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.