Effect of sulfur on the polymorphism and reactivity of dicalcium silicate of Portland clinker

Abstract

The present study regards the effect of sulfur in dicalcium silicate (Ca2SiO4), a major crystalline phase (20 to 40 wt.%) of the ordinary Portland cement clinker. Dicalcium silicate is also known as C2S (2CaO.SiO2) or belite. The synthesis of the C2S samples was made with high purity reactants with addition of sulfur as CaSO4.2H2O, mixed according to the stoichiometric proportion 2Ca:(1-x)Si:xS, in which x corresponds to the cationic proportion of sulfur, with values ranging from 0 to 20%. Added amounts of SO3 in the samples were 0.23, 1.39, 2.77, 4.60 and 9.71wt.%. Chemical composition of the samples was determined by X-ray fluorescence before and after sintering. Large-scale sulfur loss by volatilization leads to an excess in calcium and consequently to formation of increasing amounts of tricalcium silicate with increasing sulfur content in the starting mixture. Mineralogical composition of the samples and cell parameters of C2S polymorphs were determined by X-ray diffraction and Rietveld refinements. Structural analysis of diffraction data indicates that the presence of sulfur stabilizes the intermediate temperature polymorph β C2S, with increasing unit cell volume. The reactivity with water (heat of hydration) of the samples was measured by differential scanning calorimetry, which was strongly influenced by the highly reactive tricalcium silicate.