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.     

Influence of SO3 on the Phase Relationship in the System CaO–SiO2–Al2O3–Fe2O3

The influence of the additive SO₃ on the phase relationships in the quaternary system CaO-SiO₂-Al₂O₃-Fe₂O₃ was investigated by observing the change of volume ratio of 3CaOSiO₂ (C₃S) to 2CaOSiO₂ (C₂S) + CaO (C) in the sintered material with the increase of SO₃ content. The primary phase volume of C₃S in the quaternary phase diagram shrank with the increase of SO₃ and disappeared when the SO₃ content exceeded 2.6 wt% in the sintered material. Changes in the peritectic reaction relationship between CaO (C), 2CaOSiO₂ (C₂S), 3CaOSiO₂ (C₃S), 3CaOAl₂O₃ (C₃A), 4CaOAl₂O₃Fe₂O₃ (C₄AF), and liquid were also observed and discussed.     

Thermal Study of the Effect of Additives on the Hydration of Tricalcium Silicate

The rate of paste hydration of 3C_aO·SiO₂ (C₃S) and the effects of additions of CaCl₂, CdI₂, and CrCl₃, were studied by differential thermal analysis and thermogravimetry. X-ray analyses were used to identify the synthesized C₃S. The salts CaCl₂, CdI₂, and CrCl₃, accelerated the hydration of C₃S. The degree of hydration was estimated by the amount of Ca(OH)₂, formed, as determined by TG.     

Structure Change of Ca2SiO4 Solid Solutions with Ba Concentration

A series of Ba-bearing Ca₂SiO₄ solid solutions (C₂S( ss )), (Ba _x Ca_{1− x} )₂SiO₄ with 0.075 x 0.30, were prepared and examined by X-ray and electron beam diffraction. They are all made up of orthorhombic domains 120° different in orientation around the common c axis of the former α phase. The C₂S solid solution with x = 0.075 shows a superstructure incommensurate along the a axis with λ (modulation wavelength) = 3.5 and commensurate along the c axis with Δ= 3. With x = 0.15, modulation is observed only along the a axis and Δ= 3.4. No evidence of superstructure is found with x = 0.24; the space group and cell dimensions are comparable with those of pure α '_H-C₂S. The C₂S( ss ) with x = 0.30 gives a superlattice with the cell-edge length of 3 b . All the C₂S( ss ), when reheated at 1000°C for 24 h, produced lamellae of the trigonal phase T nearly in parallel with (001) of the host α '_L phase. The crystallographic orientation between the two phases is

This indicates that the above Ba-bearing C₂S( ss ) phases occur as precursors to the thermodynamically more stable two-phase mixtures.     

Characterization of Ca2SiO4:Eu2+ Phosphors Synthesized by Polymeric Precursor Process

The green emitting Ca₂SiO₄:Eu²⁺ (C₂S:Eu) phosphors were synthesized by the polymeric precursor process (Pechini-type), and the effects of calcination temperature and europium (Eu) doping concentration on the luminescent properties were investigated. The crystalline β-C₂S was obtained in the calcination temperature of 1100°–1400°C, and Eu was reduced into Eu²⁺ by annealing in 5% H₂/N₂ atmosphere. The obtained C₂S:Eu²⁺ phosphors exhibited a strong emission at 504 nm under the excitation of λ_exc=350 nm. The highest photoluminescence (PL) intensity was observed in the C₂S:Eu²⁺ phosphors either calcined at 1300°C or doped with 3 mol% Eu. The obtained PL properties were discussed in terms of crystal structure, particle size and shape, surface roughness, and effect of concentration quenching.     

Direct Synthesis and Hydration of Calcium Aluminosulfate (Ca4Al6O16S)

Calcium aluminosulfate (Ca₄Al₆O₁₆S or C₄A₃̄) was prepared by direct synthesis from calcium and aluminum nitrates, and aluminum sulfate. CaAl₄O₇(CA₂) formed as an intermediate at 900°C, and C₄A₃̄ was the main phase after calcination at 1100°C. The specific surface areas after calcination at 1100° and 1300°C were ∼2.5 and 1 m²/g, respectively. Hydration was investigated using XRD, DSC, SEM, conduction calorimetry, and solid-state ²⁷Al MAS-NMR spectroscopy. Calorimetry showed that the induction period was longer than that of a sample prepared using conventional solid-state sintering, and this was attributed to the formation of amorphous coatings. Crystalline hydration products, principally calcium monoaluminosulfate hydrate and aluminum hydroxide, appeared subsequently. Although the induction period was very long, complete hydration occurred as early as 3 d in the sample calcined at 1100°C and was 91% complete in the sample calcined at 1300°C.     

Effect of Phosphorus Impurity on Tricalcium Silicate T1: From Synthesis to Structural Characterization

Alite is the major compound of anhydrous Portland cement: it is composed of tricalcium silicate Ca₃SiO₅ (C₃S) modified in composition and crystal structure by ionic substitutions. Alite is also the main hydraulic phase of cement and the most important for subsequent strength development. Using raw meals (rich in Ca₃P₂O₈) as alternative fuels in cement plants raises the question about the effect of phosphorus on C₃S and its consequences on reactivity with water. This paper deals with a systematic study of C₃S triclinic T₁ polymorph doped with P₂O₅ in the range 0–0.9 wt%. All the samples were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and electron-microprobe analysis. The appearance of a phase rich in phosphorus is shown. It displays a structure derivative of the α'_H–Ca₂SiO₄ polymorph, noted α'_H–C₂S(P). As phosphorus content increases, C₃S is more and more decomposed into free lime and α'_H–C₂S(P). The α'_H phase was detected from 0.1 wt% P₂O₅ and located at the interfaces of C₃S grains. Two identification keys are proposed in order to highlight the α'_H–C₂S(P) phase: the XRD angular window at 2θ_Cu=32.8°–33.2° and a smooth aspect on SEM micrographs.     

Kinetics of the α-to-α'H Polymorphic Phase Transition of Ca2SiO4 Solid Solutions

The α-to-α'_H transition of Ca₂SiO₄ solid solutions (C₂S(ss)) is a nucleation and growth process. This process was shown on time–temperature–transformation (TTT) diagrams for C₂S(ss) with different concentrations of foreign oxides (Na₂O, Al₂O₃, and Fe₂O₃). The kinetic cutoff temperature and the activation energy for growth of the α'_H phase increase steadily with increasing concentration of impurities. The activation energy for nucleation also increases above 950°C. The α'_H phase, which exists in equilibrium with the α phase at 1280°C, is formed at a maximum rate at around 1100°C regardless of the chemical composition. The TTT diagrams enable us to predict, as a function of cooling rate, the phase constitution of C₂S(ss) at ambient temperature.     

Melt Differentiation Induced by Zonal Structure Formation of Calcium Aluminoferrite in a CaO–SiO2–Al2O3–Fe2O3 Pseudoquaternary System

The phase stability in part of the P₂O₅-bearing pseudoquaternary system CaO–SiO₂–Al₂O₃–Fe₂O₃ has been studied by electron probe microanalysis, optical microscopy, and powder X-ray diffractometry. At 1973–1653 K, the α-Ca₂SiO₄ solid solution [α-C₂S(ss)] and melt coexisted in equilibrium, both chemical variations of which were determined as a function of temperature. The three phases of melt, calcium aluminoferrite solid solution (ferrite), and C₂S(ss) coexisted at 1673–1598 K. On the basis of the chemical compositions of these phases, a melt-differentiation mechanism has been, for the first time, suggested to account for the crystallization behavior of Ca₃Al₂O₆ solid solution [C₃A(ss)]. When the α-C₂S(ss) and melt were cooled from high temperatures, the melt would be induced to differentiate by the crystallization of ferrite. Because the local equilibrium would be continually attained between the rims of the precipitating ferrite and coexisting melt during further cooling, the melt would progressively become enriched in Al₂O₃ with respect to Fe₂O₃. The resulting ferrite crystals would show the zonal structure, with the Al/(Al+Fe) value steadily increasing up to 0.7 from the cores toward the rims. The C₃A(ss) would eventually crystallize out of the differentiated melt between the zoned ferrite crystals in contact with their rims.     

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.     

Phase Stability Study on the Remelting Reaction in Ca2SiO4 Solid Solutions

A pseudobinary phase equilibrium diagram has been established for the P₂O₅-bearing Ca₂SiO₄-CaFe₄O₇ system to confirm the occurrence of remelting reaction in α-Ca₂SiO₄ solid solutions (C₂S(ss)). The reaction started at 1290°C immediately after the α-to-α'_H transition and finished at 1145°C. The reaction products were made up of about 1 wt% of liquid and 99 wt% of solid α'_H-C₂S(ss). The liquid exsolved at 1290°C was rich in Fe₂O₃, consisting of about 30 wt% of Ca₂SiO₄ and 70 wt% of CaFe₄O₇. The liquid coexisting with α-C₂S(ss) precipitated new α'-phase crystals in association with the remelting reaction.     

Thermoelastic Behavior in Ca2SiO4 Solid Solutions

Dicalcium silicate solid solutions (C₂S(ss)) doped with Na₂O, A1₂O₃, and Fe₂O₃ were examined by high-temperature optical microscopy. Surface deformation caused by a possible martensitic transformation between a'_L and β phases was observed in situ under the microscope during temperature changes, indicating that the transformation was thermoelastic. Both the start and finish temperatures of the a'_L-to-β and β-to-a'_L transformations decreased with increased Na:(Na + Ca) ratio. Because of the athermal nature of the a'_L-to-β transformation, the a'_L phase, when cooled below the finish temperature, should have been completely converted to the β phase.     

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.     

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.     

Slurry Consistency and In Situ Synchrotron X-Ray Diffraction During the Early Hydration of Portland Cements With Calcium Chloride

Class A and H oil well cements are compared at 25° and 50°C with 0%, 1%, 2%, and 4% CaCl₂. Up to 4% CaCl₂ accelerated Class A thickening, but 4% led to slower thickening than 2% for Class H. C₃S hydration in the two cements responded differently to CaCl₂. CaCl₂ always accelerated aluminate hydration. For Class A, CaCl₂ accelerated early Ca(OH)₂ precipitation, but sometimes reduced the amount at longer times. This may be coupled to C–S–H gel composition changes. For Class H, Ca(OH)₂ precipitation changes nonlinearly with CaCl₂ concentration. Ettringite to monosulfate conversion and Friedel's salt formation were sometimes seen.     

Early Hydration of Tricalcium Silicate

The hydration of tricalcium silicate (C₃S) in the preacceleration stages was studied. The C₃S particles carry a positive charge during the early stages of hydration. Following a rapid hydrolysis of C₃S, calcium ions adsorbed on the Si-rich surface of C₃S particles, greatly reducing their further dissolution, thus initiating the induction period. The [Ca²⁺] and [OH^-] continue to increase at lower rates and, because Ca(OH)₂ crystal growth is inhibited by silicate ions, become supersaturated with respect to Ca(OH)₂. When the supersaturation reaches a value of ∼1.5 to 2.0 times the saturation concentration, nuclei are formed, and rapid growth of Ca(OH)₂ and C-S-H is initiated. These products act as sinks for the ions in solution, thus enhancing the further dissolution of C₃S.     

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.     

Application of Selective 29Si Isotopic Enrichment to Studies of the Structure of Calcium Silicate Hydrate (C-S-H) Gels

Selective isotopic enrichment of SiO₂ with ²⁹Si in a mixture with tricalcium silicate (C₃S) has allowed the Si from this phase to be effectively labeled during the course of the hydration reaction, thus isolating its contribution to the reaction. A double Q₂ signal has been observed in ²⁹SI solid-state MAS NMR spectroscopy of C-S-H gels of relatively low Ca/Si ratio, prepared by hydration or by carbonation of a C₃S paste. The origin of the weaker, downfield peak is discussed and tentatively attributed to bridging tetrahedra of a dreierkette silicate chain structure.     

Microstructural Characterization of C60-Doped Zirconia

Zirconia powders doped with C₆₀ molecules were prepared from an aqueous solution of zirconium oxynitrate dihydrate, C₆₀ and C₁₆TMA, and sintered at 600°C under 5.5 GPa for 2 h. C₆₀ was found to be retained in the sintered specimens by HRTEM, and carbon was observed to be uniformly dispersed by the SEM-EDX analysis. HRTEM observations of the sintered specimens exhibited the formation of ZrO₂ crystal grains covered with thin graphitic or amorphous carbon films.