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.     

Hydration Kinetics and Phase Stability of Dicalcium Silicate Synthesized by the Pechini Process

Reactive dicalcium silicate (Ca₂SiO₄) has been synthesized by the Pechini process, and hydration kinetics studied. With increasing calcination temperature, the amorphous product first crystallizes to α'_L-phase and subsequently to the ß- and γ-phases. The specific surface area, ranging from 40 to 1 m²/g, strongly depends on the calcination temperature of 700°-1200°C for 1 h. Samples with a high surface area have a high water demand; a water/cement ratio >2.0 is required to produce formable pastes in some instances. Hydration kinetics are determined by XRD, ²⁹Si magic-angle spinning nuclear magnetic resonance (MAS NMR), and differential scanning calorimetry/thermogravimetry (DSG/TG). The hydration rate depends only on the surface area, not on the polymorph. Complete hydration occurs in as early as 7 d. Very little calcium hydroxide (Ca(OH)₂) is formed in the most reactive specimens (calcined at 700° and 800°C), which indicates the Ca/Si ratio in C-S-H gels is ∼2.0, but more Ca(OH)₂ forms from samples calcined at higher temperature. The silicate structure of the hydrated Ca₂SiO₄ pastes is investigated using ²⁹Si MAS NMR spectroscopy and trimethylsilylation analysis.     

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.     

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.     

Transformation Toughening in Composites Containing Dicalcium Silicate

By taking into account solid-state Compatibility relations in the system CaO-SiO₂-ZrO₂, a series of dense, tough CaZrO₃-β-Ca₂SiO₄ composites were obtained. The increase in K_IC and σ_f values observed is interpreted in terms of the β-Ca₂SiO₄→γ-Ca₂SiO₄ polymorphic transformation.     

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.     

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.     

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.     

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.     

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.     

Effect of Substituent Ions on Martensitic Transformation Temperatures in Dicalcium Silicate Solid Solutions

Five types of Ca₂SiO₄ solid solutions, doped with either P⁵⁺, Ge⁴⁺, Fe³⁺, Mg²⁺, or Ba²⁺, were prepared and examined by high-temperature X-ray diffractometry up to 800°C. The starting and finishing temperatures were determined for the α'_L-to-β martensitic transformation during cooling and the reverse (β-to-α'_L) transformation during heating. These four types of transformation temperatures for the preparations doped with either P⁵⁺ or Ge⁴⁺ steadily decreased with increasing substituted fraction. The effect of the substitution on the decrease for each transformation temperature was quantitatively evaluated by Δ T / x , where Δ T is the difference in the transformation temperatures between the solid solutions and pure Ca₂SiO₄, and x represents the fraction substituted for Si⁴⁺ or Ca²⁺ in the α'_L-phase structure. The evaluated value for the substitution of P⁵⁺ was more than 3 times that of Ge⁴⁺. The effect of the substituent ions mentioned above, together with Na⁺ and Sr²⁺, on the lowering of the starting temperature of the α'_L-to-β transformation was principally determined by the differences in the ionic radius between the interchanging cations.     

Preparation and Phase Transformations of Dicalcium Silicate-Alkali Fluoride Complexes

Complexes of dicalcium silicate and lithium, sodium, and potassium fluorides were prepared by solid-state reactions. X-ray diffraction powder pattern data are presented for the complexes. Phase transformations were found below 1000°C for the Li and K complexes, but not for the Na complex. The inversion point for the Li complex is between 915° and 920°C and that for the K complex is between 805° and 810°C. Between 940° and 950°C the Li complex fuses to a colorless glass-like melt which on cooling shows evidence of partial conversion to γ-2CaO·SiO₂.     

Superstructure in a Triclinic Phase of Tricalcium Silicate

A triclinic phase of tricalcium silicate (TI) was investigated using transmission electron microscopy (TEM). Electron diffraction patterns were analyzed by introducing a subcell with the cell parameters of a = 7.081 Å, b = 7.043 Å, c = 25.230 Å, α= 89.97°, β= 90.37°, and γ= 119.44°. It was proven that the coordinates of all the reflections can be expressed to be ha *+ kb *+ lc *± m /6( a *+ 2 b *+ 2 c *), where m = 0, 1, 2, and 3. The result indicates that the structure modulation in TI is a one-dimensional type with a structural modulation normal to (122). The modulated structure could be observed in a high-resolution TEM image as wavy contrast streaking parallel to the plane with an interval of six times the (122) spacing.     

Formation and Local Structure Analysis of High‐Valence Chromium Ion in Dicalcium Silicate

Dicalcium silicate (2CaO·SiO₂) is a typical silicate compound that exists even at elevated temperatures. Different types of elements can be dispersed in dicalcium silicate to form a solid solution phase. However, the dispersion behavior of transition elements that can display different ionic valences is not well understood. Herein, we investigate the local structure of chromium ions dispersed in dicalcium silicate of different phase states under inert and air atmospheres using Cr K‐edge X‐ray absorption near‐edge structure (XANES) spectroscopy and first‐principle calculations. The measured XANES spectra indicated that Cr ions exist as high‐valence states when dispersed in dicalcium silicate in air atmosphere. Specifically, Cr⁶⁺ ions were detected in γ‐dicalcium silicate, which was prepared by annealing a mixture of 2CaO·SiO₂ and Cr₂O₃ at 973 K in air atmosphere. In addition, the XANES spectra obtained via first‐principle calculations revealed that Cr ions, with high‐valence states between Cr⁴⁺ and Cr⁶⁺, in dicalcium silicate, occupy the tetrahedral Si⁴⁺ sites.     

Phase Transitions and Therrnoluminescence of a Plasma-Sprayed Zinc Silicate Phosphor

A plasma-sprayed zinc silicate phosphor was heat-treated at ∼800°C informing gas; the major phase present was the metastable β phase, as revealed by bright yellow fluorescence under uv excitation. On heat treatment at ∼1000°C in air, a green, fluorescence developed as a result of the formation of the stable a phase. However, its thermo luminescence characteristics were different from those of the original P1 phosphor used for spraying     

Physical Stabilization of the β→γ Transformation in Dicalcium Silicate

It has been shown that the monoclinic β -phase of dicalcium silicate (Ca₂SiO₄) can be stabilized against transformation to the orthorhombic γ -phase by physical rather than chemical factors. Stabilization was studied in different types of microstructures fabricated under various processing conditions such as different powder or grain sizes, chemical additives, cooling kinetics, or high-temperature annealing treatments. The observations can be explained in terms of a critical particle size effect controlling nucleation of the transformation. Rapid quenching through the high-temperature hexagonal ( α ) to orthorhombic ( a' _H) transformation at 1425°C, which is accompanied by a −4.7% volume decrease, causes periodic fracture of β -twins due to accumulated strains. Chemical doping with K₂O or Al₂O₃ promotes the formation of amorphous phases which mold themselves around β -Ca₂SiO₄ grains. Annealing treatments cause crystallization of the glass and subsequent transformation to the γ -phase.