Alite calcium sulfoaluminate cement: chemistry and thermodynamics

Calcium sulfoaluminate (C$A) cement is a binder of increasing interest to the cement industry and as such is undergoing rapid development. Current formulations do not contain alite; however, it can be shown that hybrid C$A-alite cements can combine the favourable characteristics of Portland cement with those of C$A cement while also having a lower carbon footprint than Portland cement clinkers. This paper presents two results on the formation of alite calcium sulfoaluminate (aC$A) clinkers. The first is a thermodynamic study which demonstrates that the production of a-C$A clinker is possible without the use of mineralizers, doping with foreign elements, or using multiple stages of heating. It is established that a-C$A clinker can be readily produced in a standard process by controlling the oxygen and sulfur dioxide fugacities in the atmosphere. This allows for the stabilization of ye'elimite to the higher temperatures required for alite stability. The second result establishes that when using fluorine to mineralise a-C$A clinker production at 1250°C the iron content in the clinker is also an important variable. Although the exact mechanism of alite stabilisation is not known, it is shown that alite formation increases with the presence of both CaF2 and Fe2O3 in the raw mix.