Quantitative Control of C2S Crystal Transformation

Crystal transformation of dicalcium silicate (2CaO center dot SiO2, C2S) has influences on the microstructure, hydraulicity, and grindability of cement clinker. The transformation of beta-C2S to gamma-C2S can destroy the clinker nodule integrity by volume expansion due to the low density of the gamma-phase. It can be used to pulverize cement clinkers for saving grinding energy, but the hydraulicity of gamma-C2S is lower than beta-C2S. Quantitative control of C2S crystal transformation could balance the energy saving and hydraulicity. The influences of sintering temperatures, cooling methods and chemical impurities on the transformation were investigated. The results show that the appropriate sintering temperature of gamma-C2S was 1250 similar to 1500 degrees C, and the rate of cooling should not be higher than 500 degrees C/min to guarantee beta-C2S transform to gamma-C2S. Chemical impurities (Na2O, P2O5, B2O3, Cr2O3 and K2O) were stabilizers of beta-C2S, which could prevent beta-C2S transforming to gamma-C2S. The stability effect was related to their concentrations. The characteristic concentrations of Na2O, P2O5, B2O3, Cr2O3 and K2O were approximately 1.2%, 0.3%, 0.3%, 1.0% and 1.5%, respectively. Above these concentrations, most of beta-C2S could be stabilized rather than transforming to gamma-C2S. In "transition zone", C2S crystal transformation could be controlled quantitatively by adjusting the stabilizer concentration.