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Calcium Aluminate Cements - Raw Materials, Differences, Hydration and Properties
Abstract
Calcium aluminate cements their mineralogy and hydration is described. Also differencies in mineralogy and properties are included.
... First, the hydration process in CAC cement is entirely diferent from that of the Portland cement. During the hydration process of CAC cement, stable phases such as AH 3 (gibbsite) and C 3 AH 6 , metastable phases such as CAH 10 and C 2 AH 8 , or a combination of these two phases can be produced [11][12][13]. As expressed in equations (1) and (2), the metastable phases are gradually converted into stable phases over time, known as conversion processes [11,13,14]. ...
... During the hydration process of CAC cement, stable phases such as AH 3 (gibbsite) and C 3 AH 6 , metastable phases such as CAH 10 and C 2 AH 8 , or a combination of these two phases can be produced [11][12][13]. As expressed in equations (1) and (2), the metastable phases are gradually converted into stable phases over time, known as conversion processes [11,13,14]. As a result, the porosity is increased, and the compressive strength is reduced [15]. ...
... Similarly, Heikal et al. [48] reported that adding slag to the CAC composite could inhibit the conversion process by forming strätlingite or gehlenite. As illustrated by equations (10) and (11), this is because the silica content of SCMs reacts with the main hydrate phases (CAH 10 and C 2 AH 8 ) and forms the stable phase of strätlingite. ...
The current research aims to investigate the influence of pumice, zeolite, and limestone powder as supplementary cementitious materials (SCMs) on the characteristics of calcium aluminate cement (CAC) composites. For this purpose, SCMs were used in substitution levels of 5%, 15%, 25%, 40%, and 60% of CAC. The results indicate that the active SCMs had a great influence on enhancing the characteristics of the cement composites. For instance, the mixtures with 40% pumice and zeolite outperformed the plain mixture in the compressive strength test by about 45% and 90% at 90 days, respectively. At the age of 90 days, the rapid chloride migration coefficient for the optimal mixture of Z40 (containing 40% zeolite) was reduced by about 93%, and the electrical resistance was increased by about 70% in comparison to the age of 28 days; however, at the same ages, for the plain mixture, the rapid chloride migration coefficient was increased by about 74%, and the electrical resistance was decreased by about 60%. At the age of 90 days, the electrical resistivity of the Z40 mixture was 685% higher compared to the plain mixture. The results show that the high cost of CAC composite could be significantly lowered by utilizing SCMs. Moreover, using SCMs could significantly lower greenhouse gas emissions. In addition, the tests including the modulus of rupture, modulus of elasticity, permeable pore space, XRD, and microstructural analyses were also carried out to study the mechanical and durability properties. It must be mentioned that the effect of high dosages of pumice and zeolite on the durability properties of this type of cement has not been studied previously, which can be considered an innovation of this study. Furthermore, the obtained results could be beneficial to develop applications of CAC.
... Calcium aluminate cement (CAC) has been explored as a sustainable alternative to PC [5][6][7]. The main chemical phases of CACs are calcium aluminate (CA)* and mayenite (C12A7), whereas gehlenite (C2AS) and calcium di-aluminate (CA2) are the minor phases [8,9], where C = CaO, A = Al2O3, H = H2O and S = SiO2. During the production of PC, the calcination of limestone is a significant contributor to CO2 emissions, accounting for approximately 60% of the total CO2 emissions [10,11]. ...
... When the temperature reaches 60 °C, C3AH6 and AH3 are formed without forming any metastable phases [8,18,19]. If the CAC contains silica, C2ASH8 (straetlingite) may also be formed [9]. Straetlingite is a strength-providing phase that can improve the compressive strength of the CAC. ...
... In mixture designs of CAC binders, water-to-cement ratios were 0.2, 0.25, and 0.3; the Li2CO3 contents were 0, 0.001, 0.003, 0.005, 0.007, and 0.01%mass. The ages of CAC binders were 1, 2, 3, 4, 5, 6,7,8,9,24,72, and 168 h. The compressive strength measurement was conducted based on ASTM C349 [63] and ASTM C109 [64]. ...
Calcium aluminate cement (CAC) has been explored as a sustainable alternative to Portland cement, the most widely used type of cement. However, the hydration reaction and mechanical properties of CAC can be influenced by various factors such as water content, Li2CO3 content, and age. Due to the complex interactions between the precursors in CAC, traditional analytical models have struggled to predict CAC binders’ compressive strength and porosity accurately. To overcome this limitation, this study utilizes machine learning (ML) to predict the properties of CAC. The study begins by using thermodynamic simulations to determine the phase assemblages of CAC at different ages. The XGBoost model is then used to predict the compressive strength, porosity, and hydration products of CAC based on the mixture design and age. The XGBoost model is also used to evaluate the influence of input parameters on the compressive strength and porosity of CAC. Based on the results of this analysis, a closed-form analytical model is developed to predict the compressive strength and porosity of CAC accurately. Overall, the study demonstrates that ML can be effectively used to predict the properties of CAC binders, providing a valuable tool for researchers and practitioners in the field of cement science.
... Although they have use in construction and high water mine backfilling, refractory applications are mostly targeted by CAC producers. Several classes of CAC are distinguished based on the purity, mostly defined by the wt% of Al 2 O 3 [234,235]. The most commercialised and most relevant to this review paper is the "Standard" grade CAC, which is the lowest in alumina content (36-42 wt% Al 2 O 3 ) [234]. ...
... It can therefore incorporate the highest levels of iron (12-20 wt% of Fe 2 O 3 ) [234,236], as it is intended for use at less-elevated temperature (whereas the higher-alumina CACs are used in higher-temperature refractory applications), and so the fluxing effect of the iron is not strongly detrimental to key performance characteristics. Similar to Portland cement, the iron is mostly present in the ferrite phase [234,235], but also in pleochroite [237]. Reducing conditions in the kiln during CAC production often result in the presence of minor contents of magnetite or wüstite in addition to the ferrite phase [234]. ...
... The main hydration reactions in the CAC system result from the reactions of monocalcium aluminate (CA) and mayenite (C 12 A 7 ) or grossite (CA 2 ) to form the calcium aluminate hydrates: CAH 10 , C 2 AH 8 , C 3 AH 6 and AH 3 [234,238]; however, only the latter two are stable at normal temperature and pressure. The presence of the ferrite phase results in a substitution of aluminium for iron in these hydrates and the formation of FH 3 [235], but less is known about the stability of the Febearing hydrates. ...
The bulk of the cement industry's environmental burden is from the calcareous source. Calcium is mostly available naturally as limestone (CaCO3), where almost half of the mass is eventually released as CO2 during clinker manufacture. Iron (Fe) is the fourth most common element in the Earth's crust surpassed only by oxygen, silicon, and aluminium; therefore, potential raw materials for alternative cements can contain significant amounts of iron. This review paper discusses in detail the most abundantly available Fe-rich natural resources and industrial by-products and residues, establishing symbiotic supply chains from various sectors. The discussion then focusses on the impact of high iron content in clinker and on ferrite (thermo)chemistry, as well as the importance of iron speciation on its involvement in the reactions as supplementary cementitious material or alkali-activated materials, and the technical quality that can be achieved from sustainable Fe-rich cements.
... The main mineral phases of the significantly cheaper low alumina CAC, responsible for the early hydraulic activity, are CA and C 12 A 7 , with C 12 A 7 content being strictly limited, as it might cause early stiffening [16,225]. Other phases frequently observed in CACs include, besides others, calcium silicates such as C 2 S, calcium aluminum silicates such as C 2 AS and calcium aluminum ferrites, which have in common that they have a much lower reactivity than CA and C 12 A 7 [225,226]. In high alumina CACs, CA 2 , which also dissolves significantly slower than CA, is another major component [226][227][228]. ...
... Other phases frequently observed in CACs include, besides others, calcium silicates such as C 2 S, calcium aluminum silicates such as C 2 AS and calcium aluminum ferrites, which have in common that they have a much lower reactivity than CA and C 12 A 7 [225,226]. In high alumina CACs, CA 2 , which also dissolves significantly slower than CA, is another major component [226][227][228]. Major hydration products of CAC at 20 • C are the metastable phases CAH 10 and C 2 AH 8 , which might be noted as C 2 AH x , as different water contents are possible, depending on relative humidity, temperature and pressure [226,229]. ...
... In high alumina CACs, CA 2 , which also dissolves significantly slower than CA, is another major component [226][227][228]. Major hydration products of CAC at 20 • C are the metastable phases CAH 10 and C 2 AH 8 , which might be noted as C 2 AH x , as different water contents are possible, depending on relative humidity, temperature and pressure [226,229]. The metastable CAH 10 and C 2 AH x convert to stable C 3 AH 6 , which causes a decrease in solid volume, an increase in porosity and a decrease in strength; an effect that is most prominent at high w/c [16]. ...
The desire to accelerate cement hydration for faster construction processes is almost as old as cement itself. In recent years, however, new requirements for the acceleration, and the precise control of the setting behavior and the early strength development of cementitious materials have emerged. This is due to the increasing proportion of cements with reduced clinker content and new technologies, such as additive manufacturing.
As the use of accelerating admixtures offers high flexibility in controlling of early cement hydration, these admixtures might play a key role in developing fast-hydrating green cements and cementitious materials suitable for application in additive manufacturing. A detailed understanding of the mechanisms by which different accelerating admixtures influence cement hydration and their influence on setting and hardening is thus required. In this study, the most widely used accelerators for cast and sprayed concrete are reviewed, focusing on their influence on the aluminate and silicate reaction of cement hydration as well as the setting and hardening depending on the dosage of the admixture.
... Calcium Aluminate Cements represent an important type of cement significantly having many advantages respect to Ordinary Portland Cement (OPC), including early rapid hardening, enhanced durability properties, resistance to abrasion and alkali-silica reaction and sulphate attack, yet they are currently mainly used in refractory and building chemistry applications [1][2][3][4][5]38]. Three kinds of CACs are usually reported in literature and differ each other according to their chemical composition: i) standard CACs with low iron content (48-60% wt. ...
... Fe 2 O 3 , <0.5% wt. SiO 2 ) [1]. HACs represents the most important CACs for refractory castables and concrete with high early strength and sea water resistance, which is mostly based on high purity raw materials, such as pure limestone and high-quality synthetic alumina [6]. ...
... HACs represents the most important CACs for refractory castables and concrete with high early strength and sea water resistance, which is mostly based on high purity raw materials, such as pure limestone and high-quality synthetic alumina [6]. HACs is usually obtained by heating at 1500-1600 • C a mixture of bauxite and limestone [7], even though hydrated lime, laterite and different types of alumina can also be used as alternative raw materials [1]. The most common heating process involves the sintering of the raw meal at high temperature, followed by an air quenching [1]. ...
In the present study the influence of minor elements (Na2O and SiO2) on the mineralogy, chemistry and microstructure of High Alumina Cements (HACs) has been investigated. HACs have several advantages respected to Ordinary Portland Cement (OPC) but the shortage of Al-rich raw materials represents a limiting factor: re-use of Al-rich waste as raw material represents a solution but it will add minor elements to the raw meal that could change HACs properties. For the first time, four commercial HACs, doped with sodium and silicon, and one synthetic HAC, only highly doped in sodium, were studied through a multidisciplinary approach by combining conventional and unconventional analytical techniques. Results highlighted that (i) sodium and silicon were mainly incorporated in a sodium-rich phase (Na-phase, NCA2, Na1.9CaAl3.9Si0.1O8), (ii) no minor phases such as gehlenite and/or mayenite occurred, and (iii) CA (CaAl2O4) and CA2 (CaAl4O7) revealed a limited ionic substitution.
... Calcium Aluminate Cements (CACs) represent an important type of cement with many significant advantages compared to Ordinary Portland Cement, such as rapid hardening, chemical (alkali-silica reaction and sulphate attack) and abrasion resistance. Consequently, they are mainly used in refractory and building chemistry applications [1][2][3][4][5]. CACs are usually classified according to their chemical composition: (i) standard CACs with low iron content (48-60% wt. ...
... Fe 2 O 3 , <0.5% wt. SiO 2 ) [1]. HACs represent the most important CACs for refractory castable and concrete with high early strength and sea water resistance [6]. ...
... However, hydrated lime (Ca(OH) 2 ), laterite, bauxite and different qualities of alumina (i.e. impure synthetic alumina) can also be used as alternative starting raw materials [1]. Currently, the use of alternative raw materials with a wide range of chemical composition and origins, such as laterite, iron-rich bauxite, low-grade alumina and blast furnace slags, appears the most challenging frontier in CACs manufacturing due to the limited supply of bauxite [8,9]. ...
Na2CaAl4O8 represents a crystalline phase occurring in metal-slag and calcium aluminate cements. Despite the industrial implications of these materials, its crystal structure had not been yet solved. We have solved the crystal structure of Na2CaAl4O8 by means of Single Crystal X-ray Diffraction from grains obtained from synthesis. From single Crystal analysis we demonstrate that Na2CaAl4O8 is orthorhombic, with P212121 space group, a = 7.2541(1), b = 10.4301(2), and c = 10.4348(2) Šand V = 1287.92 ų. The structure is characterized by a framework of corner-sharing aluminium centred tetrahedra, described as a stacking along [101] of (101) layers of composed of six-membered tetrahedral distorted rings; layers are linked by sharing some AlO4 tetrahedra apices with Al tetrahedra forming the adjacent layers along [010], along with the Ca-centred octahedra. Na atoms occupy extra-framework spaces in two types of coordination, one being asymmetric, representing a hydrophilic point, explaining the water reactivity.
... In general, CACs (also high alumina cements) were initially developed during the 1900s with the main purpose of being sulfate resistance [25]. Compared to Portland cements, CACs were developed using limestone, iron ore, and bauxite, or other materials rich in aluminum, iron and even magnesium, the ratio of which, can define the overall quality of the produced cement [26]. Among the many benefits, CACs are reported to hydrate in lower pH environments, have relatively high weathering stability and achieve high strength values even in lower temperatures, when compared to their Portland cement components [26]. ...
... Compared to Portland cements, CACs were developed using limestone, iron ore, and bauxite, or other materials rich in aluminum, iron and even magnesium, the ratio of which, can define the overall quality of the produced cement [26]. Among the many benefits, CACs are reported to hydrate in lower pH environments, have relatively high weathering stability and achieve high strength values even in lower temperatures, when compared to their Portland cement components [26]. ...
Over the last few decades there has been a growing interest in the use of blended or hybrid rapid hardening cements with optimized performance for use in a variety of applications. Blended cements can help decarbonize the construction industry and address the growing need for retrofitting the aging infrastructure. Among the most used cements, ordinary Portland cement (CEM) and calcium aluminate cements (CAC), from ordinary and rapid hardening family, are commonly on the list for use in many repair and rehabilitation applications. Using various grades of the mentioned cements, this study provides a detailed and comparative analysis of the influence of the type and content of the cement, compositions, and water-to-cement (w/c) ratio on the properties of lightweight mortars. Five types of ordinary and rapid hardening cements with different chemical compositions were used to produce a total of 20 mixes with w/c ratios ranging from 0.4 to 1.8. Samples have been tested for a variety of physical, mechanical, and thermal properties. Results show that CACs exhibit considerably higher sensitivity to water content due to stoichiometric water requirement that results in embedded cracks and structural defects, whereas the ordinary cements (CEM I) develop a more compact and crack free microstructures at longer ages, which can be the result of differences in the hydration phases. Finally, the results provide valuable information for the use based on physical–mechanical properties of lightweight mortars produced with a variety of cementitious compositions.
... The figure also indicates that increasing the steam pressure from 3 to 9 bars led to the disappearance of the characteristic peak of calcium aluminate hydrate at 2θ = 55.87 • . This behaviour may be attributed to the phase transformation [12] of metastable hydrocalumite (C 4 AH 13 ) to stable tricalcium aluminate hydrates (C 3 AH 6 ) or stable straitlingite/hydrogarnet phases (CASHs) [73]. ...
... The microstructure development appears in the form of stacked-plates from C-A-S-H interlocked with rod-shaped from cancrinite; the Geo-0.25 W/3bars/4hrs has compressive-strength higher than Geo/3 bar/4hrs and Geo-0.25-W/28-days by 39.1% and 45.3%, respectively. At 9 bar/4hrs (Fig. 14 [c1, c2]), the cracks observed in the Geo specimen are approximately disappeared in Geo-0.25 W due to the formation of an additional amount of C-S-H, staked-rods zeolitic phases from cancrnite/analcime and hexagonal hydrogarnet [73], reflecting on a significant strength retention in Geo-0.25 W cured at high steam pressure. These findings emphasize the synergistic role of WO 3 -NPs and hydrothermal curing in developing microstructures. ...
According to the sustainability concept, this work developed a green geopolymeric composite (Geo) prepared by mingling 50 wt% slag + 50 wt% brick-waste (BW) as an alternative eco-friendly and low-cost cementitious material. The main target of this study is to find a solution to the problem of poor characteristics of binding materials containing high proportions of BW; most previous studies recommended using only 10–20 wt% BW. The compressive-strength results showed that replacing slag with 50 wt% BW reduced the strength from 47 to 24.6 MPa at normal curing conditions for 28-days, referring to the detrimental impact of BW on mechanical performance. In an endeavour to enhance the mechanical performance of this composite, different doses from laboratory-prepared tungsten oxide nanoparticles (0.25, 0.5, 1 wt%WO3-NPs) and hydrothermal curing at various steam-pressure/periods were used. From an economic point of view, hydrothermally treated Geo-paste modified with 0.25 wt%WO3-NPs at 3 bar/4hrs was selected as an ideal composition/curing-conditions; the compressive-strength reached 54.5 MPa exceeding the standard limit of Portland cement (42.5 MPa). This clearly shows the synergistic role of using WO3-NPs and hydrothermal curing to enhance the compressive-strength, which was confirmed using different analysis techniques. XRD, TGA/DTG and SEM/mapping proved that the catalytic performance of WO3-NPs/hydrothermal-curing participates in augmenting binding hydrates, creating a cubic-stable-phase of tricalcium-aluminate-hydrate (C3AH6) and different types of zeolitic-like structure (spherical Zeolite-NaP, rods analcime and stacked-plates cancrinite). To maximize the benefits of employing WO3-NPs in the developed composites, their anti-microbial activity was studied. The measured inhibition zone around specimens containing WO3-NPs proved that these composites have a superior self-cleaning efficiency against Candida albicans, Mucor circinelloides, Salmonella typhi and Staphylococcus aureus due to the WO3- NPs’ photocatalytic activity.
... During the last decade within the EU, the extraction of Al from calcium aluminate slags has attracted renewed attention, partly because of the inclusion of bauxite in the most recent critical raw materials list (Regulation of the European Parliament and of the Council establishing a framework for ensuring a secure and sustainable supply of critical raw materials and amending Regulations (EU) 168/2013, (EU) 2018/858, 2018/ 1724 2019/1020E. Commission, 2023. ...
... 10.1) and its native database of thermochemical data (HSC Chemistry®, 2022). Calcium aluminates are hydraulic solids, i.e., they participate in hydration reactions with water or aqueous solutions, leading to the formation of hydrates that possess cementitious properties (Pöllmann, 2012). A number of metastable calcium aluminate hydrates can form but, eventually, they all transform to the only thermodynamically stable hydrate of the system, Ca 3 Al 2 (OH) 12 (or C 3 AH 6 in cement chemistry notation) (Scrivener and Capmas, 1998;Edmonds and Majumdar, 1988). ...
... F corresponds to the previously undeterminable product of density and volume squared (Eq. 3) and is explained in the course of the next paragraph. The diffractogram of C 2 AH x synthesis sample #5 (Table 3) was chosen because of its highest scale factor after the refinement with the C 2 AH x -hkl-phase model as proposed earlier by Hüller et al. [31] on the basis of data published in [8,41]. The c-lattice parameter of the chosen C 2 AH x sample (c 0 = 62.76 Å) fits the form C 2 AH 7.5 when compared to the lattice parameters of the two other forms [8,41]. ...
... The diffractogram of C 2 AH x synthesis sample #5 (Table 3) was chosen because of its highest scale factor after the refinement with the C 2 AH x -hkl-phase model as proposed earlier by Hüller et al. [31] on the basis of data published in [8,41]. The c-lattice parameter of the chosen C 2 AH x sample (c 0 = 62.76 Å) fits the form C 2 AH 7.5 when compared to the lattice parameters of the two other forms [8,41]. For calibration of the C 2 AH x the diffractogram of sample #5 was analysed and refined with the Rietveld evaluation software TOPAS, Bruker AXS, V5 using the described routine: ...
This work presents a novel way to use X-ray diffraction (XRD) to quantify the plate-like calcium-aluminate-hydrate phases C2AHx (2CaO·Al2O3·xH2O), for which three different basal spacings are detected during early hydration at ambient temperatures. By combining Rietveld refinement and the G-factor method, C2AH7.5 was calibrated using a variety of syntheses and preparations for XRD. Since cell volume and X-ray density (that are necessary for the G-factor method) of C2AH7.5 are not known, an approach similar to [1], [2] was used for the generation of a hkl-phase model: A “F-factor” – a factor that replaces the product of these two unknowns, which was verified by the multitude of syntheses – was successfully determined within this paper. The model was applied for the absolute quantification of C2AHx during in-situ XRD of CA (CaO·Al2O3) hydration at 40 °C. The results demonstrate the potential of the hkl-phase model in practical use and reveal the accuracy of the determined F-factor.
... The literature of hydration reactions of calcium aluminates and the properties of their solid products has been reviewed by H. Pöllmann [31]. The thermodynamically stable solid phases in the system Al2O3-CaO-H2O are C3AH6 and Al(OH)3 (also referred to as AH3 in the literature of cements). ...
... The solubilities of the metastable hydrates are higher than the solubility of C3AH6 and the driving force of its precipitation is the dissolution of the metastable hydrates. Both metastable phases are more soluble in increased temperatures, which favors the progression of the conversion reaction for the production of C3AH6 and AH3 [31]. ...
Sustainable utilization of Bauxite Residue (BR) is currently one of the greatest challenges being tackled by the alumina industry, due to its high production rates and limited reuse options. The present work is concerned with the use of BR as a candidate metallurgical raw material for iron (Fe) production and aluminum (Al) extraction. In more detail, at first, BR undergoes reductive smelting to extract its Fe content and produce a slag of mainly calcium aluminate composition. In a second step, Al contained in the calcium aluminate phases is extracted hydrometallurgically by leaching with a Na2CO3 aqueous solution. The focus of the current study is the optimization of this leaching process, and it was performed in two stages. The first was a laboratory scale investigation on the main parameters affecting the extraction rate of Al. The second stage was performed in pilot scale and incorporated observations and suggestions based on the laboratory scale investigation. Laboratory work showed that more than 50% of aluminum could be easily extracted in less than 1 h, in 5% S/L, at 70 °C and with an 20% excess of Na2CO3. Pilot scale work, by successfully applying the suggestions derived from laboratory scale work, achieved an average Al extraction of 68% from a 10% S/L pulp, with a slag of optimized composition in relation to the one used in the laboratory scale.
... Another important reactive phase is C 12 A 7 , (mayenite) and helps to shorten the setting time. Other phases are CA 2 -grossite, C 5 A 3 -pentacalcium trialuminate, C 4 A 3 -tetracalcium trialuminate, C 2 A -dicalcium aluminate, CA 6 -hibonite, -C 2 S (belite), C 2 AS (gehlenite), A 3 S 2 -mullite, spinel phase, ferrite solid solution, etc. (Touzo et al. 2001, Pöllmann 2012) -contains silicon and/or iron and other minor elements in addition to calcium and aluminium. ...
... Simultaneously, facilitating an approach to confer adequate strength by bridging the elongated CA 6 grains is also important because most studies focus only on the formation of CA 6 platelets [18][19][20][21][22]. Calcium aluminate (CaO·Al 2 O 3 , CA) cement, which acts as a binder by the formation of different hydration products in the presence of water, can be used for this purpose [23,24]. Because CA cement can eventually be transformed to CA 6 at high temperatures by adjusting the CaO/Al 2 O 3 molar ratio, refractory aggregates mainly composed of highly stable CA 6 phases can be synthesized [14,25]. ...
The synthesis of macro-porous calcium-hexaluminate (CaO∙6Al2O3, CA6) aggregates composed of CA6 platelets was examined using various Ca- and Al-precursors. The effects of annealing temperature, time, atmosphere, and AlF3 as a mineralizer on the formation of plate-shaped CA6 were also studied. The Ca:Al ratio was adjusted for full conversion to CA6, which is the most stable phase among the CaO-Al2O3 intermediate compounds. The conditions for the synthesis of plate-shaped CA6 aggregates with adequate strength were uncompromising, requiring heat treatment at ≥ 1500 °C for 15 h, which was also significantly affected by the type and size of the precursors. CA6 aggregate synthesized directly using coarse Al2O3 and CaO with 10 wt% CaO∙Al2O3 cement as a binder phase revealed the optimal macro-porous microstructure for castable refractory applications, showing porosity and compressive strength of 65% and 2.9 MPa, respectively.
... The main limitation of the aluminate is its phase conversion when subjected to body temperatures and humidity. This phase change leads to a reduction in strength [41][42][43]. Addition of additives can improve the properties of calcium aluminates [44,45]. ...
Objectives
The introduction of hydraulic cement sealers has increased the popularity of single cone obturation where the chemistry and properties of hydraulic cement sealers are crucial. This article has investigated the materials present on the market by reviewing the chemistry aiming at understanding whether these materials are optimized or have been tested appropriately.
Methodology
A market search on materials called bioceramic and hydraulic sealers was undertaken. The safety data sheet and manufacturer details for every material were searched and the components were checked. The literature was searched for information about the properties of these materials based on their composition.
Results
The safety data sheets and manufacturer details were imprecise with some manufacturers providing little detail on composition. From the publications reviewed, it is apparent that the materials used clinically are not optimized, and there is little evidence that the material chemistry and presentation aid the clinical technique in any way.
Conclusions
There has been a rapid increase in materials identifying as bioceramics on the market. These materials have diverse chemistries, and some of the constituents are not declared. This may affect the clinical performance of these materials.
Clinical significance
Smart materials developed on the clinical need which are appropriately tested are necessary for a paradigm shift in root canal obturation. It is important to use reputable materials that have been adequately researched in clinical practice.
... The rate of strength development at an early age is very high due to the high reactivity of the CA phase, which is the main cement compound (Zhang vd. 2018;Pöllmann, 2012). ...
... • Belite cements (BCs) contain mainly C 2 S, C 3 S and calcium aluminates and produce high amounts of C-S-H gel, which leads to very good durable properties [14,15]. • Calcium aluminate cements (CAC) [16][17][18][19][20], are manufactured from limestones and bauxites. Their main oxides are CaO and Al 2 O 3 , with a variable content of Fe 2 O 3 , TiO 2 , and MnO. ...
The manufacture of Portland cement entails high energy and environmental costs, and various solutions have been implemented in recent years to mitigate this negative impact. These solutions include improvements in the manufacture of cement clinker or the use of supplementary cementitious materials (SCMs), such as fly ash (FA) or slag as a replacement for a portion of the clinker in cement. The incorporation of these SCMs in cement may increase its radiological content as they are naturally occurring radioactive materials (NORMs). The Activity Concentration Index (ACI) is a screening tool established in the European EURATOM Directive 2013/59 to determine the radiation protection suitability of a final construction material. The ACI is determined by the activity concentrations of 226Ra, 232Th and 40K, usually determined by gamma spectrometry. The methodology of gamma spectrometry is accurate and appropriate, but this technique is not available in all laboratories. For this reason, and taking into account that there is a relationship between the chemical and radiological composition of these building materials, a new approach is proposed to determine the radiological content of these materials from a chemical analysis such as X-ray fluorescence (XRF). In this paper, principal component analysis (PCA) is used to establish the relationships between the chemical composition and radiological content of cements, FAs, and slags of different natures. Through PCA it was possible to group the cements based on two variables: CaO content and Fe2O3–Al2O3–TiO2 content. A lower correlation was observed for the FAs and slags, as the sample scores were centered around the origin of the coordinates and showed greater dispersion than the cements. The clusters obtained in the HJ–Biplots allowed the determination, using multiple regression, of models relating the activity concentration of 226Ra, 232Th (212Pb), and 40K to the oxide percentages obtained for the three matrices studied. The models were validated using five cements, one FA and one slag with relative percentage deviations (RSD(%)) equal to or less than 30% for 89% of the activity concentrations and 100% of the ACI determined.
... Calcium aluminate cements (CACs) have been produced commercially since the early 20 th century and are used in niche applications today. They were initially developed as "Ciment Fondu," a high-sulfate resistance material (Pöllmann 2012). The term fondu ("molten" in French) was derived from the molten state of the material that is manufactured at ~1550 °C by firing a mix of bauxite or other aluminium-bearing rocks with low SiO 2 contents and limestone. ...
A background on non-Portland cementitious binders is presented, followed by a review of the key alternative binders that are currently of interest. The mineralogy of these cements is described, along with phases present in the reacted/hardened materials. The similarities and differences between the setting processes, as controlled by reactions at the solid-liquid interface, provide insight into the ways in which different classes of binder develop their performance and thus offer value to society.
... In contrast to the 40 • C sample, where only crystalline-bound water is present in addition to the free water, the protons in the 5 and 23 • C samples are present in several reservoirs and thus with different confinements. In C 2 AH x , protons are bound in the main layer [Ca 2 Al(OH) 6 ] + , the interlayer [Al(OH) 4 ]and also in the weakly bound interlayer H 2 O [33,49,50]. The situation is similar with CAH 10 , where some of the protons are firmly bound in the structure and some are present as water molecules in channels of the structure [32,51,52]. ...
When refractory castables are dried, hydrothermal conditions may result inside the bodies if the H2O cannot escape from the material. Under such high-pressure conditions, problems such as explosive spalling can arise. As different curing temperatures during the hydration of calcium aluminate cement (CAC)-bound castables lead to the formation of different hydrate phases, different microstructures can develop in the hardened material. This study presents the changes in porosity and in the mineralogical composition of a refractory castable model system under hydrothermal conditions depending on the curing temperature (5, 23 and 40 °C).
Quantitative X-ray diffraction (QXRD) measurements show that different hydrate phases are formed during curing, while C3AH6 and boehmite are formed in the same quantities after hydrothermal treatment in an autoclave at ∼11 bar/180 °C. Although the mineralogical composition after autoclaving is not different, the three samples differ in their microstructure. Mercury intrusion porosimetry measurements reveal that although the total porosity after autoclaving is the same, the 40 °C samples have a higher proportion of large pores. SEM images also show that the appearance of C3AH6 in the 40 °C autoclaved samples varies, which originates from the starting phase composition and microstructure after curing.
... Katoite (Ca 3 Al 2 (OH) 12 ) is a typical silicon-free endmember of grossular (Ca 3 Al 2 (SiO 4 ) 3 ), where [(OH) 4 ] 4groups substitute all the [SiO 4 ] 4groups [28,29]. With hydroxyl groups and AlO 6 octahedra in the cubic structure [30], katoite has a large potential to selectively adsorb phosphate through ligand exchange [31]. ...
Phosphate removal and recovery from wastewater is an effective approach to alleviate eutrophication and overcome the future scarcity of phosphorus. Katoite, a kind of calcium aluminate hydrate, was synthesized and examined as a novel adsorbent to remove phosphate from aqueous solution and recover phosphate as potential slow-release fertilizer. Batch experiments showed that the maximum phosphate adsorption capacity of katoite reached 111.51 mg P/g. Also, a high equilibrium adsorption capacity of 80.14 mg P/g was obtained at the initial concentration of 10 mg P/L. The adsorption could be well described by the Elovich kinetic model and Redlich-Peterson isotherm model. Thermodynamic study suggested that the adsorption was an endothermic process and occurred favorably and spontaneously. Moreover, the adsorption was almost independent of the ionic strength and showed high selectivity for phosphate in solutions containing common coexisting anions. The phosphate adsorption on katoite could be attributed to inner-sphere complexation and surface precipitation, which were further suggested by the SEM-EDS, FTIR and XPS analysis. Desorption experiments results indicated the possible plant availability and successive release property of the phosphate adsorbed on the surface of katoite. Overall, the study suggested that katoite is a promising candidate for phosphate adsorption and subsequent recovery as a slow-release phosphorus fertilizer.
... CAC is well known and used in various applications (Pöllmann, 2001;Pöllmann, 2012). One of the known special characteristics of (CAC) is its rapid hardening (RH) property. ...
Quantitative X-ray diffractometry using a Rietveld-based computational method was carried out for a series of Calcium Aluminate Cement (CAC) samples. This indicated that the CA content ranged between 37.7% to 47.7% while Brownmillerite (C4AF) amount varies between 11.0% to 23.6%. Magnetite was found in all the samples, ranging from 0.7% to 3.9% while Gehlenite amount varies between 0.5% and 6.5%. The amount of spinel varies between 0.5% and 0.1% and its average value is 1.3%.. The amorphous content of CAC is ranged between 12.0% and 32%. The Mayenite and amorphous content could be a good indicator of the Rapid Hardening (RH) property of CAC. Samples with the high Mayenite content showed less RH properties, whereas RH increased as the content of amorphous material increased. The RH properties of CAC based on its mineralogical composition was predicted through various neural network techniques. The R 2 value of the models was 0.39 for Linear Regression analysis model (LR), 0.56 for feed forward neural network (ANN) and 0.78 for Generalized Regression Neural Network (GRNN) approaches. The best prediction approach for RH value of the CAC with an Al2O3 content of 40% was GRNN that can be applied to predict RH.. Ural ve Aydın 2022, 54 (1), 1-14 72 ÖZ Bir dizi Kalsiyum Alüminat Çimentosu (CAC) numunesi Rietveld tabanlı hesaplama yöntemi kullanılarak nicel X-ışını difraktometresi yöntemi ile incelenmiştir. Analiz sonuçları, CAC içeriğinin %37.7 ile %47.7 arasında, Brownmillerit (C4AF) miktarının ise %11.0 ile %23,6 arasında değiştiğini göstermiştir. Tüm örneklerde %0,7 ile %3,9 arasında değişen manyetit bulunurken, Gehlenit miktarı %0,5 ile %6,5 arasında değişmektedir. Spinel miktarı %0.5 ile %0.1 arasında değişmektedir ve ortalama değeri %1.3 tür.. CAC'nin amorf içeriği %12.0 ile %32 arasında değişmektedir. Mayenit ve amorf içerikleri, CAC'nin Hızlı Sertleştirme (RH) özelliğinin iyi bir göstergesi olabilir. Mayenit içeriği yüksek olan numuneler daha az RH özelliği gösterirken, amorf malzeme içeriği arttıkça RH artmıştır. CAC'nin mineralojik bileşimine dayanan RH özellikleri, çeşitli sinir ağı teknikleri ile tahmin edildi. Modellerin R 2 değeri, Lineer Regresyon analiz modeli (LR) için 0.39, İleri Beslemeli Sinir Ağı (ANN) için 0.56 ve Genelleştirilmiş Regresyon Sinir Ağı (GRNN) yaklaşımları için 0.78'dir. %40 Al2O3 içeriğine sahip CAC'nin RH değeri için en iyi tahmin yaklaşımı, RH'yi tahmin etmek için uygulanabilen GRNN yöntemi olmuştur.
A new approach to prepare highly dispersed calcium aluminate at temperatures from 900°C with desired properties (bulk density starting from 0.015 g/cm3, particle size falling in the range of 7–42 described, which consists of step-by-step heat treatment of a concentrated aqueous solution of Al(NO3)3, Ca(NO3)2, and C6H8O7 in the molar ratio CaO : Al2O3 = 1 : 2. The main stages of the synthesis X-ray powder diffraction, IR spectroscopy, as well as scanning and transmission electron microscopies. dispersed calcium aluminate obtained using the developed approach has pronounced luminescent features.
As a byproduct of the steelmaking process, ladle slag has the potential to be used as an auxiliary cement material in the construction field. However, ladle slag generated after secondary refining is typically handled by air cooling and stacking, leading to the presence of the typical mineral phase mayenite (Ca 12 Al 14 O 33 , abbreviated as C 12 A 7 ) in a crystalline form within the slag. This reduces its early hydration activity, which adversely affects the compressive strength of concrete and consequently lowers the resource utilization rate of ladle slag. Based on this, this article provides a comprehensive review of the generation process and composition of ladle slag. By discussing the hydration process and hydration products of the typical mineral phase C 12 A 7 in ladle slag, as well as the mutual transformation of hydration products, it is shown that hydration products undergo transformation with increasing temperature. Compared to crystalline C 12 A 7 , amorphous, C 12 A 7 exhibits excellent hydration activity. Building upon this, methods for amorphizing C 12 A 7 are elucidated, wherein thermal activation or chemical activation is employed to alter the ordered arrangement of atoms within the crystal structure, thereby reducing the stability of the crystal structure to achieve amorphization of C 12 A 7 .
The effect of the ratio CaO.Al2O3/12CaO.7Al2O3, AlF3 and CaF2 on the formation of the structure and phase composition of clinker minerals of aluminate cement during hydration has been studied. It is found that after 24 hours of hydration, the main hydraulic phases in the studied cement stones are CAH10, C2AH8, AH3. It has been shown that the introduction of AlF3 and CaF2 fluorides reduces the liquidus temperature of the systems and increases the rate of CA formation. Scanning electron microscopy has revealed that the process of hydration of calcium aluminates of cement at low temperatures can be considered as simultaneous reactions.
Free water available in calcium aluminate cement (CAC)-bonded castables is crucial for the hydration of CAC and the conversion of hydration products in the curing and drying processes, as both the hydration and conversion reactions are dissolution–precipitation reactions. To elucidate the effect of different levels of free water loss upon the hydration of CAC, the conversion of hydration products and the mechanical strength of the CAC-bonded castables, the CAC-bonded castables were subjected to sealed and unsealed curing conditions at 50 °C and drying at 110 °C. The results demonstrate that the fast removal of free water during unsealed curing would hinder the conversion from 2CaO·Al2O3·8H2O to 3CaO·Al2O3·6H2O and consequently prevent the deterioration of strength. As a comparison, although sealed-cured samples have less water loss and high degree of hydration of CAC, they still show lower strength than the unsealed samples after curing. The following drying process further accelerates the hydration of residual calcium aluminate clinkers for both the sealed and unsealed samples, but still does not favor the conversion from 2CaO·Al2O3·8H2O to 3CaO·Al2O3·6H2O in the unsealed-cured samples.
Calcium sulfate (CaSO4), an essential retarder in cement, retards the hydration of tricalcium aluminate (C3A) and tetracalcium aluminoferrite (C4AF) phases. However, its retarding mechanism remains unclear. This paper focused on the adsorption of CaSO4f on C4AF and C3A surfaces based on isothermal calorimetry, the measurement of the ionic concentrations in a diluted system, and density functional theory to enhance the understanding of the retardation mechanism. The results showed that the retarding effect of CaSO4 on C4AF was stronger than that on C3A due to the slower CaSO4 consumption rate, lower driving force for CaSO4 adsorption, and surface coverage of Fe(OH)3 gel. The adsorption of CaSO4 hindered Ca dissolution more markedly on C4AF than C3A, which was pronounced on Fe‐free C4AF surfaces. The adsorption of CaSO4 weakened the affinity of water on C4AF and C3A surfaces, lowering the driving force for H2O adsorption. The adsorption of H2O and CaSO4 promoted the dissolution of Al on the [AlO6] octahedral surface of C4AF, which may be responsible for the maintenance of a higher Al concentration in the solution. Based on the above results, the adsorption of CaSO4 on initial C4AF and C3A hydration was explained.
This chapter provides a compact overview of the durability of calcium aluminate cements (CAC) in comparison to Portland cements (PC) in the context of (bio)chemically aggressive sewers. Special focus is on the CAC dissolution—precipitation reactions of alumina phases during acid attack and corresponding overall material response. Furthermore, the H2S adsorption behaviour and the impact of high Al3+ content on microbial growth is discussed.
Carboaluminate hydrates have become increasingly important hydrate components in cement-based materials and the monocarbonate dominated paste is expected to be an alternative binder. This study investigates the hydration of C12A7-based cement (CAC) in a mix with precipitated calcium carbonate (PCC) with a multi-method approach including X-ray diffraction (XRD) analyses, in-situ XRD, derivative thermogravimetric (DTG) analyses, isothermal calorimetry and compressive strength test. The results indicated that during early hydration, C2AH8 precipitated as C12A7 dissolved quickly and then monocarbonate and hemicarbonate precipitated at the expense of C2AH8. The hydration reaction in all systems studied was fast and the heat was released mainly during first 24 h. With enough content of PCC added, monocarbonate dominated, leading to a dense microstructure and high early strength development and stable development of strength for a long age.
Artificial rockfill (AR) instead of natural rockfill can be used in rock-filled concrete (RFC) to increase the utilization ratio of solid waste. However, research on the placement style of ARs in RFC is still lacking. Artificial rockfill ratio (ARR) and exposed surface ratio (ESR) are taken into account when designing the placement style of ARs. The placement style with the row and column alignment is recommended for single-layer AR placement. The ARR and ESR of ARs with various sizes as influenced by placement styles without/with cushion blocks are compared. The ARR of the two styles both increases as rockfill size increases. The ARR of RFC without cushion blocks is greater than that of cushion blocks for each rockfill size. The ESR of RFC without cushion blocks decreases as rockfill size increases, whereas the ESR of RFC with cushion blocks increases. Placement style with cushion blocks is highly recommended when the particle size of ARs exceeds 300 mm. ARs with solid waste produced with the pressure forming method and self-compacting concrete (SCC) designed using the paste rheological threshold theory are used to produce scaled solid waste RFC specimens. Cracks tend to appear at the interfaces of the ARs and the SCC due to insufficient bonding force. Furthermore, wide cracks appear near the outer layer of the specimen's side wall. The ARR approaches 53.6% by ignoring the formwork sidewall effect and results in a higher solid waste content of 70.2%.
This study evaluated the corrosion resistance of pure calcium aluminate cement (CAC) concrete specimens made with water-to-cement (w/c) ratios of 0.4, 0.5, and 0.6 and cured at 4, 21, and 90 °C following the ASTM G109 standard. Thermogravimetric analysis results showed increased CAC conversion with increased curing temperature. Compressive strength and bulk electrical resistivity measurements showed that the CAC specimens had lower strength and higher bulk electrical resistivity than ordinary portland cement (OPC) concrete. Macrocell corrosion currents measured after cyclic exposure to chloride solutions showed that the CAC specimens were highly permeable and less resistant to chloride-induced corrosion than OPC concrete. Among the different CAC specimens tested in this study, the specimens made with a w/c of 0.4 and cured at 90 °C showed the highest corrosion resistance.
Microbial-induced corrosion of ordinary Portland cement concrete due to its low biogenic acid resistance is still a global concern, requiring scientists to develop sustainable alternative binders. Accordingly, this study investigates the biogenic acid resistance of alkali-activated glass powder/GGBS mortars with various amounts of calcium aluminate cement (CAC) to replace GGBS while fixing the glass powder (GP) at 75% by mass. The results revealed that incorporating CAC to replace GGBS at 15% in alkali-activated cement (AAC) mortars improved the resistance to microbial-induced corrosion due to incorporating more Al into the AAC gels and can be proposed as a sewer repairing material.
Industries such as metal, ceramics and petrochemicals suffer from high temperature spills. Such events exert a unique form of loading in concrete structures that cannot be accurately simulated by heating of samples in an oven. Calcium aluminate cement (CAC) based concrete is the industry standard for such environment, and while much is known regarding its heating, literature considering hot spills on concrete surfaces is scarce. In this paper, slag is heated up to the same temperature as in a steel factory and then poured on top of cement paste samples with W/C ratios of 0.20 and 0.40. A combination of FEM, TGA, XRD and SEM/EDS was used to investigate the effects of hot spill on the samples. The rapid expansion caused by the thermal shock generated cracks in only some of the samples, while the high temperature environment and unidirectional escape of water caused chemical changes in all samples.
Composite cements belonging to the CaO–Al2O3–ZrO2 (C-A-Z) and CaO–SrO–Al2O3–ZrO2 (C–Sr-A-Z) systems were evaluated as alternatives to calcium aluminate cements (CACs) for use in the refractory castables technology. The mineralogical phases in the as-synthesized C-A-Z and C–Sr-A-Z clinkers confirmed by XRD and SEM-EDS were CaAl2O4, Ca7ZrAl6O18 and CaZrO3 or (Ca,Sr)Al2O4, (Ca,Sr)Al4O7 and (Ca,Sr)ZrO3, respectively. The present work aimed at developing novel Zr-containing cements with excellent mechanical performance, i.e. modulus of rupture and cold crushing strength. Both hydration behavior of the C-A-Z, C–Sr-A-Z, CAC and their hydration products, were studied with isothermal calorimetry, XRD, SEM-EDS and NMR. Impedance camera instrument was firstly successfully applied for time-resolved electrochemical impedance spectroscopy measurements under nonstationary conditions and record the time-dependent evolution of the Nyquist plots for hydrating cement pastes. A sharp increase in the real part of impedance Z’ versus hydration time were corresponded to the second exothermic peak detected on the calorimetry curves.
The early hydration process of calcium aluminate cement (CAC) was characterised by low-field nuclear magnetic resonance (LF-NMR), isothermal calorimetry (IC), X-ray diffraction (XRD) and scanning electron microscopy (SEM). It was revealed that LF-NMR characterisation of the hydration process gives results very similar to those of IC; both show that the hydration reaction is faster in about 10–17 h. In addition, the hydration process of CAC could be quantitatively divided into four stages according to the second derivative of LF-NMR signal amplitude. LF-NMR is also used in the study of CAC microstructure. The results of the T 2 distribution curve prove that there are mainly two kinds of pores of different sizes in the slurry, and they gradually become smaller, especially in more than 10 h. The weighted average T 2 and specific surface area decreased and increased, respectively, due to the gradual densification of the slurry. Owing to the conversion of CAH 10 and C 2 AH 8 to C 3 AH 6 , the solid specific surface area decreased, resulting in the LF-NMR specific surface area curve slowing down or even decreasing in about 20–50 h of hydration. XRD and SEM results further validate the characterisation of hydrated CAC microstructure by LF-NMR.
The Rietveld method is a powerful and relatively new method for extracting detailed crystal structural information from X-ray and neutron powder diffraction data. Since such structural details dictate much of the physical and chemical attributes of materials, knowledge of them is crucial to our understanding of those properties and our ability to manipulate them. Since most materials of technological interest are not available as single crystals but often are available only in polycrystalline or powder form, the Rietveld method has become very important and is now widely used in all branches of science that deal with materials at the atomic level. This book, a collaboration by many authorities in the field, has tutorial and advisory value for those who already have some experience with this important method, but an introductory chapter enables the book to be used as a first text for researchers starting in this area of science.
Silicon dioxide is the main component of the crust of the earth. Combined with the oxides of magnesium, aluminum, calcium, and iron, it forms the silicate minerals in our rocks and soil. Over millions of years silicon dioxide, or silica, has been separated from the original silicate rocks by the action of water to appear as quartz. In a few places it was deposited in the amorphous form as opal. Our English word silica has a very broad connotation: it includes silicon dioxide in all its crystalline, amorphous, soluble, or chemically combined forms in which the silicon atom is surrounded by four or six oxygen atoms. This definitely excludes all the organosilicon compounds made by man in which carbon atoms have been linked directly to silicon atoms - commonly referred to as “silicones,” which do not occur in nature. Silica is soluble enough in water to play important roles in many forms of life. It forms the skeletons of diatoms, the earliest form of life that absorbed sunlight and began to release oxygen into the atmosphere. Many plants use silica to stiffen stems and form needles on the surface for protection. As animals developed, the role of silica became less obvious. But each one of us contains about half a gram of silica, without which our bones could not have been formed, and probably also not our brains. Silica has played a key role since the beginning of civilization, first in flint for tools and weapons and in clay and sand for pottery. The high strength and durability of Roman cement 2000 yr ago is now known to be due to the use of a special volcanic ash that is an almost pure form of amorphous colloidal silica. Today there is active research on the use of the somewhat similar silica fume from electric furnaces to make a super-strong Portland cement. Our present technology would be very different without the silica for the catalysts of our oil refineries, for the molds for casting the superalloys in our jet engines, for modern glass and ceramics, electronic microcircuits, quartz crystals, and fiber optics.
Diagram of system is constructed from literature and new experimental data; solid solution formation on CaO-rich side was studied by electron microprobe; solubility of Fe(II) oxide as found to increase from 6. 1% at 1700 C to 11. 9% at 1160 C; laboratory data are compared with measurements on LD slags.
By using structure chemical results taken from IR-spectroscopic analyses, the CaO-activity was calculated. In liquid state the aluminates dissociate into Ca2+ and into the complexes AlO2- and AlO33-. By application of the Temkin-theory the CaO-activity can be calculated and compared with measured values.
Lafarge remains today the principal manufacturer of calcium aluminate cements with a range of products from 40%-80% Al//20//3 content distributed worldwide from three integrated plants in France, one in England and grinding facilities in the USA and South Africa. The manufacture of calcium aluminates nowadays is not exclusive to Lafarge Fondu International and a number of other suppliers offer various products of this type. The two principal methods used today for the manufacture of calcium aluminate cements (CAC) are fusion and sintering.
Experience and operating characteristics associated with the production and use of high-alumina cement are reported. An example of the possible installation of a compact small production unit in an existing portland cement or lime manufacturing plant is presented, and attention is drawn to the required raw material quality. Not only the chemical composition but also the physical properties of the raw materials are of major importance in determining their suitability for utilization in the tank shaft kiln.
The hydration mechanism of C//5A//3 at 10 degree , 20 degree and 35 degree C was studied. Process evolution was followed by X-ray diffraction, differential scanning calorimetry (DSC) and isothermal calorimetry tests.
The problems of calculating the phase composition of high alumina cement clinkers are reviewed. Data on the stability and composition of pleochroite are described. Eleven experimental clinkers have been prepared under oxidizing conditions and analysed by microscopy and X-ray diffraction. Data are presented on these clinker compositions and microprobe analyses given for the CaAl2O4 phase. Significant differences occur in iron and, occasionally, in Si contents. These arise as a consequence of cooling conditions: faster cooling enhances iron and, to a lesser extent, Si substitution in the aluminate.
The article deals with the subject of establishing a quantification routine for Fe-rich Calcium Aluminate Cements (CAFC). Two different synthesized Fe-rich Calcium Alu-minate Cements (SCAFC) were produced according to the chemical composition of the "standard grade"-CAFC, but in the simplified system CaO-Al2O3-Fe2O3-SiO2. From refined structure models, a single quantification routine (QXRD) was established. The amounts of phases and the chemical composition of major phases Monocalciumalumi-nate (CA), Ferrite phase and Gehlenite were determined by refining occupancy factors. The quantification result was calibrated by calculating the amounts of phases combined with their individual chemical compositions determined by QXRD and EPMA and shows the good accuracy of the established quantification routine. The flexibility of the routine makes it applicable to the industrial "standard grade"-CAFC.
The Building Research Establishment (BRE) in England is working on blended high alumina cement (HAC) which may lead to its use in structural concrete. The BRE is currently studying cements made by blending HAC with granulated blast furnace slag for which the name BRECEM has been registered. The compressive strength of such concretes increases with time when kept in hot, wet conditions.
High alumina cements are usually made by a fusion process in which the components are melted, cast into ingots and allowed to crystallise. It has proved difficult to predict and control the mineralogical composition of the melt-ceramic product, termed clinker. The clinker mineralogy is known to be sensitive to the bulk SiO2 content, which is typically only 2-6 wt-%. This study provides data on laboratory simulant clinkers, melted and allowed to cool at rates of 5 and 300 k min-1. The charges were selected to yield, upon crystallisation, a melilite close to gehlenite, Ca2 (Al,Fe)2SiO7. Analyses of the melilite actually present in undercooled charges, containing 2-6% wt-% SiO2, disclose that melilite crystallises with a substantial deficit of Si relative to its nominal composition: this deficit is approximately charge balanced by additional trivalent ions, Al3+ and Fe3+. The implications to clinker mineralogy and reactivity are discussed.
La structure pseudo˗hexagonale du silicate tricalcique, à la température ambiante, est mise en évidence à l'aide de deux méthodes : construction du réseau réciproque d'un monocristal microscopique à l'aide d'un diagramme de cristal oscillant et d'un diagramme de Weissenberg puis méthode du pseudo˗réseau hexagonal grâce aux raies de poudre.
L'indexation des raies de diagrammes Debye˗Scherrer obtenus à l'aide d'une chambre à haute température a permis d'identifier six phases du silicate tricalcique entre la température ambiante et 1 100° soit :
Triclinique I →(à 620°) Triclinique II →(à 920°) Triclinique III →(à 980°) Monoclinique →(à 990°) Orthorhombique →(à 1 050°) Hexagonal
Ces transitions, confrontées avec celles de la micro˗analyse thermique différentielle, peuvent correspondre à des changements de symétrie ou à des surstructures.
A la température ambiante, la forme triclinique aurait les paramètres :
a = 28,16 Å b = 28,42 Å c = 25,10 Å
α = 90°10 β = 90°22 γ = 120°
Eleven experimental high alumina cement clinkers were prepared in which iron was partially reduced to mixtures of FeO and Fe2O3. Crystallizations products of the melts were determined as a function of composition and cooling rate. The most significant discovery was the occurrence of the mixed layer intergrowths of the gehlenite and pleochroite, nominally Ca20Al32-xFex2+SixO68 (x nearly equal to 2.5-3.5). These intergrowths are indistinguishable from ordinary pleochroite by optical microscopy and powder X-raay diffraction but are revealed by high resolution electron microscopy. Intergrowth formation is assisted by similarities between the two structures. In some composition and crystallisation regimes mixed intergrowth formation removes much of the calcium, aluminium, and silicon which would otherwise yield monocalcium aluminate and dicalcium silicate.
The Q phase has the composition Ca20Al32-2xMgxSixO68, 2.5 = or < x = or < 3.5. Unit-cell dimensions are given for four compositions within this range, and the indexed XRD pattern for Ca20Al26Mg3Si3O68. The subsolidus phase assemblages with Q as a phase in the section 3CaO.Al2O3 - CaO.Al2O3 - MgO.SiO2 are presented. Compositions Ca22Al26Mg3Si4O72 and Ca6Al8MgSiO21, previously suggested as formulae for Q, decompose on heating to give poly-phase assemblages, and Ca21Al28Mg7SiO72 is also thermally unstable.B.C.M.B.
Full structure refinements based upon single-crystal X-ray diffraction data, have been completed upon a suite of natural and heat-treated anorthite-rich feldspars with compositions between An68Ab32 and An100. The complete set of 20 refinements provides an internally consistent database for the detailed characterization of the structural variations arising from changes in composition and cation order. Multiple regression analysis has been used to demonstrate how observed tetrahedral T-O bond lengths reflect not only (Al,Si) occupancies, but are perturbed by M-O bonding, T-O-T bond angles, and a linkage factor dependent upon neighboring tetrahedral sites. Differences in mean bond lengths between tetrahedra are used to define a thermodynamic order parameter, QOD, for the Cl̄-Il̄ transition. Refined thermal parameters of both the large cation sites and those atoms forming the tetrahedral framework result in probability ellipsoids that are oriented in the same way as those obtained from average structure determinations of Pl̄ structures. -from Authors
CaSO4.0.53H2O was prepared by dehydration of CaSO4.2H2O in conc. HNO3 at 36 + or - 3oC and rehydration at 35% relative humidity. The crystals are monoclinic, space group I2. The composition depends on the water vapour pressure, varying from CaSO4.0.53H2O at 35% to a limiting composition of CaSO4.O.62H2O (trigonal, space group P3121) at 0.92% relative humidity. Soluble anhydrite of composition CaSO4.0.03H2O was produced by dehydration of the hemihydrate for 4 days at 54 + or - 4oC in a vacuum of 1 X 10-4 bar. The heats of reaction DELTA H295 for the 0.53, 0.62 and 0.03H2O preparations were measured as -17709 + or - 18, -16981 + or - 11 and -28001 + or - 33 J/mole, respectively. From these values, the heats evolved in the transition of monoclinic CaSO4.0.53H2O to trigonal CaSO4.0.62H2O and in the hydration of soluble anhydrite CaSO4.0.03H2O to hemihydrate CaSO4.0.53H2O were calculated to be -728 and -10272 J/mole, respectively. The hydration of CaSO4.0.03H2O to hemihydrate does not occur by continuous increase in the amount of channel water in the structure but by direct hydration to CaSO4.0.53H2O.-R.A.H.
Crystalline anorthite (CaO·Al2O3·2SiO2) powder was synthesized below 1000°C by a solution process employing a polymer carrier. Polyvinyl alcohol (PVA), having different molecular weights, was used as an organic carrier for preparation of the precursor ceramic gel. The PVA content, its degree of polymerization and type of silica sol had a significant influence on the specific surface area and morphology of the powder, and its crystallization behavior. The polymer content and its molecular length had an effect on the cation distribution in the solution and resulted in different crystallization behavior. Abnormally, omisteinbergite (CaO·Al2O3·2SiO2), which has the same composition as anorthite, was observed prior to crystallization of anorthite. A more porous morphology and higher specific surface area were obtained with a higher content of the PVA polymer. The milled, amorphous-type anorthite powder was densified to a relative density of 94% below 1000°C.
Calcined magnesia and two high-alumina cements, i.e. Secar 51 and Secar 71, were combined with solutions of Na polyphosphate with chain lengths between n = 4 and n = 30. The resulting pastes exhibited setting time and strength development that depended on the polyphosphate chain length and curing conditions. A distinct chemical shrinkage accompanied the hardening process.
Manganese can interact in several ways in High Alumina cements. By the addition of manganese sulphate to High Alumina cements a control of hydration similarly as by the use of gypsum can be obtained, despite other hydration products are formed. A new method of introducing manganese in High Alumina cements can be obtained by using manganese raw materials already in mixing up the raw meal before sintering. By the use of elevated amounts of manganese (up to 30 %) some manganese cements can be obtained. The addition of manganese can be performed by Mn-ore, Mn-bearing residues, Mn-wastes or Manganese fines from productions. Different manganese-containing phases are formed during the sintering process. Also the hydration products can contain manganese. The properties of manganese cements and the advantages are described.
The hydration behaviour of calcium aluminate cements can be controlled by using different additives. A combination of additive type and crystallochemical influence was studied by using different concentrations of calciumsalts of acetate [Ca(CH3COO)2], monochloroacetate [Ca(CH 2ClCOO)2], dichloroacetate [Ca(CHCl2COO) 2] and trichloroacetate [Ca(CCl3COO)2] at varying temperatures (10°C, 20°C, 45°C and 60°C). For these studies a specially modified isothermal heat-flow calorimeter together with a modified sample preparation technique was used. The additive free cement samples showed a unique setting behaviour with temperature rise from 10°C to 20°C. Calorimetric data of acetate containing samples indicates that hydration is strongly influenced by the parameters temperature, additive and its concentration hence different hydration effects such as light acceleration, strong retardation or even suppression of hydration. Therefore different acting mechanisms can be proposed. Temperature as one of the modified parameters seems to play a very important role. The best retarder in the investigated system was Calciummonochloroacetate at every investigated temperature. The development of different hydration products was followed by XRD-measurements. XRD analysis of additive free reference cement samples showed temperature dependent hydration products. Samples treated at low temperatures showed CAH10 and C 2AH8. Higher temperatures promoted the formation of C 2AH7,5, C2ASH8 and C 3AH6. In the additive containing samples besides known phases like CAH10, C2AH8, C2AH 7,5, C2ASH8 and C3AH6 also C4AH13 and C4ACcH11 as well as Acetate containing calcium/aluminium hydrates were formed. Also the formation of acetate bearing gel phases can be proposed. The formation of C 3AH6 at higher temperatures was only suppressed in the case of Calciumonochloroacetate. The use of cluster analysis of X-rays of hydration products is given. Also for the first time cluster analysis on calorimetric data is included.