Article

Calcium Aluminate Cements - Raw Materials, Differences, Hydration and Properties

Authors:
To read the full-text of this research, you can request a copy directly from the author.

Abstract

Calcium aluminate cements their mineralogy and hydration is described. Also differencies in mineralogy and properties are included.

No full-text available

Request Full-text Paper PDF

To read the full-text of this research,
you can request a copy directly from the author.

... 1,2 Calcium aluminate cement contains CA (monocalcium aluminate) as the main cementing compound, with C 12 A 7 , CA 2 , and CA 6 as the minor phases, where CCaO, AAl 2 O 3 , and HH 2 O in the cement chemistry notation. 3,4 The rapid hydration of CA produces CAH 10 and C 2 AH 8 + AH 3 at ambient temperature and pressure condition, which contributes to the fast-strength development in the first 1−2 days after mixing with water. 5 The hydration route of CA is strongly temperaturedependent; see eq 1: 1 ...
... 5 Investigating the mechanism of the conversion reaction has pronounced significance both to the fundamental knowledge of cement chemistry and its engineering applications. The crystal structures of CAH 10 and C 3 AH 6 are well established; 4,9,10 however, that of C 2 AH 8 is still not completely defined, although it is known to be a layer-double-hydroxide (LDH) compound with Al(VI) in the main layer and Al(IV) in the interlayer space. 11,12 The missing information on the C 2 AH 8 structure and the unclear relationship between the chemical environments of the metastable and the stable phases add to the difficulty in solving the following fundamental questions regarding the conversion reactions: ...
... These phases were verified by a PANalytical X'Pert Pro TR diffractometer, according to the characteristic x-ray diffraction (XRD) peak positions of each phase. 4,8 The pure CAH 10 , C 3 AH 6 , and gibbsite samples were used to collect reference spectra at the Ca L 2,3 -edge and Al K-edge. The hexagonal-shaped C 2 AH 8 single crystals are readily identified from amorphous AH 3 under STXM and are used to collect the reference spectra for C 2 AH 8 . ...
Article
The main binding phases of calcium aluminate cement (CAC) concrete, CaO∙Al2O3∙10H2O (CAH10) and 2CaO∙Al2O3∙8H2O (C2AH8), slowly convert to 3CaO∙Al2O3∙6H2O (C3AH6) and Al(OH)3 (AH3). This reaction significantly speeds up at a temperature higher than ~30°C, and over time leads to significant strength loss in concrete. Due to the lack of direct evidence that simultaneously probes morphological and chemical/crystallographic information, intense debate remains whether the conversion is generated by a solid-state or through-solution reaction. The conversion of CAH10 at an elevated temperature is studied herein using synchrotron-radiation-based X-ray spectromicroscopy capable of acquiring near edge x-ray absorption fine structure (NEXAFS) data and ptychographic images with resolution of ~15 nm. We show that, when stored at 60 °C CAH10 first converts to C2AH8 by solid-state decomposition, followed by the through-solution formation of C3AH6. The C3AH6 crystallizes from both the relics of dissolved C2AH8 and from the surface of existing C3AH6 crystals. The solid-state decomposition of CAH10 occurs in multiple sites inside the CAH10 crystals; the spatial range of each decomposition site spans a few tens of nanometers, which overcomes the kinetics barrier of ion transportation in solid-state. Our work provides the first nanoscale crystal-chemical evidence to explain the microstructure evolution of converted CAC concrete.
... 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]. ...
Article
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.
... Regarding calcium aluminate cement hydration, amorphous/nanocrystalline aluminum hydroxide coexists with other crystalline materials as a result of the hydration reactions [6]. For instance, CaAl 2 O 4 is the main phase of calcium aluminate cements [6] and its hydration mechanism has been widely studied [6][7][8][9]. ...
... Regarding calcium aluminate cement hydration, amorphous/nanocrystalline aluminum hydroxide coexists with other crystalline materials as a result of the hydration reactions [6]. For instance, CaAl 2 O 4 is the main phase of calcium aluminate cements [6] and its hydration mechanism has been widely studied [6][7][8][9]. The mechanism of this hydration reaction strongly depends on temperature, as different crystalline phases can be formed with temperature [10]. ...
... Regarding calcium aluminate cement hydration, amorphous/nanocrystalline aluminum hydroxide coexists with other crystalline materials as a result of the hydration reactions [6]. For instance, CaAl 2 O 4 is the main phase of calcium aluminate cements [6] and its hydration mechanism has been widely studied [6][7][8][9]. The mechanism of this hydration reaction strongly depends on temperature, as different crystalline phases can be formed with temperature [10]. ...
Article
There are many commercially important multiphase materials which contain amorphous phases, such as cement pastes, porcelains, glass-ceramics or pharmaceutical compounds. However, the analysis of amorphous phase(s) within cement matrices that contain high amounts of crystalline phase(s) is a challenge. Here, we report measurements of total scattering data quantitatively analyzed by Pair Distribution Function (PDF) minimization and Rietveld methodology to determine phase compositions including both amorphous and nanocrystalline phase contents in cement-related samples. Furthermore, laboratory techniques were used to complement the sample characterization. In addition to five reference materials, the main phase of calcium aluminate cements, CaAl2O4 hydrated at 50 °C to yield crystalline hydrogarnet, Ca3Al2(OH)12, (43 wt%) and nanocrystalline aluminum hydroxide gel, Al(OH)3·0.1H2O (50 wt%) was also investigated. The PDF analyses revealed that the hydroxide gel has a gibbsite local structure with an average particle size close to 5 nm. PDF and Rietveld quantitative phase analysis results fully agree.
... 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. ...
Article
Full-text available
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.
... This evidence further supports the thermodynamic stability of this composition. Figure 4 also reports volumes of some synthetic katoite samples: (1) empty circles represent the hydrothermal samples 1 and 12 from ; (2) asterisks are samples by Jappy and Glasser (1991); (3) empty rhombi are taken from Pöllmann (2012). By inspection of the lower panel of Figure 4, it turns out that most of the above mentioned katoite samples display a negative excess volume. ...
... The authors relied on the chemical formula calculated from the structure refinement, namely Ca 3 Al 2 (SiO 4 ) 1.53 (H 4 O 4 ) 1.47 , despite their own microprobe analysis revealing a distinctly higher Si content: Ca 2.98 Mg 0.05 Fe 0.02 Al 1.93 (SiO 4 ) 1.77 (H 4 O 4 ) 1.23 . Indeed, in view of the remarkable agreement with the composition previously obtained by Pabst (1942) Pöllmann (2012). All these points occur below the solid red line joining the two end-members. ...
... At intermediate compositions (4 < n < 8) we cannot compare precisely, because the available experimental data are scarce, and because those on hibschite from Crestmore are located right at the discontinuity of the calculated functions DV(n) and DH(n). We may just remark that samples reported by Pöllmann (2012) with positive excess volume lie very close to the corresponding calculated points. Lager et al. (1987a), n = 0; Sacerdoti and Passaglia (1985), n = 2.56; Ferro et al. (2003), n = 4.48; Basso et al. (1983), n = 7.08; and Novak and Gibbs (1971), n = 12. ...
Article
Full-text available
Structural and energetic properties of the grossular-katoite solid solution are studied with a full ab initio quantum chemical approach. An all-electron basis set and the hybrid B3LYP functional are used. Calculations are performed within the primitive cell of cubic garnets. The hydrogarnet substitution , SiO 4 ↔ H 4 O 4 , yields 136 symmetry-independent configurations ranging from triclinic to cubic symmetry. All of them have been structurally optimized, the relaxed geometries being characterized by pseudo-cubic conventional cells. At the present level of approximation, the most stable configurations constitute by far the largest contributions to the system properties. Considering only the most stable configurations, average geometrical features of the actual solid solution are closely approximated. The excess volume displays a highly non-ideal behavior that is favorably compared with carefully analyzed and selected experimental data. The excess enthalpy deviates from the regular model; it draws an asymmetric function of composition with two minima that can be associated to structures or compositions observed in nature. Geometrical variations and distribution of the tetrahedra are analyzed. Calculations provide independent support to the use of a split-atom model for experimental refinements on these compounds. The asymmetry of the enthalpy of mixing can be associated with two distinct distribution patterns of the tetrahedra. Hydrogen interactions also contribute to the asymmetry of the excess enthalpy, as it turns out by comparison between compositions close to fully hydrated katoite and those close to grossular. Hydrogen interactions in Si-free katoite are found to be weak as suggested by dramatic changes in the H environment associated with the introduction of SiO 4 tetrahedra.
... 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]. ...
Article
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.
... Because Q phase possesses advantages of high early strength, relatively stable hydrates and high chemical resistance [9,11], it shows a potential to improve the early strength of Portland cement paste. Also, previous study showed that mortar prepared with cement containing Q phase clinker and Portland clinker showed a high early compressive strength [12]. ...
... Also, previous study showed that mortar prepared with cement containing Q phase clinker and Portland clinker showed a high early compressive strength [12]. However, gypsum is usually added to regulate the rapid reaction of C 3 A and preserves the workability of Portland cement paste during the first few hours [13,14], and the presence of gypsum would also extremely influence the hydration of high alumina cement [11] as well as Q phase. Then, investigation on the hydration of Q phase with gypsum is important to promote the usage of Q phase in Portland cement. ...
Article
Since the quaternary (Q) phase (Ca20Al26Mg3Si3O68) shows a potential to improve early strength and reduce the shrinkage of Portland cement paste, understanding the hydration of Q phase with and without sulfate is of great importance. In this work, the hydration of Q phase-gypsum systems were investigated in terms of the kinetics, phases evolution and microstructure development. The test results show that the hydration kinetics, microstructure and composition of hydration products depends on the gypsum addition. The observation of heat evolution indicates that the hydration rate is accelerated by gypsum, and formation of ettringite (AFt) and monosulfoaluminate (AFm) is probably the controlling factor at early age. The main hydration products are AFt, AFm, calcium aluminate hydrate and AH3 phase. And the calcium aluminate hydrate and AH3 phase will continue to form as the hydration proceeds. The compressive strength increases significantly with increasing gypsum content, which is due to the increasing hydration degree of Q phase-gypsum samples. Microstructural study shows that the formation of AFm phases occurs in the space between the Q phase grains but also within the “inner” regions around the Q phase grains. The AFm platelets become small and the “inner” regions around grains disappear in the samples with high gypsum. In addition, the AFt or AFm is closely intermixed with AH3 and calcium aluminate hydrate, microcracks occur in samples with high gypsum content due to the formation of AFt.
... As the hydrates densify during conversion, see reactions (4) to (6), they release secondary water into the binder and therefore (new) porosity is formed. It is well documented that the development of porosity results in a significant loss of strength [1,2]. ...
... Secondly, anhydrous CA component is identified in three tomograms (20 °C, 4 °C and 4 → 50 °C) as it has largest electron density value, see Fig. 2, and it perfectly matches the expected crystallographic The paste hydrated at 50 °C does not contain anhydrous CA, see Fig. 2a, and it has residual electron density in the water porosity region. Very importantly, the 4 → 50 °C sample, see Fig. 2d, contains anhydrous CA, likely because a low w/s ratio in the initial stage of hydration, but it also shows a significant electron density contribution in the water porosity range, very likely originated in the conversion stage when heating at 50 °C, see Eqs. (4) to (6). Additional details can be extracted from the study of the electron density histograms, see Fig. 2, by combining the information with that present in the bivariate histograms of the n e (r) and β(r) datasets, see Fig. 3. ...
Article
Calcium aluminate cements are used for special applications but are nowadays banned for general structural purposes due to the calcium aluminate hydrate conversion, that has led, for concretes fabricated with high water contents, to building collapses. The stoichiometries of these conversion chemical reactions are relatively well established but the consequences in porosity, key to predict durability, were unknown. Here, we have used hard X-ray ptychographic nanotomography to study the hydration of CaAl2O4 at different temperatures and chiefly, at 4 °C and then at 50 °C to provoke conversion similar to field conditions. The mass densities of the resulting Al (OH)3 gels were 1.94, 1.98 and 2.23 g·cm⁻³, for samples hydrated at 4, 20 and 50 °C, respectively. These values are lower than that of gibbsite, 2.42 g·cm⁻³. Above all, this 3D imaging technique has allowed measuring the secondary water porosity developed in the conversion, which has an average pore dimension close to 140 nm.
... En second lieu, un phénomène appelé «conversion» se produit pendant l'hydratation des CAC, à travers lequel les hydrates métastables au vu de la thermodynamique sont convertis en hydrates stables. Comme les masses volumiques des hydrates sont différentes, cette transformation provoque l'augmentation de la porosité totale qui est à l'origine de la perte de résistance (Pöllmann, 2012;Scrivener & Capmas, 1998). Par conséquent, l'utilisation des ciments alumineux dans les mortiers et les bétons nécessite des précautions particulières. ...
... Les réactions possibles au cours de l'hydratation du CA sont présentées ci-dessous. Des réactions analogues peuvent aussi être écrites pour la phase C12A7 (Pöllmann, 2012;Scrivener, 2003). Pour des températures inférieures à 15 °C, la formation de CAH10 est prédominante (l'équation (1)). ...
Thesis
Full-text available
Les chapes autonivelantes sont souvent composées de ciment alumineux et de sulfate de calcium pour assurer une prise rapide. Le ciment alumineux est généralement à base de monocalcium aluminate (CA). Récemment, un nouveau ciment alumineux riche en mayénite (C12A7) a été conçu pour optimiser l’application des chapes par l’augmentation de la quantité d’ettringite formée. Le développement de l’hydratation au jeune âge (pendant les premières 24 heures) de ce type de produit est pourtant très peu connu. L’objectif principal de cette étude est d’évaluer l’interaction entre l’hydratation au jeune âge et le séchage naturel à travers un modèle de couplage. Ce modèle est appliqué à l’étude d’une pâte pure ettringitique composée de ciment riche en C12A7 et de plâtre alpha. Tout d’abord, un modèle cinétique d’hydratation, initialement développé en système dilué, a été proposé pour étudier l’hydratation d’une pâte pure ettringitique en condition endogène. Il permet de décrire l’évolution en fonction du temps de la porosité, de la teneur en eau, des quantités de phases (réactifs et hydrates). Un modèle de distribution de rayon de pores a été aussi proposé. Ce modèle permet d’obtenir la courbe de pression capillaire au jeune âge nécessaire pour la modélisation des transferts hydriques. Ensuite, une modélisation complète des transferts hydriques a été présentée. Elle a montré que l’hypothèse d’une pression de gaz constante conduit à une surestimation de perte de masse pour des matériaux à faible perméabilité. Pourtant, pour le cas des ciments alumineux, cette hypothèse peut être retenue pour simplifier le couplage avec le modèle cinétique d’hydratation. Enfin, une modélisation des couplages entre l’hydratation et la dessiccation a été développée. L’originalité de ce modèle porte sur l’intégration de la courbe isotherme issue du modèle de distribution poreuse dans la modélisation des transferts hydriques via une fonction Matlab. Le modèle a été capable de reproduire la cinétique de perte de masse d’un échantillon de pâte pure ettringite réactive soumis au séchage précoce. La nécessité de la prise en compte de l’évolution de température a été mise en évidence. De plus, le modèle a prévu un arrêt précoce de l’hydratation à la surface séchante après 10 heures d’hydratation. L’effet de la dessiccation atteint une profondeur de 5 mm sur un échantillon de 3 cm d’épaisseur.
... This reaction is known as conversion, and is accompanied by the formation of AH 3 gel, and the release of water [10][11][12]. The conversion process is driven by temperature as well as access to moisture [10,13,14]. At low curing temperatures (T< 15°C) the initial metastable hydrate favored to form is CAH 10 . ...
... This process is thermodynamically inevitable, and the rate at which it occurs is a function of the temperature history of the material during hydration. Increasing the curing temperature and exposure to moisture will result in faster conversion from the metastable hydrates to the stable hydrates [13,14,17]. The density of the metastable hydrates (density (g/cm 3 ): CAH 10 =1.72, density of C 2 AH 8 =1.95) is lower than that of the stable hydrates (density (g/cm 3 ): C 3 AH 6 =2.52 AH 3 =2.4) ...
Article
Full-text available
There is renewed interest in North America for the use of calcium aluminate cement (CAC) in infrastructure repair because of its ability to gain strength rapidly even at low temperatures, the ability to customize its fresh workability, and its durability in adverse environments. Conversion of the hydration products of concrete where CAC is the only binder is a well-known phenomenon that is typically accompanied by strength loss, the rate and extent of which is dependent on the temperature history of the concrete, the w/cm, the cement content, and the concrete mixture design. An accelerated method of determining the converted strength of CAC concrete that is convenient for use in the field is presented. This test allows the samples to be cast in the field and left at ambient field temperatures for the first 24 hours; then the specimens are moved to the laboratory and placed in a water bath at 50°C to promote conversion. Robustness testing results are presented for the test method. The effects of the water to cement ratio, the initial (first 24 hours) curing temperature, the length of time before placing the specimens in the 50°C water bath, and the aggregate type are examined. The impact of the low replacement rates (up to 10 %) of finely ground limestone (FGLS) for CAC in concrete mixtures on converted concrete strengths was also studied. Results from testing showed that the majority of mixtures converted 48 hours after submersion in the 50°C water bath; however, differences in the initial curing temperature or aggregate type can delay conversion up to 11 days when specimens are cured following this procedure. Results also indicate that FGLS at replacement rates of 5 % and below in CAC systems may help increase the converted strength observed without reducing initial early strength gain.
... Such cementing performance leads to the wide application of CAC in civil engineering practices [1,2]. However since the 1970s the concrete community suggested to limit the use of CAC in structural application, due to increasing numbers of failure reports of CAC concrete over the long-term service life [1][2][3][4]. In recent years, standards have been proposed to guide the proper use of CAC concrete in civil engineering [2,5]. ...
... The formation of C 3 AH 6 is, on the contrary, more likely through solution. Since C 3 AH 6 and AH 3 have higher densities than CAH 10 and C 2 AH 8 , the conversion reaction significantly increases the porosity of the hardened matrix, and is therefore the major cause of the strength-loss of CAC concrete. ...
Article
Calcium aluminate cement (CAC) has wide application in civil engineering and castable refractory materials. The main binding phases of hardened CAC paste, CaO·Al2O3·10H2O (CAH10), inevitably converts to 2CaO·Al2O3·8H2O (C2AH8), 3CaO·Al2O3·6H2O (C3AH6) and Al(OH)3 (AH3), leading to a significant change in the mechanical properties of the CAC matrix. This work investigates the mechanical properties of the main crystalline components in hydrated and/or converted CAC systems, using synchrotron-radiation-based high-pressure X-ray diffraction. The anisotropic deformations of CaAl2O4 (CA), CAH10 and C3AH6 along each crystallographic direction are investigated, along with their bulk moduli. The density-driven stiffening hypothesis is validated for the studied phases and other cement-based minerals. An atom-scale topological analysis is proposed to explain the unusually high stiffness of CAH10. The results provide fundamental information to understand the mechanical properties of single cement-based phases at molecular level, and enable predicting the changing mechanical properties of converted CAC matrix using homogenization models.
... As a result, the amount of C 3 AH 6 formed quickly increases with increasing age. The crystal structure of C 3 AH 6 is very dense (Pöllmann, 2012) and thus difficult to chemically combine with chloride ions. On the other hand, Friedel's salt has a layered crystal structure and elevated temperature may lead to the decomposition of Friedel's salt (Sanjuán, 1998). ...
... This may result from the accelerated phase conversion at higher curing temperatures. As the C 3 AH 6 phase is denser than the CAH 10 and C 2 AH 8 phases, the conversion from CAH 10 and C 2 AH 8 to C 3 AH 6 is accompanied by the formation of massive harmful pores (Pöllmann, 2012). With the addition of 3% sodium chloride, the pore structure distribution was not improved and the proportion of harmful pores increased. ...
... 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]. ...
Article
Full-text available
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.
... A variety of impurities can be encountered in refractories, such as iron, silicon, or sodium, as a result of processing or as remnants from the raw materials. 6 The main sources of sodium impurities in refractories derive from the use of caustic soda in the Bayer process to access alumina from bauxite ore, as well as the addition of additives with a sodium counterion to modify the setting and hardening properties of the green refractory. The influence of sodium-containing additives on the room-temperature properties is strongly determined by their acidity and has been shown to have a dramatic effect. ...
... The influence of sodium-containing additives on the room-temperature properties is strongly determined by their acidity and has been shown to have a dramatic effect. [6][7][8][9] Much work has gone into the optimization of setting time and placing conditions of castables with the help of various additives. Sodium citrate has been shown to form a gel with the ions from dissolved cement particles which hinders the subsequent precipitation of hydrates thereby acting as a retarder, 10,11 whereas the reaction of alkaline additives, such as sodium carbonate, accelerates hydrate precipitation by forming a nucleation substrate with the aluminate ions in solution. ...
Article
The effect of sodium on refractory phase formation in a model Calcium Aluminate Cement-bonded refractory was investigated from 700°C to 1500°C. Sodium reacts with α-alumina to form sodium β-alumina (β-Al2O3) via the intermediate NaAlO2. Formation of β-Al2O3 disrupts the reaction path of calcia with alumina, delaying crystallization of calcium hexaluminate, CaO·6Al2O3, from 1350°C to 1500°C. β-Al2O3 is also shown to reduce Young's modulus and delay sintering. The presence of NaAlO2 and β-Al2O3 result in an increase in internal friction. Increased linear expansion of up to 47% is observed when 1 wt% Na is added. The expansion is shown to scale with the amount of dopant with only 0.3 wt% Na leading to an additional 31% linear expansion. On cooling, the presence of β-Al2O3 can be demonstrated by a peak in internal friction between 1200°C and 1000°C which could be caused by Na+ ion hopping along the spinel-like planes.
... 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: ...
Article
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.
... En second lieu, un phénomène appelé «conversion» se produit pendant l'hydratation des CAC, à travers lequel les hydrates métastables au vu de la thermodynamique sont convertis en hydrates stables. Comme les masses volumiques des hydrates sont différentes, cette transformation provoque l'augmentation de la porosité totale qui est à l'origine de la perte de résistance (Pöllmann, 2012;Scrivener & Capmas, 1998). Par conséquent, l'utilisation des ciments alumineux dans les mortiers et les bétons nécessite des précautions particulières. ...
... Les réactions possibles au cours de l'hydratation du CA sont présentées ci-dessous. Des réactions analogues peuvent aussi être écrites pour la phase C12A7 (Pöllmann, 2012;Scrivener, 2003). Pour des températures inférieures à 15 °C, la formation de CAH10 est prédominante (l'équation (1)). ...
Thesis
Full-text available
Self-leveling flooring compounds (SLC) are often composed of calcium aluminate cements (CAC) and calcium sulfate to ensure rapid setting. The mineral composition of calcium aluminate cements is usually designed around monocalcium aluminate (CA). Recently, a new cement with the main compound of mayenite (C12A7) has been designed to optimize the application of SLC by increasing the amount of ettringite in the hydration product. However, there is a lack of knowledge related to early-age hydration (during the first 24 hours) of this type of product.The main objective of this study is to evaluate the interaction between early-age hydration and natural drying through a coupling model. This model is applied to the study of an ettringite binder composed of cement rich in C12A7 and plasterFirst, a kinetic model of hydration, initially developed in a diluted system, has been proposed to study the hydration of an ettringite paste under endogenous conditions. It allows to describe the evolution as a function of time of the porosity, the water content, and the quantities of phases (reactants and hydrates). A model of pore size distribution has also been proposed. This model allows us to obtain the capillary pressure curve at early-age, which is necessary for the modeling of moisture transport.Then, a complete modeling of water transfers was presented. It has shown that the assumption of constant gas pressure leads to an overestimation of mass loss for weakly permeable materials. However, for the case of CAC, this hypothesis can be retained to simplify the coupling with the kinetic model of hydration.Finally, a modeling of the coupling between hydration and desiccation has been developed. The originality of this model concerns the integration of the isothermal curve resulting from the pore size distribution model in the modeling of water transfers via a Matlab function. The model was able to reproduce the kinetics of mass loss of a sample of ettringite binder subjected to early drying. The role of temperature was also elucidated. In addition, the model predicted an early stop of hydration on the drying surface after 10 hours of hydration. The effect of desiccation reaches a depth of 5 mm on a sample with a thickness of 3 cm.
... Katoite is included in a group of minerals known as hydrogarnet (Ca 3 (Al,Fe) 2 (SiO 4 ) y (OH) 4(3−y) ; 0 < y < 3), where the tetrahedra are partially or completely replaced by OH − . The Al-containing hydrogarnet includes the hydrogrossular solid solution (Ca 3 Al 2 (SiO 4 ) y (OH) 4(3−y) , 0 < y < 3), katoite being the Si-free end-member (C 3 AH 6 in chemical notation 2 ) (Pollmann, 2012;Dilnesa et al., 2014). Thenardite is easily separated from the mixture of thenardite and katoite by gentle rinsing with pure water ("PGA washed" in Fig. 9.4; Ca 3 Al 2 (OH) 12 ) within a purity of 95%, as estimated by XRF. ...
Preprint
Industrial wet phosphoric acid production in Huelva (SW Spain) has led to the controversial stockpiling of waste phosphogypsum by-products, resulting in the release of significant quantities of toxic impurities in salt marshes in the Tinto river estuary. A two-fold objective is proposed: removing hazardous and unpleasant wastes and mitigating the emissions of green house gasses in the atmosphere. Thus, the first aim of this research is to synthesize materials with the ability for CO2 sequestration from wastes as preliminary work for designing industrial processes, which involve a reduction of CO2 emissions. In this regard, phosphogypsum from the fertilizers industry, caustic soda solutions, and liquid alkaline wastes aluminum industries have been considered as precursors. Our results demonstrate the high efficiency of portlandite precipitation by phosphogypsum dissolution using alkaline solutions. Carbonation experiments performed at ambient pressure and temperature resulted in total conversion of the portlandite into carbonate, confirming that it may act as a carbon sequestration agent. Therefore, wastes, which are unable to fix CO2 by themselves, can be successfully turned into effective CO2 sinks. The fate of trace elements present in the phosphogypsum waste was also investigated, and trace impurities were found to be mainly transferred to the final calcite. We believe that the procedure proposed here should be considered not only as a solution for reducing old stockpiles of phosphogypsum wastes, but also for future phosphoric acid and other gypsum-producing industrial processes, resulting in more sustainable production. Finally, to enhance the technological potentialities of the procedure, preliminary studies for the application of one of the obtained precipitates, the portlandite (lime paste), as construction material, have been carried out.
... Portland cement is not the only cement that is commercially available. Calcium aluminate cements (CAC) [18] are another kind of cements that is currently used for several special applications. The main component of CAC cements is monocalcium aluminate, or CaAl 2 O 4 . ...
Article
Full-text available
The analysis of atomic ordering in a nanocrystalline phase with small particle sizes, below ≈5 nm, is intrinsically complicated because of the lack of long-range order. Furthermore, the presence of additional crystalline phase(s) may exacerbate the problem, as is the case in cement pastes. Here, we use the synchrotron pair distribution function (PDF) chiefly to characterize the local atomic order of the nanocrystalline phases, gels, in cement pastes. We have used a multi r-range analysis approach, where the ~4-7 nm r-range allows determining the crystalline phase contents; the ~1-2.5 nm r-range is used to characterize the atomic ordering in the nanocrystalline component; and the ~0.2-1.0 nm r-range gives insights about additional amorphous components. Specifically, we have prepared four alite pastes with variable water contents, and the analyses showed that a defective tobermorite, Ca11Si9O28(OH)2. 8.5H2O, gave the best fit. Furthermore, the PDF analyses suggest that the calcium silicate hydrate gel is composed of this tobermorite and amorphous calcium hydroxide. Finally, this approach has been used to study alternative cements. The hydration of monocalcium aluminate and ye'elimite pastes yield aluminum hydroxide gels. PDF analyses show that these gels are constituted of nanocrystalline gibbsite, and the particle size can be as small as 2.5 nm.
... 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. ...
Article
Full-text available
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.
... To achieve this, calcium alumina cement (CAC) is used to supplement ordinary Portland cement (OPC).The major mineral phase in CAC is monocalcium aluminate (CA), which is very reactive when it gets in contact with water. The minor phases of CAC, which are less reactive, include CA2 and C12A7 [2,3].The enhanced early ettringite formation leads to an accelerated hydration in OPC/CAC mixtures [2,[4][5][6]. ...
Article
Full-text available
In this work, the effect of vinyl acetate-ethylene (VAE) copolymer on ettringite formation and behavior at early age in OPC/calcium aluminate cement/hemihydrate gypsum binder system (BS) was investigated. During the study the VAE dosage was 3 % by weight of the binder. The phase composition of the hydrated BS in air after 6 hours, 1, 3 and 7 days of hardening was investigated by XRD. The microstructure of the hydrated and hardened BS pastes were studied by scanning electron microscopy (SEM). Results from the study indicate that VAE speeds up the hydration of hemihydrate gypsum and slows down the ettringite formation. It was also observed that after 7 days of hardening, the presence of VAE in the BS appears to stabilize the ettringite. There is a change in the appearance of ettringite crystals in VAE-modified BS pastes, i.e. prismatic crystals turn into needle-like crystals, while the average crystals size decreases.
... For a few of C 2 S grains high amounts of Mg are incorporated, leading to an average composition Ca 24at% , Si 14at% and Mg 3at% , matching with that of the bredigite type mineral Ca 7 Mg(SiO 4 ) 4 [21,22]. ...
Article
The chemical and mineralogical properties of cement residues and hydration products have been assessed in historical Portland cement mortars. Results indicate a long time of burning and residence and a slow cooling rate. C2AS, CS and under burned raw material residues suggest inhomogeneous heat distribution typical for shaft kilns. Additionally, minor amounts of $-C2S, C5S2$, C4A3$ and CaS refer to the burning of a pyrite-bearing marlstone and a brown coal used as a solid fuel. Monosulfoaluminate intermixed with monocarboaluminate predominantly concentrate in the inner hydration products confirming the absence of interground calcium sulfate as a retarding agent. The distribution of AFm phases suggests the dissolution of sulfur from the sulfur bearing cement residues and subsequent carbonation in the later stage of hydration. The coarse grain size of the cement and the presumable application of a historical deactivation method resulted in the formation of high microporosity.
... High alumina cement (HAC), mainly consisting of aluminum oxide (Al 2 O 3 ) from about 50 to 85% in cement clinker, could be often used for a special application, due to both high resistance to aggressive chemical ions (i.e., sewer concrete) and rapid development of strength within 24 hours [1][2][3][4]. However, its use in concrete structures has been severely restricted by the loss of strength in the process of conversion; metastable hydrates (CaO⋅Al 2 O 3 ⋅10H 2 O, CAH 10 ; 2CaO⋅Al 2 O 3 ⋅8H 2 O, C 2 AH 8 ) are transformed to stable ones (3CaO⋅Al 2 O 3 ⋅6H 2 O, C 3 AH 6 ) as follows [5][6][7][8]: ...
Article
Full-text available
Chloride transport in different types of high alumina cement (HAC) mortar was investigated in this study. Three HAC cement types were used, ranging from 52.0 to 81.1% of aluminum oxides in clinker. For the development of the strength, the setting time of fresh mortar was measured immediately after mixing and the mortar compressive strength was cured in a wet chamber at 25 ± 2°C and then measured at 1–91 days. Simultaneously, to assess the rate of chloride transport in terms of diffusivity, the chloride profile was performed by an exposure test in this study, which was supported by further experimentation including an examination of the pore structure, chloride binding, and chemical composition (X-ray diffraction) analysis. As a result, it was found that an increase in the Al 2 O 3 content in the HAC clinker resulted in an increase in the diffusion coefficient and concentration of surface chloride due to increased binding of chloride. However, types of HAC did not affect the pore distribution in the cement matrix, except for macro pores.
... Identical features were reported by [6,7] on a historical PC (Aspdin´s cement) from 1850, burned in a shaft kiln. Fig. 2d shows a SEM image of the ADM-1 cement grains where di-calcium silicates is present as two different polymorphisms: C 2 S and Mg-rich C 2 S (with an average composition Ca 24at% , Si 14at% and Mg 3at% ). 8 Expressed using the cement chemistry notation : C=CaO, S= SiO2, A= Al2O3, F= Fe2O3, C=CO3, $= SO3, The composition of this magnesium-calcium-silicate could match with that of the Bredigite type mineral Ca 7 Mg(SiO 4 ) 4 [10,11]. The two C 2 S polymorphisms are randomly distributed within a complex solid solution (Ca 15at% , Si 11at% , Al 9at% , Fe 3at% , Mg 2at% , Na 1at% ). ...
Conference Paper
Full-text available
Early age Portland cements (EAPC) belong to the family of hydraulic binders developed from the middle of the 19 th Century until the wide dissemination of rotary kilns and ball mills in the Portland cement production (around the 1910s). The article presents the applications of EAPC mortars used for joint, cast elements and wall-cover, in Switzerland (Aventicum Roman Theatre) and Austria (Collegiate church in Admont), covering a wide period of practice (1866-1900) in restoration and reconstruction projects. The good state of the samples suggest that this type of cement seemed particularly adapted to formulate both durable construction elements (Admont) and restoration mortars (Aventicum)
... The different investigations in the system Ca 1-x Sr x Al 2 O 4 are based on the importance of these phases as cement phases and also as long term phosphorescence materials (when doped with foreign elements) Carlson, E.T. (1955), Braniski, A. (1957Braniski, A. ( ,1965Braniski, A. ( ,1967, Knopp, R.(1962), Witzmann, H. et al. (1963), Rodehorst et al (2003), Ye et al. (2006), Saines & Kennedy (2006), Chatterjee, A.K. (2009), Huang, P. (2009), Fu et al. (2010, Hwang, K. et al. (2011), Mohr (2011), Pöllmann, H. (2012. The components of the system and their structures are described by several authors (table 1). ...
Article
Alkaline earth aluminates are important phases in the field of high temperature refractory cement phases. Based on Calcium Aluminate Cements, the replacement of calcium by strontium and barium leads to cements which can be used at elevated temperatures in refractory applications. Alkaline earth aluminates also can be used in the field of phosphorescence materials when doped with rare earth elements (REE) like Dy, Eu or La. In addition to that barium aluminate cements are also suitable in the field of radiation protection materials. The replacement of calcium by strontium was already investigated, showing a complete miscibility with 3 fields of different crystal structural modifications within the Ca1-xSrxAl2O4 solid solution series. In the present work different compositions of Ca1-xBaxAl2O4 were synthesized via the Pechini method. The miscibility between the endmembers CaAl2O4 and BaAl2O4 was found to be incomplete. A Ca-rich solid solution of the monoclinic CaAl2O4 – structure type exists up to a Ba incorporation of Ca5/6Ba1/6Al2O4 and a Ba-rich solid solution of the hexagonal BaAl2O4 – structure type up to a Ca incorporation of Ba2/3Ca1/3Al2O4. Between both solid solution limits the phases Ca5/6Ba1/6Al2O4 and Ca1/3Ba2/3Al2O4 coexist within the miscibility gap.
... Mullite is also traced throughout in both samples, which is a promising coating material due to its high thermal resistance and low thermal conductivity (Arizmendi-Morquecho et al., 2012). Mullite is a stable phase, which occurs in refractory materials (Pollmann, 2012). Mullite crystals were also detected in the ESEM of the 10·00 15·00 20·00 25·00 30·00 35·00 40·00 45·00 50·00 55·00 60·00 65·00 70·00 75·00 80·00 85·00 90·00 2θ Figure 11 (point 4). ...
... The different investigations in the system Ca 1-x Sr x Al 2 O 4 are based on the importance of these phases as cement phases and also as long term phosphorescence materials (when doped with foreign elements) Carlson, E.T. (1955), Braniski, A. (1957Braniski, A. ( ,1965Braniski, A. ( ,1967, Knopp, R.(1962), Witzmann, H. et al. (1963), Rodehorst et al (2003), Ye et al. (2006), Saines & Kennedy (2006), Chatterjee, A.K. (2009), Huang, P. (2009), Fu et al. (2010, Hwang, K. et al. (2011), Mohr (2011), Pöllmann, H. (2012. The components of the system and their structures are described by several authors (table 1). ...
Article
Calcium monoaluminate is the main phase in calcium aluminate cements and participates in the hydration, forming calcium aluminate hydrates. The amount of incorporation of foreign ions influences the hydration behaviour. Strontium aluminate is an important phase in producing phosphorescent materials when doped with REE as like Eu, Dy, La. These monoaluminates occur in different modifications. Monocalcium aluminate forms a monoclinic and an orthorhombic modification, whereas monostrontium aluminate forms a monoclinic low temperature and a hexagonal high temperature form. Monoclinic monocalcium aluminate and monoclinic strontium aluminate form a partial solid solution series. Also the hydration behaviour of the different solid solutions was investigated using calorimetry. The newly formed strontium aluminate hydrates could be identified and similar strontium aluminate hydrates are formed. Solid solutions of strontium-and calcium-aluminate hydrates will be investigated.
... Katoite is included in a group of minerals known as hydrogarnet (Ca 3 (Al,Fe) 2 (SiO 4 ) y (OH) 4(3 À y) ; 0 < y < 3), where the SiO 4 4À tetrahedra are partially or completely replaced by OH À . The Al-containing hydrogarnet includes the hydrogrossular solid solution (Ca 3 Al 2 (SiO 4 ) y (OH) 4(3 À y) , 0 < y < 3), being the katoite the Si-free end-member (C 3 AH 6 in cement notation 2 ) [36,39]. Thenardite is highly soluble in water under room conditions, so gentle rinsing with pure water led to the separation of the thenardite and katoite ("PGA washed" in Fig. 2; Ca 3 Al 2 (OH) 12 ) within a purity of 95% (XRF, see Table S2, Supplementary material). ...
... Mullite is also traced throughout in both samples, which is a promising coating material due to its high thermal resistance and low thermal conductivity (Arizmendi-Morquecho et al., 2012). Mullite is a stable phase, which occurs in refractory materials (Pollmann, 2012). Mullite crystals were also detected in the ESEM of the 10·00 15·00 20·00 25·00 30·00 35·00 40·00 45·00 50·00 55·00 60·00 65·00 70·00 75·00 80·00 85·00 90·00 2θ Figure 11 (point 4). ...
Article
Elevated temperatures, specifically in the event of fire, are likely to cause extreme deterioration in fibre-reinforced polymer (FRP) strengthened reinforced concrete (RC) structures. Various types of high-performance cementitious composites (HPCC) have been explored for the protection of RC structural members against elevated temperature, but there is inadequate information in this regard for ultra-high performance fibre-reinforced cementitious composites (UHPFRCC) containing high-alumina cement (HAC) and ground granulated blast furnace slag (GGBS) in conjunction with hybrid fibres - a prospective fire-resistant UHPFRCC for structural members. In this study, the change in mechanical strength of UHPFRCC was examined before and after heat treatment, followed by thermal and microstructural analysis. Besides the control sample, three other samples containing up to 1.5% of basalt fibres, and 1 kg/m3 of polypropylene fibres, were prepared and tested. Another mix was also prepared with only 1 kg/m3 of polypropylene fibres. Each sample was heated to 400, 700 and 10008C. Results showed that the use of hybrid fibres significantly improved the room temperature mechanical strengths of UHPFRCC, which were found to be 80 MPa and 14.3 MPa, respectively. However, the optimum residual compressive and flexural strength was attained by UHPFRCC with only PP fibres and hybrid fibres, respectively. ICE Publishing: All rights reserved.
... Hydration reactions in Ciment Fondu lead to formation of AH 3 (gibbsite), C 2 AH 8 (dicalcium aluminate octohydrate) and CAH 10 (calcium aluminate decahydrate) compounds. Later, during the conversion reaction, CAH 10 and C 2 AH 8 re-dissolve and C 3 AH 6 (katoite) was formed (Taylor, 1997;Pöllmann, 2012). The main mineral phases of calcium-aluminate concrete are katoite, gibbsite, and portlandite. ...
Article
Katoite-bearing rock was revealed in the Hatrurim Basin, Israel. The rock formed by hydration of pyrometamorphic calc-silicate assemblages at temperatures higher than 200 °C. The likely age of the hydration process is Miocene, about 6.2 Ma. The main phases of katoite-bearing rock are minerals of the katoite-grossular series, calcium hydrosilicates, fluorapatite and carbonate. Chemistry and mineralogy of katoite-bearing rock resembles belite sulfoaluminate concrete. Studied samples show two sets of porosities, which are low and high, between 22 and 28% and between 29 and 39%, respectively. Increased porosity characterizes non-weathered katoite-bearing samples, whereas decreased porosity is linked to carbonation during weathering. Katoite remained stable in spite of natural alkaline leaching. Although high porosity samples display decreasing strength parameters, most observed rock mechanic characteristics are comparable to modern-day concrete. We suggest that obtained data on natural katoite-bearing rock can simulate the longevity and durability of belite sulfoaluminate concrete.
... 1 In the refractory industry, calcium aluminate cement (CAC) has established itself as the admixture of reference. [2][3][4][5][6][7] Compared to ordinary Portland Cement (OPC), CAC has higher Al 2 O 3 percentages (about 40%-80% Al 2 O 3 ) and, thus, lower proportions of CaO (<40%) and SiO 2 (<5%). With an annual production of more than four million tonnes, CAC is the base of a broad portfolio of multipurpose cements currently available for all kinds of applications in the refractory sector, be it linings or premanufactured parts. ...
Article
Full-text available
This study describes an analytical approach to identify and quantify organic admixtures, which are increasingly being used in multifunctional additive systems, such as calcium aluminate‐cement based dry mix formulations. The investigations refer to a wide range of established organic admixtures used in high performance industrial concrete applications, such as flow agents, retarders or organic plasticisers, which are added in dosages of up to 5% (to the mass of cement). Based on specific considerations, a consistent approach was outlined by combining the output of complementary analytical techniques. These analytical methods included Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC) coupled with Mass Spectrometry (MS), in combination with Fourier Transform Infrared Spectroscopy (FTIR) and Morphologically Directed Raman Spectroscopy (MDRS), as well as Total Carbon Analysis (TC). Within the scope of the aforementioned studies the applied methods were validated for stringent detection limits (LOD <0.05%) and precision demands (SD 1% to 7% with R²> 0.99). This article is protected by copyright. All rights reserved.
... A compact microstructure, consisting of large C 2 AH 8 platelets [15] amorphous AH 3 gels and gibbsite, is seen for the paste containing the as-delivered cement (Fig. 6a). In comparison, a more porous microstructure, composing with granular C 3 AH 6 grains [16], amorphous AH 3 gels and gibbsite, is observed in the paste prepared with 2 h milled cement (Fig. 6b). These observations confirm again that the hydration products at 30°C was changed by intensive milling. ...
Article
Calcium aluminate cement (CAC) was ground for 1 and 2 h to investigate the impact of mechanical grinding on CAC hydration at 30 °C and CAC-bonded castable strength. Phase composition and microstructure of unground and ground cements after hydration for predetermined times and terminated by the freeze-vacuum drying were compared. The results indicate that the particle size and particle size distribution of CAC were reduced and narrowed, respectively by grinding, thereby favoring the hydration rate and the conversation of C 2 AH 8 to C 3 AH 6 . Then enhanced cement hydration also increases the strengths of castables bonded with milled CAC after drying and firing.
... This study covers the following issues: a nagelschmidtite formula according to [20]; b symmetry of Ca 4 Al 6 O 12 (SO 4 ) polymorphic modification according to single-crystal determination. CAS notation for sharyginite according to [34], all other notations according to [35,36]. New data of this study (Rietveld analysis of powder XRD, SEM) and published evidence [4,25-27,32]. ...
Article
Full-text available
This is the first attempt to provide a general mineralogical and geochemical survey of natural Ca2SiO4-bearing combustion metamorphic (CM) rocks produced by annealing and decarbonation of bioproductive Maastrichtian oil shales in the Hatrurim Basin (Negev Desert, Israel). We present a synthesis of data collected for fifteen years on thirty nine minerals existing as fairly large grains suitable for analytical examination. The Hatrurim Ca2SiO4-bearing CM rocks, which are natural analogs of industrial cement clinker, have been studied comprehensively, with a focus on several key issues: major- and trace-element compositions of the rocks and their sedimentary precursors; mineral chemistry of rock-forming phases; accessory mineralogy; incorporation of heavy metals and other trace elements into different phases of clinker-like natural assemblages; role of trace elements in stabilization/destabilization of Ca2SiO4 polymorphic modifications; mineralogical diversity of Ca2SiO4-bearing CM rocks and trace element partitioning during high-temperature–low-pressure anhydrous sintering. The reported results have implications for mineral formation and element partitioning during high-temperature–low-pressure combustion metamorphism of trace element-loaded bituminous marine chalky sediments (“oil shales”) as well as for the joint effect of multiple elements on the properties and hydration behavior of crystalline phases in industrial cement clinkers.
... C 2 AH x peak area reaches a plateau or decreases with the onset of monocarbonate formation. In the study monocarbonate is a not further discussed minor phase, might derive from the Li 2 CO 3 accelerator that is used or from atmospheric CO 2 [24]. At 40°C the order of first appearance of hydrates is: C 2 AH xmonocarbonate -C 3 AH 6 . ...
Article
In the present work the hydration of white CAC, inert alumina-filler and CaCO3 was investigated at different temperatures with in-situ XRD. Experiments were performed at 10 °C, 23 °C, 40 °C and 60 °C. Overcoming or mitigating the phenomena of calcium aluminate hydrate conversion by adding reactive additives to calcium aluminate cement is often described in literature. When a sufficient amount of calcite is added, carbonate-AFm (monocarbonate and hemicarbonate) is expected to be the dominant hydrate phase. Calcite however, acts as an inert filler and barely influences the calcium aluminate hydration products at 10 °C. With rising temperature monocarbonate contents increase as well as the reaction rate of cement phases. At 60 °C minor amounts of C3AH6 precipitate during the early hydration. Special attention is given to the role of CAH10 and C2AHx during hydration. While CAH10 is dominant at 10 °C, C2AHx precipitates in a temperature range between 23 and 60 °C as a precursor phase for carbonate-AFm phases. These are not found at 10 °C, which is probably due to the lack of the precursor phase (C2AHx). The hydration of the mix at 10 °C with an initial addition of C2AHx shows that carbonate-AFm-phases can be precipitated in addition to CAH10 during the first 96 h.
... Different types of hydration products, such as CaOÁAl 2 O 3 Á10H 2 O (CAH 10 ), 2CaOÁAl 2 O 3 Á8H 2 O (C 2 AH 8 ), 3CaOÁAl 2 -O 3 Á6H 2 O (C 3 AH 6 ) and Al 2 O 3 Á3H 2 O (AH 3 ), are generated depending on environmental temperature during the curing process of castables [7][8][9][10]. At lower temperatures (below 15°C), the hydration product is commonly fine prism-shaped CAH 10 ; when the curing temperature raises to around 25°C, the hydration products are mostly plate-shaped C 2 AH 8 and gibbsite (AH 3 ); and when the curing temperature is above 40°C, the hydration products are predominantly granular-shaped C 3 AH 6 and AH 3 [11,12]. Moreover, the metastable hydration products CAH 10 and C 2 AH 8 would convert to C 3 AH 6 and AH 3 with the increase of curing time and temperature [3,[13][14][15]. ...
Article
Calcium aluminate cement (CAC) is extensively applied as castable binder, and the drying process is a crucial step before castables come into service. Limited researches have focused on the hydration and phase transformation of CAC in the castables during the evaporation stage of drying process, in which the free water is gradually discharged. This work involves the investigation of the strength development of CAC-bonded alumina castables and phase evolution of the hydrates during drying at 110 °C. After curing for 24 h at 30 °C, the CAC-bonded alumina castables and matrices were dried at 110 °C for different periods of time to explore the effect of drying time on the mechanical property of CAC-bonded castables and on the phase assemblages and microstructure of the castable matrices. The results demonstrate that the initial drying period prompts the rapid conversion from metastable C2AH8 to stable C3AH6 as well as the continued hydration of the residual CA and CA2. The drying strength of the castables is significantly enhanced in comparison with demolding strength.
... First detailed investigations on the thermal behavior and microstructure evolution of CCC are subject of a recently submitted publication [13]. The CCC material is processed at temperatures of about 60 °C, forming stable hydrate phases via cement hydration reactions [14]. Comparable to PCbased systems, the CAC material sets at higher pH-values and needs to be adapted with respect to potential corrosion effects with metallization and packaging contacts. ...
Article
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.
Article
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.
Article
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.
Article
Welan gum has been widely used in oil cement and grouting materials for its excellent rheological properties and anti-bleeding, and most of all, being friendly to the environment. However, when welan gum was added, the fluidity of mortar decreased sharply, so it should be used together with a superplasticizer to enable good workability. With its powerful charge density in the molecular structure, the competitive adsorption between welan gum and other admixtures happened remarkably during the addition process. Consequently, we experimentally studied on the bleeding rate and rheological properties of cement slurry, fluidity and mechanical properties of mortar with welan gum mixed with superplasticizer, aiming at understanding the competitive adsorption phenomenon by application of welan gum mixed with superplasticizer. By measuring the hydration heat and zeta potential, the mechanism of interaction of welan gum with superplasticizer was deduced and explained. The results showed that it could ensure a good dispersion effect when welan gum is mixed with the two kinds of superplasticizer. Welan gum had little impact on the naphthalene superplasticizer, but did have a substantial influence on polycarboxylate. In practice, adding welan gum after PCE acted with cement for 2 min could effectively avoid the competitive adsorption and then achieve better performance. On this viewpoint for mortar with PCE, new delay release welan gum needs further research and development.
Article
Al2O3-CaO-Cr2O3 castables are required for various furnaces linings due to their excellent corrosion resistance. However, toxic and water-soluble Cr(VI) could be generated in these linings during service. In this study Al2O3-CaO-Cr2O3 castables were prepared and heated at 300-1500 oC in air and coke bed to simulate actual service conditions. The formations of various phases were investigated by XRD and SEM-EDS. The Cr(VI) compounds CaCrO4 and Ca4Al6CrO16 formed in air at 300-900 oC and 900-1300 oC respectively, while C12A7 and CA2 were generated rather than forming Cr(VI) compounds in coke bed at 700-1300 oC. However, at 1500 oC, nearly all the chromium existed in the form of (Al1-xCrx)2O3 solid solution in both atmosphere. As a result, the specimens treated in air contained 185.0-1697.8 mg/kg of Cr(VI) at 500-1300 oC but only 17.2 mg/kg of Cr(VI) at 1500 oC, whereas specimens treated in coke bed exhibited extremely low Cr(VI) concentration in the whole temperature range studied. Moreover, in coke bed, the mutual diffusion between Cr2O3 and Al2O3 was suppressed and a trace of Cr2O3 would even be reduced to form chromium-containing carbides on its surface, which would hindered the sintering process and hence lower the density as well as strength of the castables.
Article
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.
Thesis
Full-text available
Rapid-set concretes are currently used to repair structures such as bridge decks, substructure elements on bridges (e.g. piers and columns), pavements, and components of buildings. As the name implies, rapid-set concrete results in a high strength in a short period of time (e.g. ≥ 20 MPa in 3 hours) in order to minimize construction times and disruption to the travelling public. Although they are capable of reaching a high-early strength, they are not only meant to be a short-term solution but to survive the life of the structure. Unlike ordinary Portland cement (PC) based systems, where ettringite (C3A·3C$·H32) is a minor constituent, the hydrates in rapid-set concrete systems are primarily composed of ettringite, which forms within the first few hours of hydration. The formation of ettringite is achieved through the use of cementitious systems that are rich in calcium and aluminum bearing oxides, which include binders that contain calcium aluminate cement (CAC) or calcium sulfoaluminate cement (C$A), together with calcium sulfate (C$). High-early strength Portland cement (HEPC) with high doses of set accelerator and calcium sulfoaluminate cements (C$A) are currently being used in rapid-repair products, however, there exists limited information concerning their long-term durability in aggressive conditions often encountered in service. A new rapid setting concrete composed of a ternary blend (PC-CAC-C$) of Portland cement (PC), calcium aluminate cement (CAC), and calcium sulfate (C$) has been developed and is expected to achieve both mechanical and durability characteristics similar to that of concrete produced with either HEPC or C$A cement. This dissertation presents mechanical and durability performance data, from both laboratory and field studies, of ettringite systems based on the new ternary cement (PC- CAC-C$) and two commercially-available cements that utilize calcium-sulfo-aluminate (C$A) cement. As a comparison, the performance was compared to that of portland cement based systems including high-early strength portland cement. The effect of carbonation on both the mechanical and durability properties of the various systems was also studied to determine the stability of ettringite under various environments. The use of X-ray diffraction (XRD) and scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS) was used to investigate the microstructural properties. In addition to studying the durability performance of these systems, modifications were made to an existing chloride-penetration model to allow for the impact of carbonation on chloride ingress to be assessed. This included making adjustments for the changes in the physical (pore-structure) and chemical (chloride binding) resistance to chlorides.
Article
Solubility experiments with binary series of OH-, Cl-, SO4- and CO3-AFm were re-examined with a non-ideal, multicomponent solid solution model. The electrical double layer on the mineral's outer surface, which neutralizes about 5% of the anionic charge, can be included. The choice of the end-members in the solid solution and the reaction formulation can help to eliminate the ‘non-ideality’ for OH-SO4 and Cl-OH exchange. CO3-Cl-OH exchange is not ideal in the solubility experiments, nor can it be, because the mineral volume is not a linear combination of the end-members. The solubilities of the end-members are related to the anion charge and inversely to the anion-layer thickness. OH is part of the anion-charge, even when a pure, other anion form is the target of the synthesis. Pure OH-AFm is unstable and converts into hydrogarnet and portlandite, depending on the precipitation rate of hydrogarnet.
Article
Full-text available
The effect of NaCl on the conversion of hydration products of calcium aluminate cement(CAC) was determined by means of MIP, XRD, DSC-TG and SEM. Results show that the addition of NaCl promotes the formation of CAH 10 at early hydration stage. The total porosity and proportion of harmful pore in hardened CAC paste with NaCl are lower and the 7d-strength is 40% higher than that of the control. The later curing at 40°C accelerates the conversion from metastable C 2 AH 8 , CAH 10 to C 3 AH 6 , and the strength decreases with the increasing of curing age. After 90 days, there is no metastable hydrate detected in CAC paste. However, the addition of Niacin CAC paste can prominently delay the conversion of C-A-H. Massive CAH 10 still exists in the paste with NaCl, resulting in a slower retraction of late strength.
Article
Full-text available
Calcium aluminate cement (CAC) is gaining popularity in North America as a rapid repair material due to its ability to quickly gain strength, even at low curing temperatures. Use of CAC has been limited due to a lack of understanding of the process of conversion and the role of aggregates in CAC concrete. Conversion, which occurs only in 100% CAC systems, is a chemical process in which metastable hydrates convert into denser, stable hydrates. Presented is an examination of aggregate source impacts on this conversion process and converted CAC concrete strengths. Nine different concrete systems with fifteen varying aggregate sources were examined. Results indicated that carbonate limestone and siliceous limestone aggregate systems had significantly less strength reduction due to conversion compared to siliceous aggregate systems. Microstructural analysis of systems suggested that the carbonate limestone system had less porosity and better-formed aggregate/paste interfacial transition zones compared to the siliceous systems. Chemical analysis of the concrete pore solution indicated that the carbonate limestone system's pH and ionic concentrations of aluminum, sodium, and potassium were significantly higher than that of a siliceous system, indicating more dissolution of unhydrated cement in the carbonate limestone system. These studies are presented along with a proposed theory explaining the cause of the significant converted strength differences in CAC concrete systems made with limestone aggregates compared to siliceous aggregates.
Article
Future power electronic devices with high power density and reduced chip area need robust encapsulation materials with increased thermal conductivity and temperature stability. Especially for new SiC and GaN semiconductor technology, operating temperatures above 200 °C can increase the potential of future power electronics. This paper introduces a novel uniquely processed cement-ceramic composite material for electronic packaging, containing a calcium aluminate cement (CAC) matrix and high amounts of alumina fillers. In-situ µ-XRD 2-DTA, where X-ray diffraction patterns and DTA traces can be monitored simultaneously, was applied to investigate the dehydration behavior of this composite between room temperature and 300 °C for the first time. The results reveal the high potential of using in-situ µ-XRD 2-DTA for measurements on cement-ceramic composites. The observed microstructure evolution shows micro-crack formation and increased porosity above 210 °C, which can be correlated with the dehydration of gibbsite and katoite. At this stage, the novel cement-ceramic encapsulation material already enables miniaturized passive electronic components. Further consequences for electronic packaging of semiconductors and correlations to state-of-the-art encapsulation materials are discussed.
Chapter
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.
Article
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.
Article
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.
Article
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.
Article
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.
Article
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.
Article
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.
Article
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.
Article
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.
Article
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.
Article
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.
Article
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.
Article
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
Article
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.
Article
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.
Article
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.
Article
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.
Article
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.
Article
Pure dicalcium silicate (C2S) and tricalcium silicate (C 3S) were synthesized by solid state reaction, sol-gel synthesis, Pechini process and combustion synthesis. The investigation of the crystallization process of C2S showed, that by low temperature synthesis methods α′L-C2S crystallizes at 650°C and can be stabilized at room temperature. The hydraulic reactivity of these low sintered α′L-C2S samples is similar to a C3S and α′L-C2S is completely hydrated after 200 hours. C2S sintered at higher temperatures is, despite of low particle sizes of about 100 nm, not very reactive. The investigation of the crystallization behaviour of C3S has shown, that by all synthesis methods, sintering temperatures of 1500°C are necessary to get pure C3S. But despite the same heating and cooling condition different modifications were stabilized. The hydration reaction shows, that the degree of hydration by C3S produced with the low temperature synthesis methods is higher and that different morphologies of CSH phases occur.