Hydration of 3CaO·Al2O3 and 3CaO·Al2O3+] Gypsum With and Without CaCl2

Paste samples of tricalcium aluminate alone, with CaCl₂, with gypsum, and with gypsum and CaCl₂ were hydrated for up to 6 months and the hydration products characterized by SEM, XRD, and DTA. Tricalcium aluminate hydrated initially to a hexagonal hydroaluminate phase which then changed to the cubic form; the transformation rate depended on the size and shape of the sample and on temperature. The addition of CaCl₂ to tricalcium aluminate resulted in the formation of 3CaO · Al₂O₃· CaCl₂·10H₂O and 4CaO · Al₂O₃· 13H₂O, or a solid solution of the two. The chloride retarded the formation of the cubic phase 3CaO · Al₂O₃· 6H₂O; the addition of gypsum resulted in the formation of monosulfoaluminate with a minor amount of ettringite. When chloride was added to tricalcium aluminate and gypsum, more ettringite was formed, although 3CaO · Al₂O₃· CaSO₄· 12H₂O and 3CaO · Al₂O₃· CaCl₂· 10H₂O were the main hydration products.     

Hydration of Beta Dicalcium Silicate Alone and in the Presence of CaCl2 or C2H5OH

Beta C₂S was hydrated at room temperature with and without added CaCl₂ or C₂H₅OH by methods previously studied for the hydration of C₃S, i.e. paste, bottle, and ball-mill hydration. The amount of reacted β-C₂S, the Ca(OH)₂ concentration in the liquid phase, the CaO/SiO₂ molar ratio, and the specific surface area of the hydrate were investigated. A topochemical reaction occurs between water and β-C₂S, resulting in the appearance of solid Ca(OH)₂ and a hydrated silicate with a CaO/SiO₂ molar ratio of ≃1. As the liquid phase becomes richer in Ca(OH)₂, the first hydrate transforms to one with a higher CaO/SiO₂ ratio. Addition of CaCl₂ increases the reaction rate and the surface area of the hydrate but to a much lesser extent than in the hydration of C₃S, whereas C₂H₆OH strongly depresses the hydration rate of β-C₂S, as observed for C₃S hydration.     

Effect of Gypsum on the Hydration and Strength of C3S Pastes

The paste hydration of C₃S containing different amounts of gypsum was studied. Reaction rates and the composition of hydrated products were evaluated. The gypsum influenced the quantity and quality of the hydrated products by accelerating the hydration process and, at the same time, lowering the intrinsic strength of the gel. Variations in intrinsic strength correlated linearly with the C/S ratio of the CSH gel. The effect of gypsum on strength was interpreted in terms of its influence on the extent of hydration and on the chemical composition of the gel.     

Phase Equilibria and Structural Species in NdCl3–NaCl, NdCl3–CaCl2, PrCl3–NaCl, and PrCl3–CaCl2 Systems

Equilibrium phase diagrams for the systems NdCl₃–CaCl₂ and NdCl₃–NaCl were determined by differential thermal analysis. A simple eutectic was observed at 59 ± 1 mol% CaCl₂ and 600°± 2°C in the NdCl₃–CaCl₂ system. A compound NaCl.3NdCl₃ which melts incongruently at 545°± 5°C to NdCl₃ and a liquid containing approximately 47 mol% NaCl, and a eutectic at 68 mol% NaCl and 439°± 2°C were found in the NdCl₃–NaCl system. On the basis of agreements between the activities calculated by the Clausius–Clapeyron equation and Temkin's model using the present data for the NdCl₃–CaCl₂ system and the literature data for the PrCl₃–CaCl₂ system, the melts in the former system consist of Nd³⁺, Ca²⁺, and Cl⁻ ions and in the latter system of Pr³⁺, Ca²⁺, and Cl⁻ ions. The above approach indicates the presence of Na⁺, Cl⁻, and NdCl^2-₅ ions in the NaCl-rich melts and Nd³⁺, Cl⁻, and NdCl⁻₄ in the NdCl₃-rich melts in the NdCl₃–NaCl system. Analogous ions were indicated in the melts of the PrCl₃–NaCl system.     

Coarsening Behavior of Tricalcium Silicate (C3S) and Dicalcium Silicate (C2S) Grains Dispersed in a Clinker Melt

Microstructural evolution during the heat treatment of cement clinker was investigated. Two model specimens, which consisted of faceted tricalcium silicate (C₃S) and spherical dicalcium silicate (C₂S) grains dispersed in a liquid matrix, were prepared with 5 wt% of large seed particles. The seed particles of faceted C₃S grains grew extensively, whereas those of the spherical C₂S grains grew rather slowly, relative to the matrix grains. As a consequence, C₃S grains exhibited a bimodal size distribution that was typical of exaggerated grain growth, whereas C₂S grains retained a uniform and normal size distribution. These results suggest that the growth of faceted C₃S grains was controlled by the interface atomic attachment, such as two-dimensional nucleation, and that of spherical C₂S grains was controlled by diffusion through the liquid matrix. The dependence of growth mechanisms on grain morphology has been explained in terms of the atomistic structure of the solid/liquid interface.     

Phase Diagrams for the Systems MgCl2-MgF2, CaCl2-MgF2, and NaCl-MgF2

Equilibrium phase diagrams for the systems MgCl₂-MgF₂, CaCl₂-MgF₂ and NaCl-MgF₂ were determined by differential thermal analysis, thermal analysis, and temperature-composition equilibrium techniques. Simple eutectics were observed at 78.0±0.5 mol% MgCl₂ and 628°±2°C in the MgCl₂-MgF₂ system, at 87.5±0.5 mol% CaCl₂ and 694°±2°C in the CaCl₂-MgF₂ system, and at 95.5±0.5 mol% NaCl and 786°±3°C in the NaCl-MgF₂ system. The phase diagrams determined for these systems were compared with phase diagrams that were computed using Temkin's model. The phase diagrams of the CaCl₂-MgF₂ and NaCl-MgF₂ systems were also compared with diagrams that were computed using the expression suggested by Flood et al. for reciprocal systems. The experimentally determined and computed phase diagrams agreed for the MgCl₂-MgF₂ system but not for the CaCl₂-MgF₂ and NaCl-MgF₂ systems.     

Progressive Changes in the Structure of Hardened C3S Cement Pastes due to Carbonation

The structures of partially carbonated hardened C₃S cement pastes have been investigated by a combination of ²⁹Si magic angle spinning nuclear magnetic resonance spectroscopy and analytical transmission electron microscopy, supported by X-ray diffraction and thermogravimetric analysis. Progressive changes in structure are reported for thin slices for a paste carbonated in pure CO₂ for times from 1 to 16 h, and the results are compared with those for a paste carbonated for 2 months in air. C-S-H gel of reduced Ca:Si ratio and increased silicate polymerization was formed during the early stages of carbonation. The morphology of the original C-S-H was, in the main, retained. A cross-linked silica-rich gel formed at later times in paste carbonated in CO₂ but not up to the time of 2 months in air. Calcium carbonate took the form of microcrystals of vaterite and calcite which formed dense masses between gel fibrils and around partially reacted CH crystals, possibly accounting for the observed slowing in the rate of reaction of CH with time.     

Grain Growth of ZnO in ZnO-Bl2O3 Ceramics with Ai2O3 Additions

Grain growth of ZnO during liquid-phase sintering of a ZnO-6 wt% Bi₂O₃ ceramic was investigated for A1₂O₃ additions from 0.10 to 0.80 wt%. Sintering in air for 0.5 to 4 h at 900° to 1400°C was studied. The AI₂O₃ reacted with the ZnO to form ZnAl₂O₄ spinel, which reduced the rate of ZnO grain growth. The ZnO grain-growth exponent was determined to be 4 and the activation energy for ZnO grain growth was estimated to be 400 kJ/mol. These values were compared with the activation parameters for ZnO grain growth in other ceramic systems. It was confirmed that the reduced ZnO grain growth was a result of ZnAl₂O₄ spinel particles pinning the ZnO grain boundaries and reducing their mobility, which explained the grain-growth exponent of 4. It was concluded that the 400 kJ/mol activation energy was related to the transport of the ZnAl₂O₄ spinel particles, most probably controlled by the diffusion of O^2- in the ZnAl₂O₄ spinel structure.     

Retention of Cobalt, Cesium, and Strontium in the Hydrates of C3S, C3A, and Gypsum

The retention of cobalt, cesium, and strontium as trace elements in the hydrates of simple ordinary portland cement components has been investigated. The combined characterization of the solid by SEM, electron microprobe, XRD, and SIMS, as well as the chemical analysis of the solutions, allowed us to identify the likely localization of these trace elements in the solid. In particular, cobalt is dramatically incorporated into the solid during the hydration of C₃S; we show that this is due to the formation of a cobalt oxychloride, a compound which is unstable at temperatures ≥60°C. Cesium is retained in small amounts in the C-S-H and CH mixture. The formation of hydrated aluminates notably increases its retention. Finally, small quantities of strontium are also retained in silicates and, in the presence of gypsum, its retention is markedly higher. This is likely due to the formation of sulfoaluminates.     

Microstructural Changes During Heat Treatment of Sintered Al2O3

Samples of both high-purity and Mg-doped Al₂O₃ sintered in H₂, N₂, O₂, or vacuum were annealed at 1650° to 1850°C for times up to 64 h. In pore-free systems, grain growth is limited by the mobility of Mg-rich second-phase inclusions; in samples annealed in H₂, grain growth is limited by pore dragging with a transition toward limitation by solid-inclusion dragging at high dopant levels; in samples annealed in N₂ and O₂, grain growth is characterized by a transition from an "anchoring effect" of the pores toward a combination of pore dragging by and unpinning from the grain boundaries. Time-dependence of grain growth is insufficient to determine the mechanisms and provide an adequate foundation for model-based calculations. Observations of microstructure and its change with time, together with the rate of grain growth as a function of composition, allow elimination of alternate hypotheses and determination of the process which controls the rate of grain growth and change in pore size.     

Anisotropic Grain Growth in α-Al2O3 with SiO2 and TiO2 Additions

Microstructural changes caused by doping α-Al₂O₃ with small amounts of SiO₂ and TiO₂ added singly or together were investigated. When they were sintered at 1450°C for 120 min, singly doped samples developed equiaxed microstructures, but codoped material developed an anisotropic microstructure that contained platelike grains with an average aspect ratio of 3.4. The development of anisotropy thus resulted from a cooperative effect of silicon and titanium. Amorphous material was present at most grain boundaries in the silicon-doped sample. In the codoped sample, only boundaries that exhibited a basal facet were penetrated by amorphous material. Energy dispersive X-ray spectroscopy analysis showed strong titanium enrichment at the edges of platelets. Additional experiments demonstrated that the volume fraction of highly anisotropic platelike grains interspersed with equiaxed grains could be adjusted by using varying amounts of titanium with a constant amount of silicon content. The fracture toughnesses of such materials increased as the structure became more anisotropic.     

Phase Evolution in Heat-Treated Si3N4 with Additions of Yb2O3

The heat treatment of silicon nitride (Si₃N₄) ceramics with additions of 8, 12, and 16 wt% Yb₂O₃ was carried out at different temperatures and the evolution of grain boundary (GB) phase was investigated systematically by X-ray diffraction (XRD) as well as scanning electron and transmission electron microscopic analyses. XRD results reveal that the extent and the ease of GB crystallization increase with increasing the Yb₂O₃ content, and that high heat-treatment temperatures in general favor crystallization of the quaternary compounds such as the Yb₄Si₂O₇N₂ phase. These results provide an insight into the GB phase evolution in the Yb-system Si₃N₄ ceramics subjected to a postsintering heat treatment.     

Influence of CaCO3 on the Hydration of 3CaO·Al2O3

The influence of CaCO₃ on the hydration of 3CaO Al₂O₃ containing about 90% C₃A is studied with compacts of their mixtures proportioned to 0, 2.5, 10, 20, and 50 wt% CaCO₃. The compacts are exposed to H₂O in liquid and in vapor phase, the attendant expansion being measured as a function of time. Differential thermal analysis, infrared absorption, and X–ray diffraction are used in conjunction with this technique. It is shown that the hydration reaction of 3 CaO · Al₂O₃ is suppressed by CaCO₃ additions and that this is due primarily to the formation of the low form of calcium carboaluminate on the surface of the C₃A grains. Expansion measurements of the compacts during the reactions show that this technique indicates the progress of hydration and detects the formation of the carboaluminate at an early stage of the reaction. Using the same technique, a study at 50% relative humidity provides the basis for the conclusion that C₃A reacts with CaCO₃ under these conditions by a direct mechanism.     

Effect of Small Additions of Al2O3 and Ga2O3 on the Immiscibility Temperature of Na2O-SiO2 Glasses

Alumina and gallia were substituted separately for Na₂O in amounts of 0.2, 0.5, 1.0, 1.5, 2.0, and 3.0 wt% in three Na₂O-SiO₂ glass compositions (82, 84, and 86 wt% SiO₂) within the immiscibility region. The immiscibility regions for each system extend to ∼1.5 mol% of the added oxide. In general, the addition reduced the immiscibility temperature ( T _m), but at the edge of the immiscibility region (82% SiO₂) the Na₂O loss effect initially increased T _m. A structural model of the miscibility of Al₂O₃ added to silicate glasses is presented.     

Character of Hydration of 3CaO.Al2O3

The C₃A compacts were hydrated and the reaction was studied by DTA, X-ray diffraction, mercury porosimetry, and volume change analysis. The hexagonal hydroaluminates C₂AH₈ and C₄AH₁₉ formed at 2°, 12°, and 23°C by a direct mechanism between C₃A and H₂O. The hydration reaction at 52° and 80°C was stopped by formation of C₃AH₆ around the C₃A grains. The rate of conversion of the hexagonal hydrates to cubic C₃AH₆ increased with temperature. Volume change analysis confirmed that C₃AH₆ grows epitaxially on the surface of the C₃A grain. The reaction at this surface and the passage of water through the layer of hexagonal hydroaluminates control the overall reaction rate. The conversion of the hexagonal hydrates to C₃AH₆ accelerates the reaction by removing the layer of products from around the C₃A grain by a solution mechanism. At 52° and 80°C, C₃AH₆ may form without the intermediate formation of the hexagonal hydrate.     

Defect Chemistry of BaTiO3 with Additions of CaTiO3

The substitution of up to 5% Ca²⁺ for Ba²⁺ in BaTiO₃ results in a shift in the oxygen pressure dependence of the equilibrium electrical conductivity that is in the same direction as that caused by addition of acceptor impurities such as Al³⁺ or Ca²⁺ substituted for Ti⁴⁺. In contrast to the latter effect, however, the shape of the conductivity plot is not changed, the conductivity value at the conductivity minimum is not affected, and the amount of the shift increases with decreasing temperature of measurement. It is shown that the shift is primarily due to an increase in the enthalpy of reduction and a decrease in the enthalpy of oxidation as increasing amounts of Ba²⁺ are replaced by Ca²⁺.