Chlorellestadite (Synth): Formation, Structure, and Carbonate Substitution during Synthesis of Belite Clinker from Wastes in the Presence of CaCl2 and CO2

• Published in: Minerals (Basel) 2022-09, Vol.12 (9), p.1179
• Main Authors: Garbev, Krassimir, Ullrich, Angela, Beuchle, Günter, Bergfeldt, Britta, Stemmermann, Peter
• Format: Article
• Language: English
• Subjects: Aerated concrete; Aeration; Analytical methods; Apatite; Calcium; Calcium chloride; Carbon dioxide; Carbonates; Cell size; Cement; chlorellestadite (synth); Chlorine; Clinker; Dicalcium silicate; ellestadite; Investigations; Kilns; Limestone; Low temperature; low-temperature recycling; Raman spectroscopy; Raw materials; Spectroscopy; Spectrum analysis; Substitutes; Sulfates; Sulphates; Synthesis; Temperature; Unit cell; Wastes; X-ray diffraction
• ISSN: 2075-163X; 2075-163X
• DOI: 10.3390/min12091179

The synthesis of low-temperature belite (C2S) clinker from wastes of autoclaved aerated concrete and limestone was studied in the presence of CaCl2 as a mineralizing agent. Synthetic chlorellestadite (SCE; Ca10(SiO4)3(SO4)3Cl2) 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 CO32− groups in ellestadites formed at 800 °C and 900 °C in air. Incorporation of CO3 results in a shorter unit cell parameters and smaller cell volume similar to CO3−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.