CHEMISTRY OF DICALCIUM SILICATE MINERAL

Dicalcium silicate is of vital importance in several fields of silicate science. It exists in several polymorphic forms, of which one (the γ-form) is stable at room temperature without any stabilizer. The β-form is commonly found in ordinary portland cement (OPC) in association with stabilizing ions. Stabilization of other forms, α, α′L, α′H and α′m for structural and other studies have been reported. Theoretical structural analysis using topology has been reported to be of value in understanding the stabilization process of the polymorphs. The conversion of β→γ form is at times a problem in the cement industry, in addition to the formation of unwanted compounds, such as spurrite. The γ-form is low in hydraulic properties but in the presence of impurities such as excess CaO over the stoichiometric ratio, shows fairly high hydraulic properties. Of the other phases, the hydraulic properties of the a forms are quite encouraging but the choice of stabilizers etc. plays a dominant role. Correlation of hydraulicity with structural properties such as crystal defects, etc., has been reported but satisfactory explanation is yet to come. The hydration products of β-C2S are quite similar to those of C3S but the kinetics are fairly slow. In the presence of active silica, and at elevated temperatures, even the γ-form hydrates at a faster rate. The influence of chemical accelerators on the hydration of C2S at room temperature is well studied and NaF is found to be one of the best accelerators. The formation of ″reactive″β-C2S by different preparative methods shows a quite interesting trend for potential manufacture of low-temperature inorganic cement or OPC with low C3S; even utilization of low-grade limestone could be possible. The role of C2S in the hydration of aluminous cements is being increasingly recognized and, in fact, a newer class of cements called alumina-belite cement, etc., are being developed in which C2S is purposely maintained as a major phase. © 1979 Chapman and Hall Ltd.