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

Recently, novel gel techniques such as the metal-chelate gel method, in sit u polymerized complex method and polymer precursor method have been utilize d to prepare many kinds of ceramics. These techniques offer the potential a dvantage of achieving compositionally homogeneous and fine powders with a n arrow size distribution. Since the hydration activity of calcium silicates is affected by phase changes and the surface area, it is of vital importanc e to take into account both polymorphic forms and particle size. In this st udy, a metal-chelate gel route based on gelation of the aqueous solution of citric acid has been successfully applied to the synthesis of calcium sili cates (Ca2SiO4 and Ca3SiO5) for the first time. In addition, their phase tr ansformations and particle size are discussed in comparison to the conventi onal solid-state reaction route. The novel citrate gel route and the conven tional solid-state reaction route were found to produce beta -Ca2SiO4 (high -temperature phase) and gamma -Ca2SiO4 (low-temperature phase), respectivel y. This result can be explained in terms of the particle size effect and th e energy barrier. The nucleation and propagation of microcracks were respon sible for overcoming a comparatively high-energy barrier, DeltaG*(beta --> gamma). The particle size effect governs both the statistic of martensitic nucleation and the propagation of the beta --> gamma transformation. In con trast 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) transf ormation, DeltaG*(M-T) are considered to be small.