Synthesis of calcium silicate by a chelate gel route with aqueous solution of citric acid

Recently, novel gel techniques such as the metal-chelate gel method, in situ polymerized complex method and polymer precursor method have been utilized to prepare many kinds of ceramics. These techniques offer the potential advantage of achieving compositionally homogeneous and fine powders with a narrow size distribution. Since the hydration activity of calcium silicates is affected by phase changes and the surface area, it is of vital importance to take into account both polymorphic forms and particle size. In this study, a metal-chelate gel route based on gelation of the aqueous solution of citric acid has been successfully applied to the synthesis of calcium silicates (Ca₂SiO₄ and Ca₃SiO₅) for the first time. In addition, their phase transformations and particle size are discussed in comparison to the conventional solid-state reaction route. The novel citrate gel route and the conventional solid-state reaction route were found to produce β-Ca₂SiO₄ (high-temperature phase) and γ-Ca₂SiO₄ (low-temperature phase), respectively. This result can be explained in terms of the particle size effect and the energy barrier. The nucleation and propagation of microcracks were responsible for overcoming a comparatively high-energy barrier, ΔG_β→γ^*. The particle size effect governs both the statistic of martensitic nucleation and the propagation of the β→γ transformation. In contrast to Ca₂SiO₄, triclinic Ca₃SiO₅ (low-temperature phase) was obtained by both the citrate gel route and the conventional solid-state reaction route. The nucleation and propagation are not responsible for the transformation, thus the energy barrier of the monoclinic (M) to the triclinic (T) transformation, ΔG_M-T^*, are considered to be small.