Influence of alumina (Al2O3) crystallinity and preparation conditions on ye'elimite (4CaO•3Al2O3•SO3) synthesis

Abstract

Ye'elimite-rich cements have attracted growing research interest due to their environmental advantages, as lower sintering temperatures and reduced limestone usage lead to significantly lower CO2 emissions. Consequently, achieving high-purity ye'elimite has become increasingly important for the development of these alternative cement systems. However, solid-state synthesis is often hindered by sulfur evaporation, which reduces phase purity. This study investigated the solid-state synthesis of ye'elimite, focusing on three key factors: (1) the effect of alumina crystallinity (alpha-alumina vs. gamma-alumina) at synthesis temperatures ranging from 800 to 1300 degrees C; (2) the impact of increasing the SO3 content by 10 mol%; and (3) the effect of applying either a single or multiple quenching cycles (1-4 times) at a sintering temperature of 1300 degrees C. TG analysis revealed sulfur losses of 7.73 wt% and 8.17 wt% in stoichiometric alpha- and gamma-alumina-based mixtures, respectively. Although lower than that of raw CaSO4 (12.31 wt%), these losses indicate the necessity of SO3 compensation to ensure proper stoichiometry for ye'elimite synthesis. In stoichiometric synthesis, XRD analysis showed that gamma-alumina promoted earlier ye'elimite formation (900 degrees C vs. 1000 degrees C for alpha-alumina) and yielded higher purity at 1200 degrees C (52.0 vs. 23.0 wt%). Both mixtures reached peak purity at 1300 degrees C. Thermal analysis showed that gamma-alumina enhanced calcium aluminate crystallization above 1020 degrees C. 27Al-NMR confirmed earlier development of tetrahedral Al environments in gamma-alumina-based mixtures. In non-stoichiometric synthesis, XRD and 27Al-NMR analyses showed that, in alpha-alumina mixtures, multiple quenching cycles promoted reactions between residual CaSO4 and krotite, increasing ye'elimite purity, whereas in gamma-alumina mixtures, excessive quenching reduced purity due to CaSO4 depletion. These findings offer practical guidance for controlling raw material reactivity, sulfur compensation, and thermal treatment to produce high-purity ye'elimite at the laboratory scale, with potential scalability to larger processes.