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Effect of MgO and superfine slag modification on the carbonation resistance of phosphogypsum-based cementitious materials: Based on hydration enhancement and phase evolution regulation
... A notable influencing factor is the ambient curing moisture and improper curing methods, such as insufficient water, which are disadvantages to the hydration progress. Further, premature exposure to air (CO 2 ) leads to the decomposition of the already formed ettringite [15][16][17]. Thereafter, a comparative study on curing conditions affecting PCBA property evolution is desired so as to guide combined curing method selection for optimizing PCBA production. ...
This study compared the physical properties and mechanical strength development of PCBAs with water, sealed, standard, and open ambient air curing over 28 days to find a suitable curing method for the production of phosphogypsum-based cold-bonded aggregates. The types and relative amounts of hydration products, microstructural morphology and pore structure parameters were characterized utilizing XRD, TGA, FTIR, SEM and nitrogen adsorption methods. According to the results, water curing leads to rapid increases in single aggregate strength, reaching 5.26 MPa at 7 d. The standard curing condition improved the 28 d mechanical strength of the aggregates by 19.3% over others by promoting the generation of hydration products and the transformation of the C-S-H gel to a higher degree of polymerization and by optimizing the pore structure. Further, PCBAs achieved an excellent solidification of phosphorus impurities under all four curing conditions. This work provides significant guidance for selecting an optimized PCBA curing method for industrial production.
Little published data were effective in decreasing the setting time and improving the strength development of phosphogypsum-based supersulfate cement (P-SSC) containing an excess of 40% phosphogypsum to achieve adequate field working and mechanical properties. This study aimed to optimize the application performance of P-SSC by wet grinding, the enhancement mechanism of which was discussed further. The wet grinding mainly refined and dispersed the phosphogypsum with the large particle size, improving the formation of ettringite by increasing the supersaturation of phosphogypsum. However, the release of impurities prolonged the setting time of P-SSC pastes, leading to a lower early strength. Short-time wet grinding destroyed the surface structure of slag with the small particle size, presenting a higher hydration degree. It seemed to have a more significant improvement of generated C-(A)-S-H gel, while treating P-SSC by wet grinding slightly enhanced strength development. Increasing the aluminate concentration by incorporating active aluminum phases in this process significantly promoted the generation rate of ettringite and weakened the negative effect of impurity release. Therefore, a feasible and effective method to prepare P-SSC pastes was proposed to realize the large-scale application of phosphogypsum in the building materials industry.
This study investigated the effect of temperature (5 • C-50 • C) on the carbonation process, compressive strength and microstructure of CO 2-cured cement paste. Results showed that the carbonation process and rate were significantly affected by temperature and time. When the curing temperature increased, the rate of improvement of cement paste's properties was accelerated. The carbonation reaction was mainly kinetically controlled by product layer diffusion with an activation energy of about 10.8 kJ/mol. Temperature had greatly affected the structural transformation, morphological changes, size and amounts of calcium carbonate polymorphs (calcite, aragonite and vaterite) as well as their degree of crystallinity and decomposition temperature. Alongside calcite, vaterite and aragonite were formed at low and high curing temperatures, respectively. Apart from microstruc-ture, the compressive strength was also found to be very sensitive to temperature and carbonation products. The relationship between the amount of different carbonate polymorphs and the compressive strength was also provided.
The role of organic ligands on the formation of hydrated magnesium carbonates (HMCs) has been remaining unclear. This work reports insights into the effects of Mg-acetate on the carbonation of brucite including the kinetics of reaction, the precipitation of different HMCs, and reaction mechanisms. We found that the organic ligand increases the kinetics of brucite's carbonation and alter the formation and conversion of HMCs. A relatively unknown phase (i.e., giorgiosite) precipitates in the presence of Mg-acetate with nanowire morphology. With the presence of acetate ligand, nucleation sites formed after the breakdown of Mg-acetate complexes and be replaced by the Mg-CO 3 bonds. These sites act as a sink for Mg 2+ to grow crystals and prevent the passivation layer of HMCs on brucite's surface. Findings reported here can enable an approach to steer pore solution chemistry in the HMC-based binder for better reaction degree, durability, and mechanical properties.
The concentrations of 238U, 226Ra, 232Th and 40K maintained in phosphate rock (PR), phosphoric acid (PA) and phosphogypsum (PG) samples and its possible radiation hazards, have been measured. The radionuclides in phosphate rock have been redistributed unsymmetrically between phosphoric acid and phosphogypsum during the production process. It is obvious that Ra contributing more than 90% of the potential radiation hazards for PR and PG samples while Ra, Th, and K are contributing the radiation hazards of phosphoric acid. Phosphate rock and phosphogypsum declare higher radiological parameters than the world limit while for phosphoric acid are within the world permissible limits.
Calcium silicate hydrate (C-S-H) is the main hydration product of Portland cements. The partial replacement of Portland cements by supplementary cementitious materials can result in C-S-H with lower Ca/Si ratios and more aluminum and alkalis. The effect of equilibration time on Al uptake in C-S-H was investigated using equilibration times from 7 days up to 3 years. Lower Al concentrations were measured in the solution after longer equilibration times. In addition, a higher uptake of Al in C-S-H was observed based on the decrease in the content of secondary phases. Little further decrease in Al concentrations was observed after 2 years and longer at low Ca/Si. At high Ca/Si no significant change in solution concentrations was observed after more than 3 month, while the destabilization of secondary phases continued up to 1 year, indicating that a (meta)stable equilibrium was reached faster at higher Ca/Si ratios.
In recent years, solid waste of the production of mineral fertilizers, phosphogypsum, has been of great scientific and practical interest in construction. The results of scientific research and practical experience in many countries have convincingly proved the technical feasibility and feasibility of using phosphogypsum. The problems of reducing impurities of phosphorus and fluorine in the manufacture of building materials from phosphogypsum are considered. Comparative experimental studies of the presence of phosphorus impurities for the initial phosphogypsum of the Dnieper Mineral Fertilizer Plant, calcined phosphogypsum and mixtures of phosphogypsum were carried out. Comparative indicators of the presence of radioactive elements in phosphogypsum are given. It is shown that there are methods for reducing heavy metals in phosphogypsum.
Current demand for highly sustainable concrete, e.g. alkali-activated fly ash-slag (AAFS) concrete, urges understanding the links between microstructure and micromechanical properties of this binder. This paper presents a systematic investigation into the microstructure and micromechanical properties of AAFS paste from nano-scale to micro-scale. Nanoindentation was used to evaluate the micromechanical properties, while the microstructure was characterised using 29Si nuclear magnetic resonance, Fourier transform infrared spectroscopy, backscattered electron microscopy, and mercury intrusion porosimetry. The results indicate that N-A-S-H gels have a relatively low elastic modulus due to their high level of structural disorder and gel porosity, while the C-A-S-H gels and N-C-A-S-H gels with a low level of structural disorder and gel porosity have a relatively high elastic modulus. The elasticity of reaction products and their relative volumetric proportions mainly determine the macroscopic elasticity of AAFS paste, while the porosity and pore size distribution primarily condition its macroscopic strength.
This paper studies the mechanical properties and hydration of green concrete which is made from ground granulated blast-furnace slag (GGBS), desulfurization gypsum (DG) and electric arc furnace reducing slag (EAFRS) as cementitious materials. The green concrete with a water to cementitious materials ratio of 0.32 yields a compressive strength over 50 MPa at 28 days. The hydration mechanisms of GGBS-DG-EAFRS systems are studied using isothermal calorimetry (IC), thermogravimetric analysis (TGA), X-ray diffraction (XRD), and scanning electron microscopy equipped with energy dispersive X-ray spectroscopy (SEM-EDX). Although gypsum alone is able to slowly activate GGBS, the addition of EAFRS significantly accelerates the activation of GGBS. Compared to the portland cement paste, the ternary GGBS-DG-EAFRS pastes exhibit similar hydration heat evolution patterns but significantly lower hydration heat. The results from TGA and XRD show negligible amounts of Ca(OH)2 and Mg(OH)2, which is beneficial in the soundness of the green concrete. The results of XRD and SEM-EDX confirm that the main hydration products in the ternary GGBS-DG-EAFRS systems are ettringite, hydrotalcite and amorphous C–S–H with a low Ca/Si ratio (1.15–1.73) and a high Al/Ca ratio (0.02–0.42), which is favorable in obtaining good mechanical properties in the green concrete. The green GGBS-DG-EAFRS concrete sheds light on recycling industrial solid wastes, lowering the material cost and improving the sustainability in concrete industries.
Abstract
Fly ash is a by-product almost found in coal power plants; it is available worldwide. According to the hazardous impacts of cement on the environment, fly ash is known to be a suitable replacement for cement in concrete. A lot of carbon dioxide (CO2) is released during cement manufacturing. The investigations estimate that about 8 – 10% of the total CO2 emissions are maintained by cement production. Since fly ash has nearly the same chemical compounds as cement, it can be utilized as a suitable alternative to cement in concrete (green concrete). The current study analyzes the effect of the quantity of the two main components of fly ash, CaO, and SiO2, on the compressive strength of concrete modified with different fly ash content for various mix proportions. For this purpose, various concrete samples modified with fly ash were collected from the literature (236 datasets), analyzed, and modeled using four different models; Full-quadratic (FQ), Nonlinear regression (NLR), Multi-linear regression (MLR), and Artificial neural network (ANN) model to predict the compressive strength of concrete with different geometry and size of the specimens. The accuracy of the models was evaluated using correlation coefficient (R2), Mean absolute error (MAE), Root mean squared error (RMSE), Scatter Index (SI), a-20 index, and Objective function (OBJ). According to the modeling results, increasing SiO2 (%) increased the compressive strength, while increasing CaO (%) increased compressive strength only when the cement replacement with fly ash was between 52 and –100%. Based on R2, RMSE, and MAE, the ANN model was the most effective and accurate on predicting the compressive strength of concrete in different strength ranges. According to the sensitivity analysis, curing time is the most critical characteristic for predicting the compression strength of concrete using this database. The primary objective of this work is to explore and evaluate various machine-learning models for predicting compressive resistance. The study emphasizes these models' development, comparison, and performance assessment, highlighting their potential to predict compressive strength accurately. The research primarily uses machine learning, leveraging algorithms and techniques to build predictive models. The focus is on harnessing the power of data-driven approaches to improve the accuracy and reliability of compressive strength predictions.
This paper presents an experimental study on the effects of calcined gangue and wet co-milling on the properties of phosphogypsum-based excess-sulphate cementitious materials (PESCMs), including setting time, rheology, heavy metal solidification and strength development. For PESCMs containing 0%-50% calcined gangue, with and without wet co-milling, we examined hydration and microstructural evolution. Results indicate that the addition of calcined gangue shortens the initial and final setting time of fresh PESCM pastes, and impairs stability, fluidity and strength development. After wet co-milling, the rheological and mechanical properties of PESCM pastes are improved, where the binders with 10–20% calcined gangue show a 40% increase in 28-d compressive strength. Hydration of calcined gangue is rapid and weakens the retarding effect of impurities, providing more active aluminates and silicates for further hydration. Wet co-milling promotes physical dispersion, chemical dissolution and thus hydration, which helps refine microstructure for a better development of strength and heavy metal solidification capability.
The present study investigates the effect of carbonation curing and the water-cement ratio on the microstructural characteristics and CO2 uptake of calcium sulfoaluminate (CSA) cement. Low-pH carbonic acid promoted ettringite participation in CaCO3 formation. Different CaCO3 polymorphs, Al(OH)3, and unreacted ye'elimite occupied the higher proportion of the phase assemblage of carbonation-cured sample. Ettringite and strätlingite were the major phases of the water-cured sample. A maximum fourfold increase in the CO2 uptake capacity per gram of the carbonation-cured paste at the highest water-cement ratio was observed. ²⁷Al NMR demonstrates the strätlingite formation after 1 day of water curing, while unreacted ye'elimite was observed in the carbonation-cured sample. The dissolution of hydrated phases leading to the formation of CaCO3, moderately affects the strength. The maximum proportion of weight was reached within 1 day of water curing due to the reaction of ye'elimite and belite to the formation of ettringite and strätlingite, respectively.
In this study, two different ultra-fine ground granulated blast furnace slag referred to as GS (i.e., by dry-winnowing method) and WS (i.e., by wet-grinding method) were obtained, and the hydration and rheology of activated ultra-fine ground granulated blast furnace slag (activated-BFS) with carbide slag and anhydrous phosphogypsum (AG) as combined activator were systematically investigated by means of setting time, workability, rheological behavior, compressive strength, autogenous shrinkage, electrical resistivity, SEM, XRD and TG-DTG. The results showed that the limiting factor is the BFS type, and not the AG dosage in terms of setting time, and their rheological behavior fitted with H–B model. WS and increase in AG dosage significantly reduced the plastic viscosity, shear stress and yield stress of activated-BFS pastes. The WS presented higher initial reaction activity that generated more hydrates in the early age, restricting the further hydration of BFS particles especially when AG dosage exceeded 10%, thus leading to the different compressive strength development trends between mixtures containing WS (WS/A) and GS (GS/A). The compressive strength of GS/A mixtures increased with increasing AG dosage and reached 39.9 MPa at 3 days, while WS/A first increased and then decreased. The activated-BFS with carbide slag and AG is technically feasible to be employed as cementitious materials in the production of concrete.
Recycling of agricultural wastes to develop low-carbon supplementary cementitious materials requires the understanding of their effects on carbon sequestration, mechanical property and microstructure characteristics of cement-based materials. This work aims to disclose the mechanism of above-mentioned behavior in porous-structured rice husk ash (RHA) blended pastes using multi-technique investigations including mercury intrusion porosimetry (MIP), ²⁹Si magic angle spinning nuclear magnetic resonance (²⁹Si MAS NMR), Scanning electron microscope/Energy Dispersive Spectrometer (SEM/EDS) techniques coupled with carbonation-hydration model. It was discovered that pastes with increased dosages of RHA (5%–15%) presented an increase of CO2 uptake compared to that of control paste. It also showed an enhanced compressive strength after 12 h carbonation while simultaneously maintained a comparable strength development at 28 d. A declined permeability from water absorption experiment was also obtained especially in carbonated paste with 10% RHA. The enhanced polymerization degree of C–S–H and the decreased porosity due to the formation of calcite were the main contributors to the improved performances in RHA blended pastes.
Phosphogypsum-based excess-sulphate slag cement (PESSC) shows great resistance to sulphate and chloride ion erosion, and is expected to be an effective sink of waste phosphogypsum, to alleviate environmental pressure. Understanding the effect of magnesium ions is essential for the engineering application of PESSC in marine environment. In this paper, heat evolution, pore solution composition, phases assemblage, and mechanical properties of PESSC are characterized and analysed to reveal the effect of magnesium ions on early hydration and strength development. Results demonstrate that magnesium ions prolong the induction period and lead to a longer setting time. Increasing magnesium ions concentration from 0 mol/L to 0.2 mol/L makes the acceleration period appear earlier, and affects content of hydration products such as ettringite and C-(A)-S-H gel at early stage. Compressive and flexural strength can be significantly improved with magnesium ions addition at various degrees. It turns out that seawater could be favourable for PESSC if applied in marine environment.
In this study, an innovative wet carbonation process was developed to prepare aragonite whisker-rich materials (AWM) from cement and simulated flue gas, which subsequently served as micro-fiber additions to cement. The parameters including temperature, duration and MgCl2 concentration governing the aragonite formation were investigated, and the formation mechanism was explored. The results showed that needle-like aragonite whisker with a single crystal length of 10–30 μm and diameter of 0.5–2 μm could be synthesized rapidly when the cement suspension containing 0.05 M/L MgCl2 was carbonated at 80 °C. The formation of aragonite was favored in a suspension with a high concentration of Ca²⁺ and a minimum Mg²⁺/Ca²⁺ molar ratio >0.12. Additionally, amorphous phases including alumina-silica gel and silica gel were also detected. By mixing the AWM with cement, a new type of micro-fiber reinforced cement was developed, which showed significant mechanical performance improvement and embodied CO2 content (25.3 %).
This study aims to recycle the phosphogypsum (PG) as pavement subbase material. First, the physical and chemical properties of the PG before and after calcination treatment were characterized. Then the mix design, as well as the performance evaluation of the PG mixture, were detailed. Finally, the engineering application and economic analysis of the PG mixture for paving subbase pavement were presented. It is found that incorporation of the calcined PG (CPG) can help form gel structure to stabilize the PG. Meanwhile, addition of lime can neutralize the water-soluble phosphorus contained in the PG to improve the early strength of the CPG-stabilized PG. The optimal mix proportion of the CPG-lime stabilized PG by mass is determined as PG: CPG: lime = 89: 9: 2. In addition, as compared to the PG stabilized with the traditional inorganic binder materials, the CPG-lime stabilized PG has higher unconfined compressive strength (UCS) and better resistance to the water damage and fatigue damage caused by the water content variation. The field cores extracted from the paved CPG-lime stabilized PG subbase were experimentally observed to have a 7 d UCS as high as 3.69 MPa. Moreover, reuse of the CPG-lime stabilized PG is proven to be a highly economic and environment-friendly pathway for subbase paving, of which total cost per unit cube meter is 504.5 % lower than that of the commonly-used lime-fly ash stabilized soil. These findings identify the feasibility of utilizing the PG for pavement subbase construction.
This study investigates mechanisms of carbonation on the microstructure of phosphogypsum-based supersulfated cement (PSSC) paste at different CO2 concentrations (atmospheric, 5% and 20%). The products was characterized using XRD, TGA, FTIR and ²⁷Al MAS NMR. It is observed that the main hydration products of PSSC are ettringite, C-A-S-H gels and third aluminate hydrate (TAH), and the carbonation products are calcium carbonate (mainly vaterite and calcite), secondary gypsum, and C-A-S-H gels and separate TAH with varying degrees of decalcification. The quantitative analysis of BSEM images shows that the volume change of products occurring on carbonation significantly increases porosity and pore size of the PSSC paste. Results indicate that the carbonation of PSSC at different CO2 concentrations results in a progressive carbonation degree, 5% CO2 supports the accelerated carbonation experiment in PSSC to simulate the natural carbonation process due to the same phase composition and similar microstructure with atmospheric CO2 and 20% CO2 further loosens the microstructure of PSSC for the complete decomposition of ettringite and the severe decalcification of gel products.
Landfilled phosphogypsum would cause severe environmental issues, but the waste can be recycled for preparing excess-sulphate cement, an eco-friendly alternative to conventional cement. This paper investigates the effect of metakaolin (0–50%) on early hydration, phase assemblages and mechanical properties of the excess-sulphate phosphogypsum cementitious materials (ESPCMs). Results indicate that metakaolin is related to a new exothermic peak and significantly shortens the induction period. Setting time of ESPCM pastes is reduced by 13%–38% with 0%–50% dosage of metakaolin. More ettringite and highly disordered C-(A)-S-H gel are characterized when metakaolin dosage is below 20%, leading to 70% increase in 28-d compressive strength. With above 20% metakaolin dosage, portlandite consumption at early stage is promoted and hydration degree at late state is reduced. It turns out that 20% metakaolin dosage is efficient to optimise setting time and strength development of ESPCMs, where slag and metakaolin synergistically promote the formation of ettringite and C-(A)-S-H gel to bind the unhydrated cement particles effectively.
While traditional disposal of phosphogypsum either occupies excessive amounts of land or causes serious pollution, novel recycling of phosphogypsum is desirable. Making phosphogypsum-based cold-bonded aggregates (PCBAs) through granulation technology is a potential recycling strategy. In this study, the quality of the recycled PCBAs is assessed, considering physical properties, mechanical strength, impurity stabilization ability, and microstructure. With the increase of phosphogypsum content from 60% to 90%, 28-day bulk density of the PCBAs decreased from 1080 to 950 kg/m³, cylinder compressive strength in over dry (OD) condition decreased from 16.5 MPa to 3.9 MPa, and water absorption increased from 5.9% to 13.6%. SEM, XRD, and MIP analyses showed that as the phosphogypsum content increased, hydration products in the aggregates diminished and proportion of harmful pores (greater than200 nm) increased. Leaching test on 28-day PCBAs showed a reduced concentration of phosphorus and heavy metals in the leachate compared to the leaching of the original phosphogypsum. The study concluded that the PCBAs with the phosphogypsum content up to 80 % are qualified lightweight aggregates (LWAs) for concrete.
Reducing or replacing cement clinker is one of the most important ways for the sustainable development of the cement industry. In the present work, three types of industrial solid wastes, wet ground granulated blast-furnace slag (WGGBS), phosphogypsum (PG) and carbide slag (CS), were designed as clinker-free, high-performance ternary binders for applications such as construction and underground filling. The results show that the wet grinding process with PG and CS can synergistically excite the GGBS and make the compressive strength of the sample surpass that of the cement, reaching 45.6 MPa. However, its cost and carbon emissions are only 51% and 12% of cement, which is expected to completely replace cement as a low carbon cementitious material. In addition, with the increase of PG content and the decrease of CS content, the ettringite content increased significantly and its microstructure changed from long and thin to short and thick, and the porosity gradually decreased, which was beneficial to the development of compressive strength. The type and morphology of hydration products and the amount of crystalline hydrates are closely related to the ratio of PG/CS, and explain the development of compressive strength and microstructure of the clinker-free ternary binder.
Supersulfated cement (SSC) is a promising low-carbon binding material, however, its low hydration degree highly limits the performance and application of SSC products. In this work, aiming at a property enhancement, sodium lactate was added into mixtures of SSC, mainly composed of the hemihydrate gypsum and high-alumina slag. The experimental and theoretical results indicated that the hydration process of SSC could be finely controlled through lactates for two opposite mechanisms. At the early stage, the presence of lactates decreases the release of hydration heat and delays the hydration of SSC, which is beneficial to maintain the workability. In the long term, the molecular dynamics simulations suggest that lactates stably adsorb on the slag surface and strongly attract Ca ions, gradually weakening the structural integrity of geopolymer components in slag matrix. As a result, the dissolution of slag is accelerated, and thus more hydration product and higher mechanical properties are obtained for SSC. The findings in this study shed new lights on hydration mechanisms of SSC and help broaden the application of this low-carbon binding materials.
To ascertain the impact of carbonation on the phase evaluation of cement paste blended with metakaolin (MK), the phase composition of cement paste with different dosages of MK and at different carbonation time was characterized with X-ray diffraction (XRD) quantitative analysis technology. Meanwhile, the transformation law of Ca in different carbonation products was discussed systemically. The results show that, under accelerated carbonation, calcium hydroxide and Ettringite decrease sharply and disappear quickly, while CaCO3 increases significantly; after the complete consumption of CH, C-S-H is carbonated gradually and decreases greatly. Moreover, based on the content change of Ca in carbonation products, over 85% CaCO3 is generated through the carbonation of CH, C-S-H and unhydrated minerals. With the increase MK dosage, the ratio of the decrement of Ca in CH to the increment of Ca in CaCO3 declines gradually, while the percentage of the decrement of Ca in C-S-H grows gradually. Besides, at the early stage of carbonation, the decrement of Ca in CH constitutes the major part of the increment of Ca in CaCO3, and with carbonation time increasing, the proportion of the decrement of Ca in C-S-H rises gradually.
Carbonation hardening of Portland cement concrete can contribute to lower its CO2 footprint and to improve its strength. The reaction mechanisms involved in carbonation curing were investigated on fresh cement pastes. The quantitative results gained allowed to link the curing conditions to the reactions kinetics, phase assemblage and microstructure evolutions.
During the early carbonation curing, alite and belite react with dissolved CO2. The main carbonation products are calcium carbonate, an alumina-silica gel and a C-S-H phase with a low Ca/Si ratio. Hydration curing following the carbonation transforms the silica-rich phase into a C-S-H phase typical of hydrated Portland cements. Additionally, anhydrous calcium aluminates from clinker react to form ettringite and monocarbonate phases. The structure and composition of these phases is governed by the local equilibria in the dense microstructure. The carbonation and hydration reactions decrease the pore volume and refine the porosity resembling the microstructure development in the composite cements.
Innovations in the cement industry strive towards achieving an eco-friendly alternative to traditional cement. A low-carbon cement, calcium sulfoaluminate-activated supersulfated cement (CSA-SSC), has been recently developed. This cement consists of 80% granulated blastfurnace slag (GBFS), 15% anhydrite, and 5% high-belite calcium sulfoaluminate cement (HB-CSA) clinker. The hydration mechanism of CSA-SSC was experimentally investigated using isothermal calorimetry, X-ray diffraction, thermogravimetric analysis, and scanning electron microscopy, and was numerically studied using thermodynamic modeling. CSA-SSC shows a moderate compressive strength at the early stage, which is mainly attributed to the rapid formation of ettringite from the hydration of C4A3 S¯ with CaSO4 in HB-CSA clinker. Meanwhile, the hydration of f-CaO in HB-CSA clinker supplies an alkaline environment for the dissolution of GBFS and the formation of more ettringite. In the late stage, apart from ettringite, the hydration of GBFS forms C–S–H, leading to the continuous increase in late strength. A statistical analysis reveals that the high volume of GBFS in CSA-SSC results in a very low direct CO2 emission, which is only 8.7% of that of Portland cement.
Calcium Sulfoaluminate (CSA) cement is regarded as a plausible alternative to Ordinary Portland Cement (OPC) to reduce environmental burdens caused by the production of the latter; however, susceptibility to carbonation of the former restricts its utilization in the construction sector. This study proposes and evaluates a novel solution to address the carbonation issue of CSA by partial replacement with a fine CSA (<2.4 μm) cement-sand (1:3) mix. For this purpose, the modified CSA (MCSA) samples were prepared by substituting fine CSA cement-sand paste in conventional CSA mortar by 0 to 30 wt% and then submitted to accelerated carbonation. The performance of different MCSA mixes, OPC, and conventional CSA was compared after carbonation from the mechanical, mineralogical, and microstructural perspectives. The investigation reveals that the MCSA having 20 wt% fine CSA cement-sand mix could be regarded as optimum MCSA (OMCSA) in terms of carbonation degree, mechanical and microstructural characteristics after carbonation. The OMCSA shows a decrease of 8% and 23% in carbonation degrees and an increase of 12% and 25% in compressive strength upon carbonation as compared to OPC and CSA. Mercury intrusion porosimetric analyses (MIP) show that the porosity of OMCSA is reduced upon carbonation compared to conventional CSA. In addition, mineralogical analyses reveal that OMCSA contains the highest amount of ettringite and less carbonated contents upon carbonation than other subjected samples. OMCSA results in negligible loss of ettringite due to fewer pores as carbon dioxide is unable to penetrate the system, making it resistant to carbonation. Thus, the current solution reduces the carbonation impacts of CSA cement and is a step forward to make it more durable and practical.
In this paper, the mechanical properties of excess-sulfate phosphogypsum slag cement (PPSC) were studied by experimental data, microscopic tests and molecular dynamics (MD) simulations. Specifically, the properties of PPSC in terms of setting time, compressive and flexural strengths were investigated by experiment. The hydration products of PPSC were tested and observed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Furtherly, at the atomic scale, the relationship between nanoscale and macroscopic mechanical properties of PPSC was established. The mechanism of the effect of chemical composition on the mechanical properties of PPSC was studied at multiple scales, and the compound synergistic effect between chemical compositions was further studied. The results show that the mechanical properties of PPSC increase with the increase of CaO/SO3, and decrease with the increase of SiO2/Al2O3. CaO and Al2O3 can improve the mechanical properties of PPSC. The high mobility and structural instability lead to CaO/SO3 has greater effect on the mechanical properties of PPSC than that of SiO2/Al2O3. When CaO/SO3 is 1.8 ∼ 2.0 and SiO2/Al2O3 is 3.5 ∼ 3.7, the mechanical properties of PPSC are better. Moreover, when CaO/SO3 is 2.0 and SiO2/Al2O3 is 3.5, the mechanical properties of PPSC are the best. At this time, the compound synergistic effect of alkaline activator and sulphate activation is the best. This paper provides an important reference for the composition design and application of PPSC.
Low calcium minerals were considered as a potential sustainable binder and reported to be reactive to CO2. In this research, the carbonation characteristics of C3S, β-C2S and γ-C2S at early age were investigated in some new sights. The results show that carbonation degree and compressive strength of C3S increased most rapidly in the first 8 h carbonation, then slowed down. After 48 h carbonation, β-C2S and γ-C2S were close to C3S in compressive strength, and apparently higher than C3S in carbonation degree. The crystalline CaCO3 generated by carbonation were mainly calcite and aragonite, while vaterite was observed in β-C2S and γ-C2S within 8 h carbonation. Carbonation curing increased MCL of C-S-H and silicate polymerization. Besides, based on the combination of TG-DTG, XRD and ²⁹Si NMR, the decalcify priority of Ca sites in C-S-H was analyzed. The results indicate that interlayer Ca atoms inside C-S-H participated carbonation first, then the calcium atoms within interlayer and silicate chains of C-S-H would be removed successively.
Partial least squares (PLS) regression models were developed to quantify CaCO3 in cement and to study the CO2 effect on the material matrix. PLS results presented good correlation coefficient (R2 = 0.9995) and low estimation error (RMSEP = 3.61 mg CaCO3/g cement). From the results, it was observed that the portlandite consumption, the increase in CaCO3 content and the C-S-H decalcification-polymerization are the most relevant cement chemical transformations. Thus, it was concluded that: i) it is possible to obtain fast, low-cost, and reliable models to quantify CaCO3 by FTIR and ii) the method is applicable to study carbonated cement-based materials.
Many industrial by-products have been disposed along coastlines, generating profound marine changes. Phosphogypsum (PG) is a solid by-product generated in the production of phosphoric acid (PA) using conventional synthesis methods. The raw material, about 50 times more radioactive as compared to unperturbed soils, is dissolved in diluted sulfuric acid (70%) forming PG and PA. The majority of both, reactive hazardous elements and natural radionuclides, remain bound to the PG. A nonnegligible fraction of PG occurs as nanoparticles (<0.1 μm). When PG are used for e.g., agriculture or construction purposes, nanoparticles (NPs) can be re-suspended by Aeolian and fluvial processes. Here we provide an overview and evaluation of the geochemical and radiological hazardous risks associated with the different uses of PG. In this review, we show that NPs are important residues in both raw and waste materials originating from the uses of phosphate rock. Different industrial processes in the phosphate fertilizer industries are discussed in the context of the chemical and mineralogical composition as well as size and reactivity of the released NP. We also review how incidental NPs of PG impact the global environment, especially with respect to the distribution of rare earth elements (REEs), toxic elements such as As, Se, and Pb, and natural radionuclides. We also propose the application of advanced techniques and methods to better understand formation and transport of NPs containing elements of high scientific, economic, and environmental importance.
This study characterizes reactive MgO-modified calcium sulfoaluminate (CSA) cement upon carbonation. Paste samples were fabricated by replacing CSA cement with reactive MgO at levels of 0, 5, 10, and 20 wt%. The samples were cured for 56 days and further cured at a CO2 concentration of 3% for 28 days. MgO incorporation into the CSA cement favored the formation of monosulfate over ettringite. Externally supplied MgO in the CSA cement reduced Al uptake in C-A-S-H and formed hydrotalcite as a secondary phase, which is associated with a reduction in the carbonation degree. In addition, the incorporated MgO inhibited the carbonation of ettringite and monosulfate, while more C-A-S-H and aluminum hydroxide were formed in neat CSA cement upon carbonation, showing the nearly full decomposition of ettringite and monosulfate. The tetrahedral Al network of the MgO-modified sample was not altered upon carbonation, indicating that MgO modified the route of carbonation.
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.
The early stages of tricalcium silicate hydration has been studied by in situ ATR-FTIR spectroscopy. A method for data reduction based on background correction and subtraction of spectra of pure components is presented, which makes it possible to describe the kinetic evolution of the calcium silicate hydrate phase upon hydration with great detail. The results obtained are in full agreement with recently reported, detailed solid-state NMR spectroscopy measurements. As ATR-FTIR spectroscopy does not require any isotope enrichment in order to increase the signal intensities and is a widely available technique, we suggest that our study opens up an avenue towards detailed in situ characterization of hydrating cementitious systems and hydrating systems in general.
As a low-carbon cementitious material, the supersulfated cement (SSC) is composed of a thimbleful of cement clinkers and massive industry by-products, i. e: gypsum and slag. However, the concrete made from SSC suffers low strength, poor carbonation resistance, inferior frost resistance, etc. In such a scenario, two types of alkali activators (lactates and sodium hydroxyl) were employed to improve the microstructure and mitigate the performance of supersulfated cement concrete. The results indicate that the incorporation of lactates effectively improves mechanical performance, water impermeability, and carbonation resistance of supersulfated cement concrete. However, the addition of sodium hydroxide failed to result in a performance enhancement of SSC concrete. Furthermore, the mercury intrusion porosimetry (MIP), X-ray diffraction (XRD), fourier transform infrared spectroscopy (FT-IR) and scanning electron microscope (SEM) were comprehensively utilized to illustrate the microstructure changes of supersulfated cement concrete. It indicates the hydration degree of supersulfated cement was promoted by the addition of lactates, which refined the pore structure of concrete. A more compact matrix conduces to overall performance enhancement. This work may shed new lights on the microstructure optimization of solid-waste-based binding materials and further promote the application of SSC concrete.
Supplementary cementitious materials (SCM) are increasingly used in concrete for economical and environmental reasons. However, the durability of reinforced concretes against, for example, corrosion induced by carbonation varies. Here, the phase assemblage of various cement pastes with/without SCM (slag, fly ash and metakaolin), carbonated in accelerated conditions (1.5% CO2 and 65% RH) or not, has been investigated by various technics (XRD, TGA/DTA and ²⁹Si as well as ²⁷Al nuclear magnetic resonance spectroscopy) and compared.
Results show that, after carbonation, anhydrous phases are less decalcified than hydrated phases. In cement pastes with slag, most of the calcium remains in the non-hydrated part of the slag. In contrast, the C-A,S-H phase is deeply modified and results show a coupling between C-A,S-H and hydrated aluminate phases during carbonation. In all carbonated materials, these phases tend to become an aluminosilicate gel, a very amorphous/disordered phase, containing less water than the original hydrates.
Autogenous shrinkage of alkali-activated slag cement (AASC) paste prepared with different levels of alkali dosage and silicate modulus (Ms) is investigated. The results show that autogenous shrinkage of AASC paste increases at a decreasing rate with alkali dosage, which is attributed to enhanced capillary pore pressure and syneresis of C-A-S-H gels. Autogenous shrinkage of AASC paste with a constant alkali dosage increases as Ms increases from 0.5 to 1.0, followed with a reduction as Ms further increases to 2.0. Increasing Ms initially enhances the capillary pore pressure due to the enhanced reaction degree, and promotes the formation of saturated capillary pores and viscoelasticity, which facilitates the autogenous shrinkage of AASC paste. However, excessive silicate in the activator retards the internal moisture consumption, and subsequently decreases the autogenous shrinkage of AASC paste. Furthermore, increasing Ms can also intensify syneresis of C-A-S-H gels, contributing to the early-age autogenous shrinkage of AASC paste.
The search for cements with lower CO2 emissions and lower consumption of non-renewable materials has been a challenge for the cement industry. The aim of this study was to investigate the resistance of SSC concrete to carbonation and chloride ingress. Portland cements (CEM I and CEM III/B) were used as a comparison. The results showed that the carbonation was very advanced in the SSC concretes. This is related to the exclusive formation of C-S-H and ettringite as hydration products, which present a higher rate of carbonation than portlandite. The SSC concrete has a good performance in terms of chloride penetration, mainly due to the increased ability to combine chloride ions both physically and chemically. The service life estimation showed that SSC concretes could be suitable for rural and urban environments when low w/c ratios (0.40) are used. All SSC mixtures studied were suitable for chloride environments.
The microstructural changes of paste, mortar and concrete based on Portland and fly ash containing Portland composite cements caused by carbonation have been studied. The objective was to find out why fly ash containing mortars carbonate with double rate of the neat Portland cement mortars. The reason is partly because they contain less calcium containing species prone to carbonation, but mainly because of their different hydrate assemblage: 1) Less calcium hydroxide that gives a volume increase upon carbonation. 2) More C-S-H with lower Ca/Si that might give an overall shrinkage upon carbonation. 3) More AFt and AFm phases that yield a substantial volume decrease per mole upon carbonation since their crystal water goes back to liquid form. The third microstructure difference is thought to be the dominating reason for coarser pores in the carbonated zone of CEM II/B-V compared to CEM I resulting in a faster carbonation rate.
In present work, a novel nano-CS seed was prepared by wet-grinding method and added to cement to accelerate the early hydration. With 5% nano-CS seed, the initial and final setting time was clearly shortened by 61.4% and 51.7% respectively; the dormant period and the main hydration peak was notably advanced by 52.9% and 43.5% respectively. Early compressive strength, especially the 8-hour strength, was greatly promoted by the nano-CS seed by 3350%, with 5% nano-CS seed. It was speculated that the early-hydration and early-strength was accelerated by weakening the electrical double layer shielded around the silicate phase, thus shortening the dormant period. Results highlighted that the prepared nano-CS seed in present work is expected to be practically used as a new type accelerator.
Supersulfated cements (SSCs) are composed primarily of blast furnace slag, calcium sulfate, and a small content of alkaline activator. SSC has been discussed in the literature because it has a low content of clinker and consequently low CO2 emissions. However, the current absence of durability studies is the main challenge for the large-scale implementation of SSC as a building material. The objective of this study was to investigate the behavior of phosphogypsum-based SSC mortar under sulfate attack. SSC mortars were submitted to MgSO4 and Na2SO4 solutions. Portland cements were also used as a comparison. SSC mortar showed good durability against sodium sulfate attack but presented an expansive behavior when exposed to magnesium sulfate. This behavior in the MgSO4 solution was attributed to the low portlandite reserve, which anticipated the attack of C-S-H. Besides, the non-formation of the brucite layer accelerated the entry of this type of sulfate.
This study examines the magnesium ions effect on carbonate crystal polymorph when Ca(CH3COO)2/urea acts as cementation solution in microbially induced carbonate precipitation (MICP) process. The results of unconfined compressive strength (UCS) indicates that the involvement of Mg ions in Ca(CH3COO)2/urea cementation solution can greatly improve the physical and engineering properties of bio-cemented samples. Compared with the bio-cemented sample without any Mg ions, additional 0.01 M Mg ions in 0.5 M Ca(CH3COO)2/urea solution will contribute to 40% higher UCS, and additional 0.5 M Mg ions in 0.5 M Ca(CH3COO)2/urea solution will make the average UCS twofold higher. While, UCS of the samples treated with MgCl2/urea with poor crystallinity magnesium carbonate is one-tenth of that treated with 0.5 M Ca(CH3COO)2/urea. Furthermore, X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS) images show that acicular aragonite and rhombohedral calcite are two main types of calcium carbonate crystal precipitated when Ca(CH3COO)2/urea acts as the cementation solution, and the incorporation of Mg ions modifies the crystal polymorph, promoting more aragonite precipitation but inhibiting calcite growth. The mechanism of magnesium ions effect on crystal polymorph is also summarized in the end. The efficiency of Ca(CH3COO)2/urea with a small amount of Mg ions is particularly noteworthy in MICP process.
The action of added sodium sulfate (Na2SO4) leading to increased reactivity and early strength in slag: cement binders remains unclear. In this study, early hydration reactions and resultant compressive strength in a 50:50 slag:cement binder in the presence of Na2SO4 were investigated. Early strength increases in the presence of Na2SO4 were shown to be due to a combination of increased alite hydration and increased slag dissolution. Increased alite hydration was due to neither reduced dissolved Al concentration nor increased alite under-saturation but related to increased ionic strength. Increased slag dissolution was associated with both increased pH and decreased Ca activity with the two being connected through the portlandite solubility limit. Na2SO4 was shown to substantially enhance slag dissolution at fixed pH 13 with this action attributed to greater under-saturation of slag as a result of ettringite formation. Na2SO4 was shown to be superior to alternate activators in a slag:cement binder.
Increasing the gypsum and slag contents in the blended cements containing slag is conducive to promote its green degree. The blended cement was prepared by mixing slag, gypsum and Portland clinker. Contour maps were designed to determine the optimal proportion of slag, gypsum and clinker by comprehensive considerations of strength, expansion, hydration, microstructure and crystallization stress. The strength contour map shows that there are two separate regions with high cement strength in the ternary slag-gypsum-clinker system; and the expansion deformation of these two regions are relatively low. The Rietveld XRD and pore solution analysis reveal that the occurrence of the destructive expansion results from the high crystallization stress of ettringite. It is illustrated that the gypsum content could be increased up to 25% when the clinker content is less than 5%, while the gypsum content should be limited below about 10% when the clinker content is more than 20%. The [CaO][SO3]/[Al2O3] molar ratio is suggested to direct the optimization of the slag, gypsum and clinker in blended cement. As long as the [CaO][SO3]/[Al2O3] ratio is not up to 1.5, more gypsum and slag could be added into the blended cement to obtain relatively excellent performance.
In order to explore the difference of the activation effect on the ground granulated blast furnace slag (GGBFS) subjected to variant activators as well as to further clarify the depolymerization mechanism of GGBFS, investigation on the dissolved mechanism, hydrated characteristics and microstructural evolution were carried out based on slag immersed in solutions with variant pH, and FTIR, NMR, XRD, SEM and EDS were used to analyze structure of GGBFS, the change of valence and bond, structural coordination, hydrated products, micromorphology and elements distribution. The results show that the H⁺ destroyed the structure of GGBFS, and a small amount of akermanite and gehlenite particles were formed on the surface of slag, Ca²⁺, Mg²⁺ and Al(OH)²⁺ ions were released as 3 ≤ pH < 7. For the pH < 3, cauliflower-like chabazite particles were accumulated, and the dissolved Si and Al gradually formed monomeric siloxane and hydrated aluminium complex ion [Al(H2O)6]³⁺, respectively. The H⁺ ionized by the water eroded the slag surface and corrugated melilite particles, together with a layer of Si(OH)4 and Al(OH)3 gel attached on the slag surface for pH = 7. As 7 < pH ≤ 12, the hydrated magnesium aluminate and calcite particles were generated on the slag surface, and Ca²⁺, Mg²⁺ and Al(OH)4⁻ ions were released under the influence of OH⁻. The Si-O-Al bond was destructed and the monomers of Al(OH)4⁻ and Si(OH)4 were released for pH > 12. The Si(OH)4 and Al(OH)4⁻ performed polycondensation in the presence of the cations such as Ca²⁺, and that finally led to formation of large number C-(A)-S-H with a corrugated morphology.
This study investigates the effect of NaOH content on alkali/sulfate-activated binders from 90 wt% ground granulated blast furnace slag (GGBFS) and 10 wt% phosphogypsum (PG). Alkali activators were prepared with a NaOH molarity ranging from 0 M to 4 M. The hydration was monitored using in-situ X-ray diffraction (XRD) and isothermal calorimetry. The hydration product assemblage was investigated using XRD, thermogravimetric analysis (TGA), Fourier-transformed infrared (FTIR) spectroscopy, nitrogen adsorption/desorption and compressive strength tests. A molarity of 0 M NaOH gave rise to the highest porosity and highest strength, although setting occurred only after 7 days. From a molarity of 2 M NaOH and higher, ettringite disappeared and got replaced by a monosulfate phase (i.e., NaCa4Al2O6(SO4)1.5.15H2O) and amorphous aluminum-hydroxide. This study shows the potential of using GGBFS and PG for the development of novel by-product based cementitious binders.
As a novel low carbon cementitious material, alkali-activated slag (AAS) attracted many researchers’ interests, not only because of its environmental benefits, but also some superior properties than Portland cement (PC). Due to different chemical reactions of AAS, gypsum, a commonly used expansive agent in PC, exhibits a different behaviour in AAS. This paper reports the investigation on the shrinkage behaviour of sodium silicate activated slag (SSAS) incorporated with gypsum and PC. In addition, the early properties, including setting time and early hydration products, were determined by Vicat apparatus, XRD and TG. The hardened properties, namely flexural and compressive strength, were investigated as well. The results showed that, by incorporating gypsum and PC, the drying shrinkage of SSAS could be significantly reduced because of the formation of expansive sulfate-rich and calcium-rich hydration products in terms of ettringite and Portlandite. The initial setting was delayed by blending PC, although adding gypsum accelerated the hydration of SSAS. The addition of gypsum and blend with 20% PC slightly increased flexural and compressive strengths of SSAS at 7 days and 28 days.
This study investigated the strength and microstructural development of MgO and MgO-microsilica (MS) systems under sealed and carbonated conditions. The influence of hydromagnesite seeds on the performance of each system was also evaluated. The hydration mechanisms were studied via isothermal calorimetry. A correlation between the strength development and formation of different phases was established. XRD, TG/DTG, FTIR and SEM were used for the identification and quantification of different hydrate and carbonate phases. MgO systems relied on the conversion of brucite into carbonate phases for their strength development, whereas M-S-H was the main source of strength in MgO-MS systems. The effect of seeding was evident in MgO-MS systems, where the extra space provided by the seeds increased the rate and degree of hydration. The formation of M-S-H was responsible for strength development and denser microstructures, which could be further improved via the increased utilization of unreacted MgO and MS.
In this study, the effect of Mg²⁺ on early hydration of Portland cement is addressed. Heat flow calorimetry and ICP-OES are employed to characterize the cement hydration at early age. XRD, TG-DSC are utilized to demonstrate the changes of phase composition during early age. SEM-EDS and ²⁷Al MAS NMR are used to reveal phase transformation of aluminum related hydration products. The results show that the induction period of cement is prolonged after Mg²⁺ incorporation, resulting in decreased dissolution of clinker minerals. The effect of Mg²⁺ on acceleration period depends on its concentration. Mg²⁺ hinders the formation of Ca(OH)2 and transformation from AFt to AFm and TAH.