Hydration in the System Ca2SiO4–Ca3(PO4)2 at 90°C

Compositions along the Ca₂SiO₄–Ca₃(PO₄)₂ join were hydrated at 90°C. Mixtures containing 15, 38, 50, 80, and 100 mol% Ca₃(PO₄)₂ were fired at 1500°C, forming nagelschmidtite + a ¹-CaSiO₄, A -phase and silicocarnotite and a -Ca₃(PO₄)₂, respectively. Hydration of these produces hydroxylapatite regardless of composition. Calcium silicate hydrate gel is produced when Ca₂SiO₄≠ 0 and portlandite when Ca₂SiO₄ is >50%. Relative hydration reactivities are a -Ca₃(PO₄)₂ > nagelschmidtite > α ¹-Ca₂SiO₄ > A -phase > silicocarnotite. Hydration in the presence of silica or lime influences the amount of portlandite produced. Hydration in NaOH solution produces 14-A tobermorite rather than calcium silicate hydrate gel.     

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.     

Solid Solubility and Polymorphism in the System Sr2SiO4-Sr2GeO4-Ba2GeO4-Ba2SiO4

The reciprocal salt pair Sr₂SiO₄-Sr₂GeO₄-Ba₂GeO₄-Ba₂SiO₄ was investigated using X-ray powder diffraction and DTA. Unlimited solubility in the low-K₂SO₄ structure type (α') occurs throughout the system above 85°C. The nonlinear changes of some lattice constants of the solid solutions are discussed. A stable monoclinic low-temperature (β) form of Sr₂SiO₄ was found which converts reversibly to the high-temperature α'-modification at 85°C. The enthalpy of the β-α' transition is 51 cal/mol. In the reciprocal salt pair the β-form solid solutions occur in a very narrow region below 85°C.     

Phase Equilibrium Studies in the System CaO-Cr2O3-SiO2

Results are presented of a study in air of mixtures in the system CaO-Cr₂O₃-SiO₂. The phase equilibrium diagram shows relations at liquidus temperatures for all but the high-lime part of the system. In this omitted part chromium in the mixtures oxidizes in air to higher valence forms. The compound Ca₃Cr₂Si₃O₁₂ (uvarovite) occurs at subsolidus temperatures, decomposing at 1370°C. to α-CaSiO₃ and Cr₂O₃. The inhibiting action of chromium oxide on the inversion of high-temperature forms of Ca₂SiO₄ to the low-temperature γ-Ca₂SiO₄ is discussed in the light of new data. Evidence is presented for the existence of a pentavalent chromium compound, Ca₃(CrO₄)₂, having solid-solution relations with Ca₃SiO₄.     

Melting Phenomena in the System CaO–SiO2– H2O: I, The Join Ca2SiO,-Ca(OH)2

The liquidus-solidus relations along the join Ca₂SiO₄-Ca(OH), in the system CaO-SiO₂-H₂O have been determined at 1000 atm up to 1110°C. This join is binary and contains the calcium silicate hydrate, calciochondrodite, Ca5-(SiO₄(OH)₂. Calciochondrodite melts incongruently to Ca₂SiO₂+ liquid (composition 23 wt% Ca₂Si0₄) at 955°C. The eutectic between calcium hydroxide and calciochondrodite lies at 13% Ca₂Si0₄ and 822°C. Preliminary experiments, also at 1000 atm, in the ternary system CaO-Ca₂Si0₄-Ca(OH), indicate that the eutectic at which the fields of primary Ca(OH)₂, CaO, and Ca₂(Si0₄)₂(OH)₂ meet is close to the CaO-Ca. (OH), side of the triangle at approximately 805° C. The ternary reaction point Ca₂SiO_l+ liquid ⇌Ca₅(SiO₄)₂(OH)₂+ CaO + liquid is believed to lie in the low-CaO (<5%) high-Ca(OH)₂ (>70%) part of the system.     

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.     

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.     

Phases in the System Ba2SiO4-Ca2SiO4

The system Ba₂SiO₄-Ca₂SiO₄ was studied by heating mixtures of Ba₂SiO₄ and Ca₂SiO₄ at 1723 K. Six distinct phases resulted; they were examined by both X-ray diffraction and differential thermal analysis. The phases β -(Ba_0.05Ca_1.95)SiO₄ and α-(Ba_0.15Ca_1.85)SiO₄ are isostructural with corresponding high-temperature modifications of Ca₂SiO₄. The X phase (Ba_0.48Ca_1.52SiO₄) is orthorhombic, is a pure phase rather than a solid solution, and is defined for the first time in the present work. The T phase (Ba_1.31Ca_0.69SiO₄) is hexagonal and interpreted in terms of a unit cell with a doubled c parameter, in contrast with literature data.     

Studies in the System CaO-Al2O3-SiO2H2O: III, New Data on the Polymorphism of Ca2SiO2 and Its Stability in the System CaO-SiO2- H2O

The nature of the low-temperature inversions γ-α' and α'-β was investigated by various techniques: hydrothermal and "dry" quenching runs, differential thermal analysis at atmospheric and elevated nitrogen pressures, X-ray diffractometer patterns obtained at elevated temperatures, "static" pressure techniques, and infrared absorption spectrometry. A revised energy-temperature diagram is presented for Ca₂SiO₄, with the transition γ' to α' taking place at about 725°C. and the α'-β transition, although not reversible at an exact temperature, taking place at about 670° C. At low water pressures (2000 lb. per sq. in.) the inversion γ-α' was placed at 675°C. Attempts to extrapolate the value obtained at 2000 lb. per sq. in. to obtain a more accurate reversible inversion temperature at atmospheric pressure, although limited in accuracy by the reliability of heat-of-transition data, would indicate a temperature of about 725° C. at atmospheric pressure. Three new compounds, 8CaO.3SiO₂ -3H₂O (X), 6CaO 3SiO₂.H₂O (Y), and 9CaO-6SiO₂ H₂O (Z), were found to be stable above 700°C. at H₂O pressures greater than 7500 lb. per sq. in.     

Ternary System Al2O3—MgO—CaO: Part II, Phase Relationships in the Subsystem Al2O3—MgAl2O4—CaAl4O7

Solid-state compatibility and melting relationships in the subsystem Al₂O₃—MgAl₂O₄—CaAl₄O₇ were studied by firing and quenching selected samples located in the isopletal section (CaO·MgO)—Al₂O₃. The samples then were examined using X-ray diffractomtery, optical microscopy, and scanning and transmission electron microscopies with wavelength- and energy-dispersive spectroscopies, respectively. The temperature, composition, and character of the ternary invariant points of the subsystem were established. The existence of two new ternary phases (Ca₂Mg₂Al₂₈O₄₆ and CaMg₂Al₁₆O₂₇) was confirmed, and the composition, temperature, and peritectic character of their melting points were determined. The isothermal sections at 1650°, 1750°, and 1840°C of this subsystem were plotted, and the solid-solution ranges of CaAl₄O₇, CaAl₁₂O₁₉, MgAl₂O₄, Ca₂Mg₂Al₂₈O₄₆, and CaMg₂Al₁₆O₂₇ were determined at various temperatures. The experimental data obtained in this investigation, those reported in Part I of this work, and those found in the literature were used to establish the projection of the liquidus surface of the ternary system Al₂O₃—MgO—CaO.     

Crystalline Solution in the System MgO-Mg,SiO4,-MgAl2,O4,

Crystalline solubility relations in the system MgO-Mg₂SiO₄MgAl₂O₄ (periclase-forsterite-spinel) were studied using coprecipitated gels as starting materials. The substitution 2Al = Mg + Si was investigated along the join Mg₂SiO₄-Mg-Al₂O₄,. At 1720°C the maximum crystalline solution in forsterite is about 0.5 mole % MgAI₂O₄, and in spinel it is slightly more than 5.0 mole % Mg₂SiO₄. The solubility of MgO in forsterite was 0.5 mole % at 1860°C, whereas more than 11 mole % Mg₂SiO₄ can be dissolved in the periclase structure at this temperature. Ternary crystalline solution exists in the periclase structure to a composition of Mg_0.853Al_0.063Si_0.026O at 1710°C.     

Further Consideration of Phases in the System Ba2SiO4–Ca2SiO4

The results of an investigation of the system Ba₂SiO₄–Ca₂SiO₄ by powder X-ray and electron diffraction suggest a greater complexity than supposed hitherto. The previously recognized phases α, α' h α' l , X, T, and the newly reported Y have room-temperature structures that are modulated distortions of hexagonal (or pseudohexagonal) parent structures. Each displays characteristic and distinctive modulations. The phases are more readily distinguished in this way than by their unit-cell dimensions and compositions which, for a given phase, can vary with bulk starting composition and thermal history.     

Low-Temperature Synthesis of β-Ca2SiO4 from Hillebrandite

Experiments on hydrothermal synthesis were conducted using quartz or silicic acid and lime as starting materials at Ca/Si = 2.0. It is possible to synthesize pure hillebrandite (Ca₂(SiO₃)(OH)₂) having the theoretical composition by heating at 200°C for 10 h or at 250°C for 5 h. The synthesized product is fibrous, open at each end, and has a length of 20 to 30 μm. Calcium silicate hydrate gels are produced at the initial stage of the reaction. These react further with the unreacted lime to give hillebrandite. However, when silicic acid is used as silica, hillebrandite with tricalcium silicate hydrate is observed at 250°C because of the high reaction rate of silica. On heating, hillebrandite starts to decompose at about 500°C and produces low-crystalline β-Ca₂SiO₄, which is stable at room temperature and has a remarkably large specific surface area of about 7 m²/g. The decomposition reaction rate in a single crystal is rapid, and the reaction is considered to proceed topotactically.     

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.     

Kinetic Studies of the Thermal Synthesis of Calcium Silicates Above 1400°C: I, Dynamic Thermal Synthesis of Ca2SiO4

The dynamic reaction of SiO² with CaO (1:2 mol ratio) was studied at temperatures up to 1500°C, especially those > 1400°C. Simultaneous TG and DTA, high-temperature X-ray diffraction (horizontal sample), and high-temperature microscopy and dilatometry were used. The reactants (CaCO₃ and quartz) were finely ground and mixed in stoichiometric amounts. Thermogravimetry was used to ascertain that the samples were representative and homogeneous. The reaction mechanism in the dynamic thermal synthesis of Ca₂SiO₄ was elucidated and confirmed by high-temperature X-ray diffraction.     

Mechanical Properties and Microstructure of Ca2SiO4–CaZrO3 Composites

Three types of dicalcium silicate (Ca₂SiO₄–calcium zirconate (CaZrO₃) composites were fabricated and their microstructures correlated with their mechanical properties. In the first type, Ca₂SiO₄ was added as a minor phase. The second type consisted of a 50 vol% Ca₂SiO₄-50 vol% CaZrO₃ mixture, while in the third type, CaZrO₃ constituted the minor phase. Pure CaZrO₃ was also studied as a control and found to have a toughness which depended on its grain size. In composites with Ca₂SiO₄ as the minor phase, a toughness increase was observed and found to be a function of matrix grain size. The composite with the second type of microstructure had the highest toughness of about 4.0 Mpa. m^1/2, which was about double that of the monolithic CaZrO₃. No evidence was found for transformation toughening by the orthorhombic (β) to monoclinic (γ) transformation in Ca₂SiO₄. The main toughening mechanisms identified were crack deflection and crack branching. Microstructural observations indicated the existence of weak grain boundaries in CaZrO₃ agglomerates as well as weak interfaces between the two phases.     

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.     

Role of Cuspidine (3CaO.2SiO2.CaF2) in the System CaO–SiO2–CaF2

Cuspidine is a well-defined ternary compound with a stability field in the subsystem CaF₂–CaSiO₃–Ca₂SiO₄. Cuspidine is easily formed by solid-state reactions in the subsystem mentioned and is stable above its apparently congruent fusion point if heated in welded platinum containers. Above 1450° decomposition and the formation of a mixture of CaF₂ and Ca₂SiO₄ is observed. Cuspidine also is easily formed by secondary reactions in solid mixtures of the subsystem CaF₂–CaSiO₃ and in ternary mixtures of these with free SiO₂ if heated in open crucibles. The existence of double compounds of CaF₂ and CaSiO₃ is not confirmed.     

Low-Temperature Synthesis of Nanocrystalline α-Si3N4 Powders by the Reaction of Mg2Si with NH4Cl

Nanocrystalline α-Si₃N₄ powders have been prepared with a yield of 93% by the reaction of Mg₂Si with NH₄Cl in the temperature range of 450° to 600°C in an autoclave. X-ray diffraction patterns of the products can be indexed as the α-Si₃N₄ with the lattice constants a = 7.770 and c = 5.627 Å. X-ray photoelectron spectroscopy analysis indicates that the composition of the α-Si₃N₄ samples has a Si:N ratio of 0.756. Transmission electron microscopy images show that the α-Si₃N₄ crystallites prepared at 450°, 500°, and 550°C are particles of about 20, 40, and 70 nm in average, respectively.     

Polymorphism of Calcium Orthosilicate

Therecentobservation of orthorhombic α'-Ca₂Si0₄ (bredigite) at all temperatures between about 850° and 1450°C. leads to a rational interpretation of the polymorphism of this substance, which is very satisfactory from the crystal-chemical point of view. The so-called β phase, of as yet unknown, complex structure, exists only meta-stably, and not above but below about 675°C, where γ is the stable phase. The monotropic β phase forms on cooling from α'near 675°C. as the result of the inhibition of the α'→γ inversion, at 850°C. The inhibition is caused by the need for a considerable atomic rearrangement and a 12% volume increase which accompanies the change of the coordinations CaO₂ and CaOlo, in α', to CaO₆, in γ. Among the solid phase equilibria with, Mg₂SiO₄ and Ca₃(PO₄)₂, unlimited solid solubility between γ-Ca₂SiO₄ and Mg₂SiO₄ is predicted, whereas the solubility of Mg₂SiO₄ is a limited one in α'and still more so in a-Ca₂SiO₄, as a result of the substitution, for calcium, of the smaller magnesium.