Crystal lattice defects in di-calcium silicate

It is well documented that the impurity content of C₂S has a marked effect on its hydration properties. It is also a fact that pure C₂S at room temperature exists in a crystal configuration, γ, which is completely different from the clinker C₂S. Incorporation of impurity ions into pure C₂S inhibits the β → γ transition on cooling, even at impurity levels as low as 0.1%. Currently, there is no fully accepted model for the mechanism of the stabilization of the β phase over the γ phase by impurities. The presence of these trace impurities naturally raises the question of the location and role of the impurities in producing this marked stabilizing effect. A quantitative solution to this problem requires that the lattice defect disorder of C₂S be defined and related to the phase stabilization properties of the impurities. Definition of crystal defect disorder can be achieved through detailed measurements of the electrical conductance of the solid with the data being interpreted in terms of solid state physics. Data for the electrical conductance of pure and “intentionally” impure (doped) C₂S over the temperature range 300 to 1100°C is presented. These data are interpreted in terms of crystal defects, their formation, location and kinetics. Models are presented to account for the stabilizing action of impurity ions within the crystal lattice of C₂S.