Chlorellestadite (Synth): Formation, Structure, and Carbonate Substitution during Synthesis of Belite Clinker from Wastes in the Presence of CaCl${}_2$ and CO${}_2$

The synthesis of low-temperature belite (C${}_2$S) clinker from wastes of autoclaved aerated concrete and limestone was studied in the presence of CaCl${}_2$ as a mineralizing agent. Synthetic chlorellestadite (SCE; Ca${}_{10}$(SiO${}_4$)${}_3$(SO${}_4$)${}_3$Cl${}_2$) forms in experiments at temperatures between 700 and 1200 °C. Samples were investigated by X-ray diffraction and Raman spectroscopy. In general, the amount of SCE depends mainly on the sulfate content and to a lesser extent on the synthesis temperature. At lower temperatures of formation, a non-stoichiometric SCE seems to crystallize in a monoclinic symmetry similar to hydroxylellestadite. Rietveld refinements revealed the presence of chlorine and calcium vacancies. Raman spectroscopy proved the partial substitution of sulfate by CO${}_3$${}^{2-}$ groups in ellestadites formed at 800 °C and 900 °C in air. Incorporation of CO${}_3$ results in a shorter unit cell parameters and smaller cell volume similar to CO${}_{3-}$apatite. At low temperatures, SCE coexists with spurrite intermixed on a very fine nm scale. At temperatures above 900 °C in air, ellestadite is carbonate-free and above 1000 °C chlorine loss starts in all samples.