Ettringite instability in supersulfated cement: Investigation under different curing regimes

The global discussion on CO₂ emission reduction policies has encouraged the search for sustainable concrete binder materials, such as supersulfated cement (SSC), since this binder is composed mainly of blast-furnace slag. However, despite having relevant environmental and mechanical properties, when compared to Portland cement, little is known regarding its complex process of hydration and durability. It is known that the primary hydration products of SSC are ettringite and calcium-silicate-hydrate (C–S–H), and the instability of the former has been observed in some studies. Thus, the aim of the present study was to investigate the influence of the SSC curing process on the formation of ettringite. SSC pastes were produced and subjected to four different curing processes. The hydration process was followed by potentiometry (pH), compressive strength tests, X-ray diffraction, and thermogravimetry (TG/DTG), carried out from 7 to 92 days. The best mechanical strength results were obtained at 58 days, but at 92 days, a drop in strength was observed, accompanied by ettringite instability in all of the curing processes investigated. The results verified the formation of calcium sulfate hemihydrate and calcite, indicating the occurrence of intense natural carbonation of ettringite.