Phase Transformations in Dicalcium Silicate: I, Fabrication and Phase Stability of Fine-Grained β Phase

Fine-grained β-Ca₂SiO₄ containing small amounts of sodium was fabricated as an analogue to tetragonal zirconia polycrystals (TZP) in order to study the stress-induced β→γ transformation. This avoided the problems associated with the fabrication and evaluation of composites containing β-Ca₂SiO₄. The microstructure of dense β-Ca₂SiO₄ exhibited severe intergranular strains and twin-terminating microcracks as seen by TEM. The β-phase twin widths were quantitatively correlated with grain sizes giving an average ratio of 0.04. Stress-induced transformation was observed on ground surfaces but not on fracture surfaces. The stress–strain behavior and the mechanical properties were consistent with stress-induced microcracking and microcrack coalescence. The elastic modulus of fully dense β phase was estimated to be 123 GPa.     

Kinetics of the Hydration of Tricalcium Silicate

The hydration of tricalcium silicate was followed at a water/C₃S ratio of 0.7 between 5° and 50°C by determining free lime and combined water. Free lime was estimated by the o -cresol method, whereas combined water was calculated from the amount of free water remaining in the paste as determined by extraction with methyl ethyl ketone. The ratio of free lime to combined water was constant throughout the hydration; this ratio indicated that CSH(II) may be represented as 1.68CaO · SiO₂· 2.58H₂O. When maximum supersaturation of the solution with Ca^2+] is attained, the induction period terminates and the reaction proceeds rapidly, probably as the result of propagative surface nucleation-growth of CSH(II). Kinetic equations were derived for these reactions. When the surface of C₃S is entirely covered by CSH(II), the reaction becomes slow and is controlled by diffusion of water. Constants involved in the kinetic equations are evaluated and discussed.     

硅酸二钙烧成过程动力学参数测定

对硅酸二钙(C2S)形成过程的各项动力学参数进行了测定计算,得到了不同温度下SiO2转化率G与时间t的关系.采用了金斯特林格(Ginstling)方程作为固相反应模型,对C2S的形成过程进行描述;计算了不同煅烧温度下固相反应的反应速率常数K、钙离子的扩散系数D;得到C2S形成的表观活化能Ea为167.4 kJ·mol-1,指前因子A为20.94.     

Crystallographic Data of a New Phase of Dicalcium Silicate

Unit-cell parameters and the space group of a new phase of dicalcium silicate (Ca₂SiO₄) were determined by using powder X-ray diffractometry and selected-area electron diffraction techniques. This phase could be synthesized via the dissociation of hydrothermally synthesized alpha-Ca₂(SiO₄H)OH at temperatures of ∼500°-920°C. Crystallographic data for the sample synthesized at 600°C were as follows: Ca₂SiO₄, monoclinic; P 2₁/ c space group; lattice parameters of a = 0.82147(9) nm, b = 0.9808(1) nm, c = 0.9741(1) nm, and β= 94.642(7)°; cell volume ( V ) of 0.7857(1) nm³; Z = 8 (where Z is the number of chemical formula units in a unit cell); and a density of 2.91 g/cm³. The crystallographic data for samples synthesized at 800°C had slightly different unit-cell parameters of a = 0.82124(6) nm, b = 0.97348(7) nm, c = 0.97935(7) nm, β= 94.831(5)°, and V = 0.7849(1) nm³. Structural relationships of the new phase with the other dicalcium silicates are discussed.     

Kinetics of Tricalcium Silicate Hydration in Diluted Suspensions by Microcalorimetric Measurements

Investigations of tricalcium silicate (C₃S) suspensions using different techniques have shown the initial hydration kinetics of C₃S are strongly dependent on the water/C₃S ratio. In order to provide new experimental data on the hydration, we have adapted a Tian-Calvet heat flow isothermal microcalorimeter to study thermal flow variation released by C₃S stirred suspensions in water and saturated lime water. We observed three exothemic peaks and one endothermic peak. Their relative magnitude and duration depend on the W/C ration and the lime solution concentration. The protective layer theory appears to be consistent with our results for high W/C ratio. Moreover, portlandite precipitation seems to be a consequence and not the cause of the acceleratory period.     

Re-Examination of the Polymorphism of Dicalcium Silicate

Samples of Ca₂SiO₄ were prepared by three different methods. These samples were examined by differential thermal analysis, high-temperature X-ray diffraction, and air quenching of pellets. It was found that the β modification, during cooling, converted completely and rapidly to the γ modification and "dusted" only if it had been heated above 1420†± 10† C. If the sample was not heated above this critical temperature, a mixture of the β and γ forms always resulted at room temperature. The addition of 0.5 weight % of CaO, SiO₂, Al₂0₃, Fe₂0₃, or MgO had no effect on the sequence of phases for samples cooled from above the critical temperature. When, however, these same samples were cooled from below 1410†C, the final room-temperature product was not the same for all samples. On the other hand, the additional 0.5% of Na₂0, K₂O, Cr₂O₃, or B₂0₃ prevented the formation of the β modification regardless of the temperature from which the sample was cooled. The critical heating temperature correlated with the transformation temperature for the α-α inversion. A theory relating this inversion to the completeness of the β→γ transformation is presented.     

Effects of Particle Size Distribution on the Kinetics of Hydration of Tricalcium Silicate

An analysis of the validity of approximating selected particle size distributions by small numbers of size classes was carried out. It was determined that the selection of a small number of particle size classes is not adequate to describe a particle size distribution. One consequence of this is that the particle size distribution, if not properly accounted for, can mask the kinetics of a hydration process. The hydration rates of two finnesses of tricalcium silicate, each with known particle size distributions, were measured by isothermal calorimetry for a period of 28 d. These data were integrated, normalized, and represented as α-time curves as a basis for comparison with kinetic models of hydration. Agreement with kinetic models was found to be strongly influenced by the effect of particle size distribution. However, the rate-limiting mechanisms appear to be independent of particle size distribution No single kinetic model was adequate to describe C₃S hydration over the first 28 d. A kinetic model that assumes initial surface-area control with diffusion control dominating subsequently provided an excellent fit to the experimental data.     

A New Approach to Modeling the Nucleation and Growth Kinetics of Tricalcium Silicate Hydration

The hydration kinetics of tricalcium silicate (C₃S), the main constituent of portland cement, were analyzed with a mathematical "boundary nucleation" model in which nucleation of the hydration product occurs only on internal boundaries corresponding to the C₃S particle surfaces. This model more closely approximates the C₃S hydration process than does the widely used Avrami nucleation and growth model. In particular, the boundary model accounts for the important effect of the C₃S powder surface area on the hydration kinetics. Both models were applied to isothermal calorimetry data from hydrating C₃S pastes in the temperature range of 10°–40°C. The boundary nucleation model provides a better fit to the early hydration rate peak than does the Avrami model, despite having one less varying parameter. The nucleation rate (per unit area) and the linear growth rate of the hydration product were calculated from the fitted values of the rate constants and the independently measured powder surface area. The growth rate follows a simple Arrhenius temperature dependence with a constant activation energy of 31.2 kJ/mol, while the activation energy associated with the nucleation rate increases with increasing temperature. The start of the nucleation and growth process coincides with the time of initial mixing, indicating that the initial slow reaction period known as the "induction period" is not a separate chemical process as has often been hypothesized.     

用阶跃升温法测2CaO-SiO2固相反应的温升

探讨了2CaO-SiO2(C2S)的固相反应热效应的温升测试方法,发现阶跃升温法适合研究类似C2S大吸热小放热固相反应自放热引起的温升。通过观察阶跃升温法中C2S形成过程中的温度变化,看到在1250℃下不同尺寸的试样内部温升不同,尺寸越大,试样内部的温度越高,φ16 mm试样的温升达到92℃;同时C2S固相反应的速率提高1.6倍,转化率提高1.4倍。     

Surface Relief Induced by Martensitic Transformation in Phosphorus-Bearing Dicalcium Silicate

Crystals of (Ca_1.955□_0.045)(Si_0.91P_0.09)O₄, where □ denotes a vacancy, have been prepared and examined by XRD, optical microscopy, SEM, and AFM. At 20°C, the crystals are composed of 38%α' _L and 62%β phases. Upon cooling to −185°C, the α' _L - to β-phase transformation occurs, which increases the β-phase composition to 72%. The transformation is also accompanied by the formation of platelike surface reliefs. The surface relief angles have been determined from observations (7.9°± 0.2°) and calculations based on a phenomenological analysis (7.84°). The fair agreement of these values indicates that the transformation is martensitic and mainly governed by a shear mechanism.     

Hydration of Tricalcium Silicate

The kinetics of paste, bottle, and ball-mill hydration of 3CaO SiO₂ and the effects of additions of electrolytes and alcohols were studied. Paste and bottle hydrations proceed through periods of induction, acceleration, and decay. If 3CaO SiO₂ is hydrated in an excess of H₂O, as in bottle hydration, the reaction rate is lower than that for paste hydration. The ball-mill hydration rate is much the highest and is controlled by the removal of the hydrate layer coating the 3CaO SiO₂ particles. Electrolytes always accelerate and alcohols retard the reaction rate. Experimental results are discussed with reference to modern theories of the 3CaO SiO₂ hydration mechanism.     

High-Temperature, High-Pressure X-ray Investigation of Dicalcium Silicate

Energy-dispersive X-ray powder diffraction experiments have been investigated at high temperature and room pressure, and at high pressure and room temperature, starting from either γ- or β-Ca₂SiO₄. High-temperature studies were performed up to 1980 K, using a versatile heating cell. The high-temperature phase transformations previously described were reexamined. Volume and linear thermal expansions were measured for each Ca₂SiO₄ polymorph, γ, β, α';_L,α';_H, and α. Volume thermal expansion increases with increasing temperature except for α';_H, whose thermal expansion tends to decrease at elevated temperature. High-pressure investigations were performed in the 0–15 GPa pressure range, using a diamond anvil cell, with silicon oil as the pressure-transmitting medium. The value of the room-pressure bulk modulus K₀ , assuming a second-order BirchMurnaghan equation of state with K'₀= 4, is 140(8) GPa for γ-Ca₂SiO₄. The γ olivine form exhibits anisotropic compression, with the c axis as the most compressible. From such in situ high-pressure X-ray investigations, the γ-→Ca₂SiO₄ phase transformation induced by cold compression is clearly evidenced and extends from 2 to about 5 GPa.     

High-Temperature Microscopic Investigation of Tricalcium Silicate and Dicalcium Silicate Phases in Portland Cement Clinker

Birefringence was measured as a function of temperature up to 1400°C for (1) alite and various types of belite in portland cement clinker, and (2) alite in fused cement. The characteristic optical changes in alite and belite grains which accompany polymorphic transformations and facilitate, in certain cases, determination of the modification without necessitating the separation of the pure phase are given. The β and α'modifications were differentiated for belite from clinker and also for alite in fused cement. A method is described for measuring birefringence of minerals at high temperatures, as well as the apparatus and the preparation of samples.     

Phase Transformations in Dicalcium Silicate: II, TEM Studies of Crystallography, Microstructure, and Mechanisms

The crystallography, microstructures, and phase transformation mechanisms in dicalcium silicate (Ca₂SiO₄) were studied by TEM. Three types of superlattice structures were observed in the α'_L and β phases. Almost all β grains were twinned and strained. Symmetry-related domain structures inherited from previous high-temperature transformations were observed in β grains. Both the α→α'_H and α'_L→β transformations were considered to be ferroelastic, and spontaneous strains were calculated. In terms of the crystal structures, the major driving force for the β→γ transformation is proposed to be strains and cation charge repulsions in the β structure. This mechanism can be displacive, but it needs to overcome a comparatively high energy barrier.     

Retrograde Solubility of Periclase, Forsterite, and Dicalcium Silicate

Published phase equilibria data are used to demonstrate that for certain compositions, including some commercial magnesites, the amount of one crystalline phase present with liquid can increase as the temperature is raised. This effect is important for periclase with forsterite+spinel+liquid and for dicalcium silicate with merwinite+periclase+liquid; it may influence bonding in commercial refractories.     

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.     

Transmission Electron Microscopy of Hydrated Dicalcium Silicate Thin Films

A method of preparing calcium silicate hydration products for transmission electron microscopy is presented. Thin films of dicalcium silicate are evaporated onto substrates and hydrated in a vacuum chamber. Since the resulting hydration products are undisturbed, the interrelations of the species can be studied. The technique provides specimens suitable for high-resolution microscopy. This method produces hydration products like those reported when conventional specimen preparation techniques are used. Calcium silicate hydrate gel, two forms of C-S-H II (fans and fiber bundles), afwillite, and Ca(OH)₂ morphologies were observed. Coevaporated CaCl₂ additions improve the electron diffraction patterns obtained from the C-S-H II fan structures but increase shrinkage of these structures on drying.     

Influence of Minor Ions on the Stability and Hydration Rates of β-Dicalcium Silicate

The effects of Al³⁺, B³⁺, P⁵⁺, Fe³⁺, S⁶⁺, and K⁺ ions on the stability of the β-phase and its hydration rate were studied in reactive dicalcium silicate (C₂S, Ca₂SiO₄) synthesized using the Pechini process. In particular, the dependences of the phase stability and degree of hydration on the calcination temperature (i.e., particle size) and the concentration of the stabilizing ions were investigated. The phase evolution in doped C₂S was determined using XRD, and the degree of hydration was estimated by the peak intensity ratio of the hydrates to the nonhydrates in ²⁹Si MAS NMR spectra. The stabilizing ability of the ions varied significantly, and the B³⁺ ions were quite effective in stabilizing the β-phase over a wide range of doping concentrations. The hydration results indicated that differently stabilized β-C₂S hydrated at different rates, and Al³⁺- and B³⁺-doped C₂S exhibited increased degree of hydration for all doping concentration ranges investigated. The effect of the doping concentration on degree of hydration was strongly dependent on the stabilizing ions.     

磷硫复合掺杂对硅酸二钙晶型结构的影响

通过XRD、FT-IR测试方法,并结合Rietveld理论,深入分析了磷单掺及磷硫复掺对硅酸二钙(C2 S)晶型结构的影响规律.结果表明,磷单掺条件下,有效稳定硅酸二钙的最佳掺量为1.64％左右.当样品中含有1.88％左右SO3时,P2 O5最佳掺量为0.83％,说明SO3可以促进P2 O5的稳定活化效果.且与未掺杂样品相比,元素掺杂有效提高了[SiO4]的对称性.