structure and highest nano-mechanical strength, which is beneficial to macroscopic mechanical properties and durability. RPHP can dissipate environmental energy and enhance its strength. The results are of great significance to ensure the safety of deep mines as the deeper depths of the earth are explored.
The characteristics of the hydration products and microstructural development of reactive MgO in the presence of metakaolin (MK) and silica fume (SF), along with thermodynamic modelling, are presented in this paper. The hydration products were characterized at different ages from 1 to 90 days. The hydration of MgO-MK mixes showed the formation of hydrotalcite like phase along with brucite and M-S-H. Coexistence of M-S-H and hydrotalcite was predicted from thermodynamic modelling in a system comprising of MgO, SiO2 and Al2O3. The voluminous nature of the hydration products helped in the development of a low porosity microstructure in MgO-MK mixes. The effects of a denser microstructure were evident from higher compressive strength across all ages and mix compositions compared to MgO-SF mixes. The bound water content of the hydration products showed a reasonable relationship with the compressive strength for both SF and MK mixes.
This study aims to examine the effect of high temperatures on lightweight geopolymer concrete (LWGC) and lightweight ordinary concrete (LWOC) made of natural pumice and lightweight expanded clay aggregate (LECA) with the addition of trapped air. To this end, a geopolymer concrete matrix has been synthesized by the alkali-activation of fly ash (FA) and granulated blast furnace slag (GBFS). The geopolymer concrete samples were cured for 24 hours at a temperature of 80 °C. To study the properties of fresh concrete, slump and slump flow tests and unit weight were applied. The mechanical properties were also measured by the compressive strength, splitting tensile, flexural strength, and elastic modulus tests. High temperatures of 100 °C, 200 °C, 300 °C, 400 °C, 500 °C, 600 °C, 700 °C, and 800 °C were applied on the geopolymer concrete samples to obtain the residual compressive strength. In general, pumice and LECA can be used as an alternative to the dolomite aggregate to produce LWGC and LWOC. The mixture F-50D50P-A achieved the lowest of unit weight of 1660 kg/m, a slump flow of 555 mm, and compressive strength of 32.9 MPa at 91-days. Lightweight geopolymer concrete containing 50% fly ash and 50% GBSF achieved the best compressive strength test results. The results showed a similarity in the behaviour of LWGC with LWOC under the influence of all the variables applied in this study.
Magnesium oxide based cements may provide a promising alternative to the conventional Portland cement in many applications. This study investigates the feasibility of using calcined kaolinitic clay to produce an MgO binder. The MgO-based binder were prepared using a low kaolinite content clay and metakaolin and were compared with a silica fume system. Implications of the addition of magnesium carbonate in the binder were also investigated. Isothermal calorimetry, X-ray diffraction (XRD), thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FTIR) were used to study the hydration characteristics of the binder system whereas compressive strength and porosity were measured to determine mechanical and durability attributes. The economic and environmental aspects of the binder system is also discussed. Hydrotalcite like phases were clearly produced on hydration in the clay mixes containing carbonate additions. The compressive strength of clay mixes was at par or better compared to systems containing silica fume, and the clay systems had significantly lower porosity levels. The presence of magnesium carbonate further enhanced the physical properties of the clay mixes.
The present study investigated the microstructural evolution and carbonation behavior of lime-slag binary binders. Samples having various lime/slag ratios were fabricated and characterized with the aid of X-ray diffractometry, thermogravimetry analyses, ²⁹Si nuclear magnetic resonance spectroscopy, mercury intrusion porosimetry, autogenous and carbonation shrinkage measurements, carbonation depth measurements, and compressive strength development tests. These test results revealed that the carbonation of lime-slag binary binders yielded stable carbonates and subsequently refinement of the pore size and a corresponding strength modification. In addition, the lime-slag binary binders exhibited much less autogenous shrinkage movement than typical alkali-activated slag made with a combination of sodium hydroxide and water glass due to its low reaction rate and volumetric expansion accompanied by the hydration of lime. On the other hand, the phenomenon of the carbonation shrinkage of lime-slag binary binders resembles that of Portland cement due to the presence of portlandite, which served as a carbonation buffer.
This paper examines on the setting time and mechanical strength behaviour of slag cement pastes activated with different alkaline activators. For this purpose three alkaline solutions were used: waterglass solution (27% SiO2, 18% Na2O and 55% H2O), NaOH and Na2CO3, maintaining always a constant concentration Of Na2O (4% by mass of slag). The solutions were prepared with mixes of 0%, 80%/20% and 20%/80%. The activation process was studied at early ages by conduction calorimetric and fourier transform infrared spectroscopy (FTIR). Results show that the initial pH of the alkaline solution plays an important role in the initial slag dissolution. However the factor playing a decisive role in the acceleration or delay of setting times and in the development of mechanical strengths is the nature of the anion present in the solution. SiO42- ions act as an accelerator of the setting time, but CO32- ions delay the setting time.
The objective of this work was to determine the mechanisms that govern the setting times of alkali-activated slag (AAS) cement pastes, as a function of the nature of the alkaline activator used. For this purpose three different activators were used: waterglass (Na2SiO2.nHaOH with a SiO2/NaO ratio equal 1.5), NaOH and Na2CO3. The concentration of all solutions was constant = 3% of Na2O by mass of the slag. The pastes were studied through isothermal conduction calorimetry, FTIR and Si-29 and (27)AL MAS-NMR. Results indicate that pastes activated with waterglass develop faster setting due to the formation of an initial calcium silicate hydrate. Setting, in pastes activated with NaOH is due to the formation of a more polymerised calcium silicate hydrate. Pastes activated with Na2CO3 show longer setting times due to the initial formation of a sodium calcium carbonate which retard the reaction processes.
The composition of solid and liquid phases of alkali-activated slag systems was investigated by thermodynamic modelling. The calculations predict the formation of C(-A)-S-H, stratlingite and hydrotalcite in different alkali-activated slag systems, which agrees well with experimental findings. In addition traces of FeS and ettringite are predicted to precipitate. The type of alkaline activator (Na-silicate, NaOH) has no influence on the type of hydrates formed but influences the Ca/Si ratio of the C-S-H, The addition of sodium silicate leads to the formation of C-S-H with a higher Ca/Si ratio (1·1-1·2), whereas NaOH results in a Ca/Si ratio of 0-8 to 1·1. In contrast, the kind and concentration of the activator has strong influence on the composition of the pore solution. Low concentrations of the alkaline activator result in lower Na, Si and Al concentrations and lower pH values. In all pore solutions a high concentration of sulphide (HS-) is expected to be present.
A new synthesis of Mg A1 double hydroxides (LDHs) has been performed by hydrothermal crystallization at 80 °C of mixtures of allumina gel and MgO from decomposition at 650 °C of magnesium basic carbonate. The products are relatively low in (0.8-I.0) in CO2. The general formula [Mgl1-x~AIx (OH)2] [(OH)x(H20)0.8-x] with 0.23 ~< x ~<0.33. Cell parameters and thermal data are given. The c-axial length decreases with Al-content; this is attributed to increased electrostatic attraction between layers.
(19) A New Synthesis and Characterization of Magnesium-Aluminum Hydroxides. Available from: https://www.researchgate.net/publication/249849126_A_New_Synthesis_and_Characterization_of_Magnesium-Aluminum_Hydroxides [accessed Feb 06 2018].
Sorption, co-precipitation and re-crystallisation are important retention processes for dissolved contaminants (radionuclides) migrating through the sub-surface. The retention of elements is usually measured by empirical partition coefficients (Kd), which vary in response to many factors: temperature, solid/liquid ratio, total contaminant loading, water composition, host-mineral composition, etc. The Kd values can be predicted for in-situ conditions from thermodynamic modelling of solid solution, aqueous solution or sorption equilibria, provided that stoichiometry, thermodynamic stability and mixing properties of the pure components are known (Example 1). Unknown thermodynamic properties can be retrieved from experimental Kd values using inverse modelling techniques (Example 2). An efficient, advanced tool for performing both tasks is the Gibbs Energy Minimization (GEM) approach, implemented in the user- friendly GEM-Selector (GEMS) program package, which includes the Nagra-PSI chemical thermodynamic database. The package is being further developed at PSI and used extensively in studies relating to nuclear waste disposal.
A multi-method approach was used for the investigation and comparison of alkali-activated slag binders (AAS), pure slag and ordinary Portland cement (OPC). X-ray fluorescence, X-ray powder diffraction, granulometry, calorimetry, thermo-gravimetric analysis and environmental scanning electron microscope investigations of the microstructure with energy dispersive X-ray analyses were used to characterise the cements and their hydrate phases. In addition, the chemical composition of the pore solution, including the different sulphur-containing ions, was analysed. The precipitation mechanisms during binder hydration in the AAS and OPC systems exhibit significant differences: in AAS the formation of the 'outer product' C-S-H is much faster than in OPC, The high Si concentrations in the pore solution during the early hydration of AAS are related to the fast dissolution of Na-metasilicate. The fast reaction of Na is an important factor for the voluminous precipitation of C-S-H within the interstitial space already during the first 24 h. In addition to the Na-metasilicate component, the high fineness of the slag represents a further important factor for the fast hydration of AAS. The small slag particles (< 2 μm) are completely dissolved or hydrated within the first 24 h, whereas hydration of the larger particles is much slower. The fast formation of a gel-like matrix in AAS is the product of a fast 'through solution' precipitation, which contrasts with the slower dissolution-precipitation mechanism of a 'topotactic' growth of C-S-H in OPC. The chemical and mineralogical characterisation of solid and liquid phases and their changes with time are the basis for thermodynamic modelling of the corresponding hydration process, which is presented in a second paper.
Quantification of the progress of hydration of supersulfated cements (SSC) may be approached in two ways: (a) from recording the increasing dissolution of the slag particles directly, and (b) indirectly from quantifying the formation of the hydration phases. Image analysis based on backscattered electron imaging in a scanning electron microscope (SEM), the dissolution of hydrates (EDTA), differential thermal analysis (DTA) and sulfide concentration (SP) were used to quantify the dissolution of the slag particles; selective extraction of hydrates by sodium carbonate (SE), X-ray diffraction (XRD) with Rietveld analysis and thermogravimetric (TGA) refinement methods were used to quantify the amount of hydration products formed. SEM-based image analysis was found to be a direct and promising way for the quantification of slag particles. With the help of selective extraction by sodium carbonate (SE), it was possible to quantify the amorphous C-S-H phase in SSC. Mass balance calculations constrained by thermodynamic stability were used to calculate the amount of reacted slag in the system. XRD Rietveld and TGA methods were used to assess the amounts of specific hydration products formed in SSC but did not allow an absolute quantification of the amount of slag reacted. Other methods such as the dissolution of the hydrates by EDTA and DTA were not found to be reliable due to intrinsic problems. Thomas Telford Ltd
The crystal structure of stratlingite Ca2Al(AlSi)O2(OH)10.2.25H2O (Z = 3) from Montalto di Castro (VT), Italy, was solved and refined in space group R3m (isotopic R = 0.080). The structure contains ordered CaVII and AlVI polyhedra fully occupying "octahedral' brucite-type layers hydrogen bonded to partially occupied double tetrahedral (Si/Al = 1) layers. Cell dimensions: a = 5.745(7); c = 37.77(1) Å imply the stacking of three main ("octahedral') layers and three "interlayers' (double-T layers) along the [001] direction as compared with two main layers for the mineral vertumnite which has therefore been proved to be a polytype with a c parameter equal to two thirds that of stratlingite. New chemical data on vertumnite holotype together with those obtained for stratlingite holotype show very close analogies between the two mineral species also in terms of their crystal chemistry. -from Authors
A layered double hydroxide (LDH) hydrotalcite–pyroaurite solid-solution series Mg 3 (Al x Fe 1 − x)(CO 3) 0.5 (OH) 8 with 1 − x = 0.0, 0.1……1.0 was prepared by co-precipitation at 23 ± 2 °C and pH = 11.40 ± 0.03. The compositions of the solids and the reaction solutions were determined using ICP-OES (Mg, Al, Fe, and Na) and TGA techniques (CO 3 2− , OH − , and H 2 O). Powder X-ray diffraction was employed for phase identification and determination of the unit cell parameters a o and c o from peak profile analysis. The parameter a o = b o was found to be a linear function of the composition. This dependency confirms Vegard's law and indicates the presence of a continuous solid-solution series in the hydrotalcite–pyroaurite system. TGA data show that the temperatures at which interlayer H 2 O molecules and CO 3 2− anions are lost, and at which dehydroxylation of the layers occurs, all decrease with increasing mole fraction of iron within the hydroxide layers. Features of the Raman spectra also depend on the iron content. The absence of Raman bands for Fe-rich members (x Fe > 0.5) is attributed to possible fluorescence phenomena. Based on chemical analysis of both the solids and the reaction solutions after synthesis, preliminary Gibbs free energies of formation have been estimated. Values of ΔG° f (hydrotalcite) = − 3773.3 ± 51.4 kJ/mol and ΔG° f (pyroaurite) = − 3294.5 ± 95.8 kJ/mol were found at 296.15 K. The formal uncertainties of these formations constants are very high. Derivation of more precise values would require carefully designed solubility experiments and improved analytical techniques.
Three series of fine limestone aggregate, alkali-activated blast furnace slag (AAS) concretes were fabricated and tested;
two through activation with waterglass/NaOH solution, of which one included NaCl as a retarding agent, and one activated by
Na2CO3. Each of these series was made up of three formulae containing different amounts of Al2O3. The compressive strengths of the series activated by waterglass/NaOH after 28days were ≈65±5.3MPa, a 22% increase compared
to previously reported formulae containing no additional Al2O3. Increasing the amount of Al2O3 did not further increase strength, however. The Na2CO3-activated formulae had strengths of ≈35±3MPa after 28days, representing no increase in strength over formulae not containing
Al2O3 previously reported. X-ray diffraction showed the main binding phase to be calcium silicate hydrate (C–S–H) gel, as is commonly
found in ordinary Portland cement (OPC). Fourier transform infrared spectroscopy showed little difference from the previously
reported results for formulae not containing Al2O3 and strongly resemble the spectra reported elsewhere for C–S–H. Electron microscopy, coupled with energy dispersive spectroscopy,
showed the cementing phase to be a single homogenous phase—not a mixed system of geopolymer and C–S–H gel—with a lower volume
fraction of unreacted slag than formulae without Al2O3. The reason for the increase in strength of Al2O3-containing formulae is unclear, but is unlikely to be ascribed to the formation of large amounts of ‘geopolymers’ and may
be related to a possible increase in reaction temperature of between 2 and 5°C, depending on amount of additive.
KeywordsSlag-Alkali-activated concrete-Geopolymer-SEM
For first time, an experimental and computational study has been conducted to define a structural model for the C-A-S-H gel forming in alkali-activated slag (AAS) pastes that would account for the mechanical properties of these materials. The study involved a comparison with the C-S-H gel forming in a Portland cement paste.The structure of the C-A-S-H gels in AAS pastes depends on the nature of the alkali activator. When the activator is a NaOH, the structure of the C-S-H gel falls in between tobermorite 1.4 nm with a mean chain length of five, and tobermorite 1.1 nm with a mean length of 14. When waterglass is the activator the structure of the C-A-S-H gel is indicative of the co-existence of tobermorite 1.4 nm with a chain length of 11 and tobermorite 1.1 nm with a chain length of 14. This very densely packed structure gives rise to excellent mechanical properties.
Lime-metakaolin-quartz mixtures containing varying amounts of quartz were prepared and hydrothermally cured at 180°C. The nature of hydration products with reaction time was studied using a combination of solid-state 27Al MAS NMR, XRD, DTA-TGA, SEM and wet chemical techniques. Hydrogarnet always preceded Al-substituted 11 angstrom tobermorite irrespective of the amount of quartz used and supplied most, if not all, of the Al to form Al-substituted 11 angstrom tobermorite. The availability of silicate anions from quartz was an important factor in providing a sink for the Al species to be in tetrahedral coordination. It also offered a means for controlling the extent of Al substitution into the 11 angstrom tobermorite crystal lattice with reaction time and the extent of breakdown of the hydrogarnet. The growth of hydrogarnet was inhibited with increasing amounts of quartz, while the formation of Al-substituted 11 angstrom tobermorite was accelerated. With increasing amounts of quartz, the crystallite size in the c-direction of Al-substituted 11 angstrom tobermorite decreased and a change in its morphology was evident from platelets to aggregated petal-like, fine crystals.
This paper studies the blast furnace slag glass phase structure by a series of analysis methods. In glass phase, both Si and Al ions are confirmed to occupy only tetrahedral sites, while the [SiO4]4- and [AlO4]5- are separated by Ca and Mg. Furthermore, the glass structure corresponds to micro-crystal model, which means it contains some nano-scaled micro-crystals in the glass phase. In addition, the slag glass may separate into two phases: a calcium rich phase and a silica rich phase. According to devitrification experiment, it has been inferred that the chemical composition and structure of silica rich phase are close to that of akermanite,which means most of Si is distribute around Mg.
The objective of the present work is to know the joint influence of a series of factors (specific surface of the slag, curing temperature, activator concentration, and the nature of the alkaline activator) on the development of mechanical strengths in alkaline-activated slag cement mortars. To reach this aim, a factorial experimental design was carried out (a complete 23 × 31 design) for every age studied (3 to 180 days). Through the variance analysis, the most significant factor on the response turned out to be the alkaline activator nature. The activator used, Na2SiO3 · nH2O + NaOH, was the factor that gave the highest mechanical strengths in all tests. The next most statistically significant factor was the activator concentration, followed by curing temperature, and, finally, the specific surface of the slag. The equations of the model describing the mechanical behaviour for flexural and compressive strengths and their relationships for each age studied were established
The hydration and the microstructure of three alkali activated slags (AAS) with MgO contents between 8 and 13wt.% are investigated. The slags were hydrated in the presence of two different alkaline activators, NaOH and Na2SiO3·5H2O (WG). Higher MgO content of the slag resulted in a faster reaction and higher compressive strengths during the first days. The formation of C(−A)–S–H and of a hydrotalcite-like phase was observed in all samples by X-ray diffraction (XRD), thermal analysis (TGA) and scanning electron microscopy (SEM) techniques. Increasing the MgO content of the slag from 8 to 13% increased the amount of hydrotalcite and lowered the Al uptake by C–S–H resulting in 9% higher volume of the hydrates and a 50 to 80% increase of the compressive strength after 28days and longer for WG activated slag pastes. For NaOH activated slags only a slight increase of the compressive strength was measured.
The chemical structure of an amorphous slag with blast furnace composition was investigated by means of molecular dynamics simulation. Our calculation suggested that the slag had a depolymerized network of SiO4 and AlO4 tetrahedra with interstitial cations, Ca2+ and Mg2+. The structural properties such as average coordination number obtained at 300 K were in good agreement with a recent NMR study, supporting the feasibility of the structure prediction by such simulation technique. At 1873 K, the coordination numbers of the ions almost remained unchanged, while the intertetrahedral angles were found to be narrower, and the Q(n) distribution of AlO4 tetrahedra was slightly modified. The small amount but significant incorporation of MgO and Al2O3 influences the network connectivity, which should affect macroscopic properties such as viscosity.
This paper presents the results of an investigation into the hydration process and development of the microstructure of alkali-activated slag. Samples were examined X-ray diffraction, scanning electron microscope, X-ray microanalysis of energy dispersive spectrum, and solid-state nuclear magnetic resonance. Hydration products have been identified as calcium-silica-hydrate (C-S-H) gel of low Ca/Si ratio, hydrotalcite, AFm phases and CH. It has been found that two types of AFm phase exist, depending on the exposure conditions. The initial AFm phase is formed through the transformation of CH and, upon exposure to air, it transforms into a carbonated form, hemicarbonate. It has been proposed that aluminum plays a key role in enhancing silicate polymerization. Slag compositional effects are discussed on the formation of hydration products and the development of microstructure.
This paper presents some recent research on the microstructural development during alkaline activation of slag pastes. Pastes of different slags activated with a range of activators have been prepared and some preliminary results from the study of the pastes activated with sodium hydroxide and waterglass solutions are presented. X-ray diffraction (XRD), differential thermal analysis (DTA), backscattered electron (BSE) imaging of polished samples in the SEM coupled with X-ray microanalysis, and secondary electron imaging of fracture surfaces were used to study the pastes.This study indicates: (i) that the products form by a dissolution and precipitation mechanism during the early stages of reaction, but at later stages the reaction may continue by a solid state mechanism; (ii) regardless of the activator used, the main hydration product is calcium silicate hydrate with low CS ratio and varying degrees of crystallinity; (iii) a crystalline phase of hydrotalcite type is formed in slag activated with either NaOH or waterglass; (iv) a crystalline phase of AFm type is also formed in slag activated with NaOH; (v) no hydrates of zeolite group or mica group were formed in slag activated with either NaOH or waterglass solution after wet curing at 20 ± 2 °C up to 15 months or at 80 °C for 14 days.
Alkali-slag cements are of higher strength, and their other properties are also better than those of Portland cement. In this paper, the authors studied how the properties of alkali-phosphorus slag cement were influenced by the modulus of water glass (Na2O:nSiO2), soluble phosphates, water to solid ratio and the fineness of the slag when water glass and granulated phosphorus slag (GPS) were used to make alkali-GPS cement. The experimental results indicated that the modulus of water glass had great influence, but soluble phosphates had little influence on the properties of the cement. If the water to solid ratio was increased, it was not helpful both to the early hydration or to the strength of the cement. There would be a critical fineness after some days. If the fineness was larger than the critical value, the fineness hardly affected the strength of the cement.
The General model for CSH gel described by Richardson and Groves (1) has been extended to incorporate elements other than Ca, Si, O and H which have been detected by X-ray microanalysis of gels in hardened Portland cement and blended cement pastes.
The dissolution mechanisms of multioxide silicate minerals and glasses differ from those of single (hydr)oxides because their dissolution may require the breaking of more than one metal-oxygen bond type. A general kinetic description of major rock forming multioxide silicate dissolution is developed in the present study by assuming the following: (1) the relative rates at which various metal-oxygen bonds are broken within a multioxide structure are consistent with the relative dissolution rates of the single (hydr)oxides; (2) the difference in the rates of breaking each metal-oxygen bond type is sufficiently large such that the reaction breaking one bond type can attain equilibrium before breaking substantial quantities of slower breaking metal-oxygen bonds; and (3) those metal oxygen bonds that break before the final destruction of the structure liberate metal atoms via metal-proton exchange reactions.Multioxide dissolution proceeds via a series of metal-proton exchange reactions until the mineral or glass structure is destroyed. This metal-proton exchange reaction sequence is shown to be consistent with leached layer compositions at acidic conditions. The last metal-proton exchange reaction in the series is slowest and thus rate controlling. Of these slowest exchanging metals, those partially freed from the structure by being adjacent to previously exchanged metals are liberated faster than those completely attached to the mineral or glass and thus constitute the rate-controlling precursor complex. The identity and reactions forming this precursor complex are used within the context of transition-state theory to derive equations that describe accurately the dissolution rates of the major rock-forming multioxide silicate minerals and glasses as a function of solution composition over the full range of chemical affinity.
Local environments of all constituent elements (Si, Al, O, Mg, and Ca) in an amorphous slag were examined using multi-nuclear solid-state NMR under high magnetic field (16.4T). The amorphous slag framework structure can be generally described as a depolymerized, chain-like network of SiO4 tetrahedra branched with AlO4 tetrahedra. 17O multiple-quantum magic angle spinning (MQMAS) spectrum demonstrated that oxygens occupy structurally inequivalent sites, depending on their bonding nature. 25Mg and 43Ca MQMAS spectra also showed multi-site occupancy of the ions, and the average coordination numbers were estimated to about 6 and 7, respectively. Our results should underscore the impact of the high magnetic field NMR techniques, especially MQMAS, when applied to non-crystalline solids.
High temperature powder X-ray diffraction (HTXRD) patterns of Mg/Al-layered double hydroxides (LDH) with interlayer carbonate (Mg/Al/CO3-LDH) indicate that a solid phase having a hydrotalcite-like layered structure (Phase I) is not stable when the temperature of solid samples is elevated. Another phase (Phase II) grows eminently and simultaneously with the degradation of Phase I. Phase II has smaller basal spacing than that of Phase I and is not stable when the temperature is elevated above 380°C. Both MgO and a stable spinel phase (MgAl2O4) finally appear in the temperature range of 400–1000°C. Two endotherms associated with weight loss are observed in differential thermal analysis/thermal gravimetry (DTA/TG). Phase II appears at the endotherm of lower temperature and disappears at that of higher temperature. The temperature at which Phase II appears depends on atomic ratio Mg/Al in Mg/Al/CO3-LDH whereas the temperature at which Phase II disappears does not. Phase II is metastable at room temperature and it transforms to an amorphous phase slowly after days of storage in dry conditions although it returns quickly to Phase I in a wet atmosphere. Solid state chemistry of the synthetic Mg/Al/CO3-LDH is discussed by means of HTXRD, DTA/TG and X-ray photoelectron spectroscopy (XPS).
Within the framework of improving mechanical properties of activated blast-furnace slag cements, a set of hardened pastes of 28 days age
were analyzed by 29Si and 27Al high-resolution nuclear magnetic resonance (NMR) at 9.4 T. Structural and compositional differences among
C-S-H phases obtained with different activation and curing conditions were characterized by NMR. Activation of the slag was done with
compounds of different alkalinity (sodium silicate, sodium hydroxide, calcium hydroxide and gypsum), and under steam and wet curing.
Parameters characterizing the extent of the hydration reaction, the polymerization degree of the aluminum-silicate chains and the Al/Si ratio
in C-S-H were obtained from NMR spectra. 29Si-NMR spectra indicated that connectivites of silicate tetrahedra in all pastes are compatible
with the ‘‘dreierkette’’ structural model of C-S-H. A substantial degree of polymerization of the aluminum-silicate groups in C-S-H was
observed in pastes resulting from activation with sodium silicate and gypsum/calcium hydroxide blend. Steam curing and a higher alkali
concentration enhanced the incorporation of Al in C-S-H. Even with low concentration of alkali, through steam curing it is possible to obtain
a degree of incorporation of Al in C-S-H as high as in the case of higher alkali addition.
The hydration and the strength evolution of supersulphated cements (SSC) produced by the activation of two different ground granulated blast furnace slags with anhydrite and small amounts of an alkaline activator have been investigated. The main differences between the two mixtures are found to be the strength development, the dissolution rate of the slags, the amount and volume of the individual hydration products formed and the growth mechanisms of the ettringite. The chemical composition of the slag had a large influence on the amount of the hydrates formed and thus on the volume of the hydrated slag.Advancement of the amount of hydrates of a slag with low reactivity by adding aluminium sulphate and calcium hydroxide did increase the amount of ettringite. Nevertheless, the early compressive strength was not increased, but late strength shows a slow increase. It was concluded that the early compressive strength of an SSC using low reactive slag cannot be overcome by adding stoichiometric amounts of constituents which are used for the formation of a specific hydration product. The best way to increase early compressive strength is to increase the intrinsic dissolution rate.
Pastes of blast-furnace slag were cured for up to 90 d using sodium silicate (waterglass), NaOH, and three different mixtures of Na2CO3–Na2SO4–Ca(OH)2 to activate reactions. The highest slag reactivity was observed for NaOH activation and the least for waterglass, although nonevaporable water indicated similar amounts of hydration products formed. The main hydration products found using X-ray diffractometry in all systems were calcium silicate hydrate (C-S-H) and a hydrotalcite-type phase. Microanalysis was performed on pastes activated using 50% Na2CO3·25% Na2SO4·25% Ca(OH)2, NaOH, and waterglass; the chemical composition of the C-S-H in the waterglass case was different relative to the other two alkalis. For all alkaline agents used, the C-S-H seemed finely intermixed with a hydrotalcite-type phase of Mg/Al = 1.82, on average.
In backscattered electron (bse) images of polished cement sections, anhydrous material, calcium hydroxide, other hydration products (mainly C-S-H) and porosity can be distinguished on the basis of their grey level in the image. Using an image analyzer connected directly to the SEM, it is possible to resolve these four components and so measure their relative proportions and distributions. The effects of magnification and the number of fields measured on the accuracy of bse image analysis are examined. The volume fractions of anhydrous material, porosity and calcium hydroxide derived from bse image analysis are compared with those obtained by other techniques and good correlation was found for the measurement of anhydrous material and porosity.
Linear relations between group electronegativeity (EN) sums of ligands bonded to tetravalent silicon and silicon-29 nuclear magnetic resonance (NMR) chemical shift (δSi) are shown to exist for both type P silicon (all ligands have lone-pair electrons available for (d-p) π-bonding, e.g., in (MeO)4Si) and type S silicon (all ligands have only σ-bonding electrons available, e.g., in (CH3)4Si). For type P silicon having group electronegativity sums greater than 11, a range encompassing all minerals, we have used previously reported EN and δSi values (for aryl-, halo-, and alkoxysilanes) to describe the observed silicon-29 NMR chemical shift as δ(Si,P) = -24.336ΣEN(P) + 279.27. We then apply this correlation to a wide range of silicates and aluminosilicates (containing insular (Q0) to framework (Q4) Si sites) to predict silicon-29 NMR chemical shifts by means of a group fragment electronegativity sum approach, in which all fragments (e.g., OAl, OLi, OCa) attached to Si are assigned, on the basis of experiments on a series of model silicates and the above equation, a characteristic group (or fragment) electronegativity value. OSi group electronegativities are scaled linearly with bridging bond angle. As an example of the use of the method, the electronegativity sum value for the cyclosilicate (Q2) beryl (Al2Be3(SiO3)6) is derived as EN(OBe) + EN(OAl) + 2(EN(OSi) (168 2°)) = 15.67, which predicts a silicon-29 chemical shift of -102.1 ppm (from Me4Si), that compares favorably with the value from experiment, -102.6 ppm. On the basis of a total of 99 sites in 51 different compounds, the mean absolute deviation between theory and experiment is 1.96 ppm (correlation coefficient = 0.979). When all types of silicon are considered (Q0-Q4), this empirical approach is the most accurate method of predicting silicon-29 chemical shifts found to date.
A commercial blast furnace slag was activated using either sodium hydroxide or hydrous sodium metasilicate, and the degree of hydration was determined by 29Si magic angle spinning nuclear magnetic resonance (NMR). The results are compared with measurements made using scanning electron microscopy image analysis (SEM-IA). The results from both 29Si NMR and the SEM-IA measurements indicated a fast initial reaction with the alkali, and similar degrees of hydration throughout the reaction. The 29Si NMR results were analyzed using two different methods for fitting the residual slag in the decomposition of the 29Si NMR spectra: the first method used the spectrum of the anhydrous slag, whereas the second method used the spectrum of the dissolution residue of the hydrated sample. Only the first method provided a satisfactory simulation. The degree of hydration and the Al/Si atomic ratio within the C–S–H, deduced by 29Si NMR were in agreement with SEM-IA and EDX analyses.
The mid-, near-, and far-infrared (IR) spectra of synthetic, single-phase calcium silicate hydrates (C-S-H) with Ca/Si ratios (C/S) of 0.41–1.85, 1.4 nm tobermorite, 1.1 nm tobermorite, and jennite confirm the similarity of the structure of these phases and provide important new insight into their H2O and OH environments. The main mid-IR bands occur at 950–1100, 810–830, 660–670, and 440–450 cm−1, consistent with single silicate chain structures. For the C-S-H samples, the mid-IR bands change systematically with increasing C/S ratio, consistent with decreasing silicate polymerization and with an increasing content of jennite-like structural environments of C/S ratios >1.2. The 950–1100 cm−1 group of bands due to Si-O stretching shifts first to lower wave number due to decreasing polymerization and then to higher wave numbers, possibly reflecting an increase in jennite-like structural environments. Because IR spectroscopy is a local structural probe, the spatial distribution of the jennite-like domains cannot be determined from these data. A shoulder at ∼1200 cm−1 due to Si-O stretching vibrations in Q3 sites occurs only at C/S lessthan equal to 0.7. The 660–670 cm−1 band due to Si-O-Si bending broadens and decreases in intensity for samples with C/S > 0.88, consistent with depolymerization and decreased structural order. In the near-IR region, the combination band at 4567 cm−1 due to Si-OH stretching plus O-H stretching decreases in intensity and is absent at C/S greater than ∼1.2, indicating the absence of Si-OH linkages at C/S ratios greater than this. The primary Si-OH band at 3740 cm-1 decreases in a similar way. In the far-IR region, C-S-H samples with C/S ratio greater than ∼1.3 have increased absorption intensity at ∼300 cm−1, indicating the presence of CaOH environments, even though portlandite cannot be detected by X-ray diffraction for C/S ratios <1.5. These results, in combination with our previous NMR and Raman spectroscopic studies of the same samples, provide the basis for a more complete structural model for this type of C-S-H, which is described.
Solid-state 27Al and 29Si magic angle spinning NMR spectroscopy has been combined with electron energy loss spectroscopy carried out in the transmission electron microscope to determine the location of Al substituting in a semicrystalline C-S-H gel present in a hydrated synthetic slag glass. The gel is found to contain mainly pentameric silicate chains in which the central silicon is substituted by aluminum.
The hydration of two slags with different Al2O3 contents activated with sodium hydroxide and hydrous sodium metasilicate (commonly named water glass) is studied using a multi-method approach. In all systems, C-S-H incorporating aluminium and a hydrotalcite-like phase with Mg/Al ratio ~ 2 are the main hydration products. The C-S-H gels present in NaOH activated pastes are more crystalline and contain less water; a calcium silicate hydrate (C-S-H) and a sodium rich C-N-S-H with a similar Ca content are observed at longer hydration times. The activation using NaOH results in high early strength, but strength at 7 days and longer is lower than for the sodium metasilicate systems. The drastic difference in C-S-H structure leads to a coarser capillary porosity and to lower compressive strength for the NaOH activated than for the sodium metasilicate activated slags at the same degree of slag reaction.
A mechanical, mineralogical and microstructural characterisation of the cement pastes obtained by alkaline activation of fly ash/slag mixtures cured at different temperatures has been carried out. The pastes obtained were characterised by XRD, FTIR, MAS NMR, SEM/EDX, atomic absorption and ion chromatography, also the insoluble residue in HCl was determined.The results obtained have proved the existence of two different reaction products in those activated pastes. The average atomic ratios in the main reaction product were Ca/Si∼0.8, Al/Ca∼0.6, Si/Al∼2–3. Such analysis corresponds to calcium silicate hydrate rich in Al, which includes Na in its structure. Other reaction product which was detected in the pastes as result of fly ash activation, was an alkaline aluminosilicate hydrate with a three-dimensional structure.
This paper presents the results of the investigation of the hydration of alkali-activated slag (AAS) by nuclear magnetic resonance spectroscopy (NMR). The cross-polarization (CP) technique was used in combination with magic-angle spinning (MAS). This research was part of a systematic study of alkaline activation of slag by several different techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM) coupled with X-ray microanalysis of energy dispersive spectra (EDS), differential thermal analysis (DTA) and calorimetry. This NMR study provides information on the polymerization of silicates, the role of aluminates in cement hydration and the nanostructure of C–S–H gel.
The quantification of the fly ash (FA) in FA blended cements is an important parameter to understand the effect of the fly ash on the hydration of OPC and on the microstructural development. The FA reaction in two different blended OPC-FA systems was studied using a selective dissolution technique based on EDTA/NaOH, diluted NaOH solution, the portlandite content and by backscattered electron image analysis.The amount of FA determined by selective dissolution using EDTA/NaOH is found to be associated with a significant possible error as different assumptions lead to large differences in the estimate of FA reacted. In addition, at longer hydration times, the reaction of the FA is underestimated by this method due to the presence of non-dissolved hydrates and MgO rich particles. The dissolution of FA in diluted NaOH solution agreed during the first days well with the dissolution as observed by image analysis. At 28 days and longer, the formation of hydrates in the diluted solutions leads to an underestimation. Image analysis appears to give consistent results and to be most reliable technique studied.
Rietveld analyses on samples belonging to C–S–H and C–A–S–H series (0.8≤C/S≤1.7) were realized on X-ray powder patterns. The tobermorite M model was successfully used to refine all the powder patterns from C–S–H samples whatever the C/S value. This gives clear indication on the steady change in a unique structural description, corresponding to a ‘tobermorite M defect’ model, when passing from C–S–H(I) (C/S1.0). The possibility for both C–S–H types (from polymerized silicate chains to isolated silicate dimers) to accommodate the same structural model is explained by the continuous evolution of the occupancies of the cationic sites: the interlayer Ca atoms, the Si atoms from paired and bridging silicates. Accurate refinements of the structural and microstructural parameters evidenced the well crystallized feature of C–S–H phase combined with a small coherent domain size. Insertion of Al atoms in the C–S–H structure (C–A–S–H phase) involves a clear disruption into the layered atomic framework. The large increase of layer spacing observed when incorporating aluminum into C–S–H indicates that Al atoms should be located in the interlayer region of the structure in new crystallographic sites. Aluminum atoms are not substituted silicon crystallographic sites or interlayer calcium crystallographic sites.
The crystal structure of tobermorite 14 Å (plombierite) was solved by means of the application of the order–disorder (OD) theory and was refined through synchrotron radiation diffraction data. Two polytypes were detected within one very small crystal from Crestmore, together with possibly disordered sequences of layers, giving diffuse streaks along c*. Only one of the two polytypes could be refined: it has B11b space group symmetry and cell parameters a=6.735(2) Å, b=7.425(2) Å, c=27.987(5) Å, γ=123.25(1)°. The refinement converged to R=0.152 for 1291 reflections with Fo>4σ(Fo). The characteristic reflections of the other polytype, F2dd space group, a≈11.2 Å, b≈7.3 Å, c≈56 Å, were recognized but they were too weak and diffuse to be used in a structure refinement. The structure of tobermorite 14 Å is built up of complex layers, formed by sheets of sevenfold coordinated calcium cations, flanked on both sides by wollastonite-like chains. The space between two complex layers contains additional calcium cations and H2O molecules; their distribution, as well as the system of hydrogen bonds, are presented and discussed. The crystal chemical formula indicated by the structural results is Ca5Si6O16(OH)2·7H2O.
During the last 20 years, backscattered electron imaging of polished surfaces has become well established as a method for the study of cement and concrete microstructures. The technique has many advantages, including the visualisation of representative cross-sections over a wide range of magnifications and reproducible contrast dependent on atomic number. Nevertheless the limitations of observing a two-dimensional section of a three-dimensional structure must be borne in mind.In this paper, the general microstructural features of hydrated cement pastes are described. Although the amount of aluminate phase (“C3A”) in cement is comparatively minor, it plays an important role in determining many of the microstructural features of cement paste microstructure, for example in the formation of “Hadley” grains.Despite the very heterogeneous nature of cement paste, it is important to be able to derive quantitative measures if the relationships between microstructure and properties are to be understood. The possibilities to quantify BSE images are described. The interface between paste and aggregates in concrete is particularly variable, but average features can be measured, which help to understand the processes of packing cement grains, which gives rise to this region. Finally an example of the potential for BSE images to study concrete durability is given.
In this work, the relationship between the composition of pore solution in alkali-activated slag cement (AAS) pastes activated with different alkaline activator, and the composition and structure of the main reaction products, has been studied. Pore solution was extracted from hardened AAS pastes. The analysis of the liquids was performed through different techniques: Na, Mg and Al by atomic absorption (AA), Ca ions by ionic chromatography (IC) and Si by colorimetry; pH was also determined. The solid phases were analysed by XRD, FTIR, solid-state 29Si and 27Al NMR and BSE/EDX.The most significant changes in the ionic composition of the pore solution of the AAS pastes activated with waterglass take place between 3 and 24 h of reaction. These changes are due to the decrease of the Na content and mainly to the Si content. Results of 29Si MAS NMR and FTIR confirm that the activation process takes place with more intensity after 3 h (although at this age, Q2 units already exist). The pore solution of the AAS pastes activated with NaOH shows a different evolution to this of pastes activated with waterglass. The decrease of Na and Si contents progresses with time.The nature of the alkaline activator influences the structure and composition of the calcium silicate hydrate formed as a consequence of the alkaline activation of the slag. The characteristic of calcium silicate hydrate in AAS pastes activated with waterglass is characterised by a low structural order with a low Ca/Si ratio. Besides, in this paste, Q3 units are detected. The calcium silicate hydrate formed in the pastes activated with NaOH has a higher structural order (higher crystallinity) and contains more Al in its structure and a higher Ca/Si ratio than those obtained with waterglass.
Scanning electron microscopy was used to study the effects of the addition of ground granulated blast furnace slag (GGBFS) on the microstructure and mechanical properties of metakaolin (MK) based geopolymers. It was found that it is possible to have geopolymeric gel and calcium silicate hydrate (CSH) gel forming simultaneously within a single binder. The coexistence of these two phases is dependent on the alkalinity of the alkali activator and the MK / GGBFS mass ratio. It has been found that the formation of CSH gel together with the geopolymeric gel occurs only in a system at low alkalinity. In the presence of high concentrations of NaOH (> 7.5 M), the geopolymeric gel is the predominant phase formed with small calcium precipitates scattered within the binder. The coexistence of the two phases is not observed unless a substantial amount of a reactive calcium source is present initially. It is thought that voids and pores within the geopolymeric binder become filled with the CSH gel. This helps to bridge the gaps between the different hydrated phases and unreacted particles; thereby resulting in the observed increase in mechanical strength for these binders.
The objective of the present work is to know the joint influence of a series of factors (specific surface of the slag, curing temperature, activator concentration, and the nature of the alkaline activator) on the development of mechanical strengths in alkaline-activated slag cement mortars. To reach this aim, a factorial experimental design was carried out (a complete 23 × 31 design) for every age studied (3 to 180 days). Through the variance analysis, the most significant factor on the response turned out to be the alkaline activator nature. The activator used, Na2SiO3 · nH2O + NaOH, was the factor that gave the highest mechanical strengths in all tests. The next most statistically significant factor was the activator concentration, followed by curing temperature, and, finally, the specific surface of the slag. The equations of the model describing the mechanical behaviour for flexural and compressive strengths and their relationships for each age studied were established
Alkali activation of ground granulated blast furnace slag (GGBFS) with sodium silicate gave clinker-free binders, with high strength and early strength development, although set times were short and somewhat variable. Isothermal calorimetry detected three heat evolution peaks (wetting, gelation of activator and bulk reaction of slag). X-ray diffraction (XRD) showed no crystalline products. Hydration was investigated by scanning electron microscopy (SEM; with quantitative image analysis) and 29Si magic angle spinning nuclear magnetic resonance (MAS NMR). From early age, a uniform gel filled the initially water-filled space, and gradually densified as reaction proceeded. Microanalysis of outer product (OP) showed an Al-substituted C–S–H gel phase of widely variable (0.5–1.0) Ca/Si ratio. NMR showed long-chain substituted C–S–H with Al/Si ratio rising to 0.19 at 1 year, and also cross-linked material, consistent with a Ca- or Al-modified silica gel. Inner product (IP) regions around slag grains probably also contained hydrotalcite. Activation with KOH gave more rapid reaction of slag than for silicate activation, a less homogeneous microstructure, and lower strengths. The hydrates contained a substituted C–S–H gel of low Ca/Si ratio probably mixed with hydrotalcite, and occasional higher Al regions in the OP regions.
Alkali-activated cements as discussed here are those with compositions falling in the Me2O-MeO-Me2O3-SiO2-H2O system. This paper reviews their history of development and discusses their present status. Currently, there are major opportunities for such cements based upon (a) substantial knowledge of properties and mechanisms; (b) good track record of field performance in various applications and; (c) future orientation as environmentally friendly materials in accord with making use of substantial amounts of by-product and waste materials, thereby consuming less energy and generating less waste. The equivalent performance to Portland cement materials is one target for these cements, but, in many cases, the properties of alkali-activated cements actually are superior. It is important for assuring long-term durability to characterize more fully the complex solid phases, including determining the combined state of alkali in the solid hydration products, and of the residual soluble species in the pore fluids as a function of time.
This paper examines the early hydration of alkali-slag cements activated by different sodium compounds, such as NaOH, Na2CO3, Na2SiO3.5H2O, Na3PO4, Na2HPO4 and NaF, at 25 and 50 °C. A conduction calorimeter was used to monitor hydration kinetics. It was found that the initial pH of activator solution has an important role in dissolving the slag and in promoting the early formation of some hydration products. However, further hydration of alkali-slag cements is dominated by the reaction of the anion or anion group of activator and the Ca2+ dissolved from slag rather than the initial pH of the activator solution. Finally, three models were proposed to describe the early hydration of alkali-slag cements based on heat evolution measurements.
The adsorption of small amounts of alumina on the surface of amorphous silica reduces the rate of solution as well as the equilibrium solubility of silica in water even when much less than a monolayer of aluminum ions is present. It is postulated that negative aluminosilicate ion sites on the surface prevent the approach of hydroxyl ions which are required to catalyze the dissolution of silica, and that the solubility depends on the proportions of silica and alumina on the surface at equilibrium.
Steady state basaltic glass dissolution rates were measured as a function of aqueous aluminum, silica, and oxalic acid concentration at 25° C and pH 3 and 11. All rates were measured in mixed flow reactors, performed in solutions that were strongly undersaturated with respect to hydrous basaltic glass, and exhibited stoichiometric Si versus Al release. Rates are independent of aqueous silica activity, but decrease with increasing aqueous aluminum activity at both acidic and basic conditions. Increasing oxalic acid concentration increased basaltic glass dissolution rates at pH 3, but had little affect at pH 11. All measured rates can be described within experimental uncertainty using