Investigation of ettringite instability in supersulfated cement: curing influence

With the exception of water, concrete is the most consumed material in the world in terms of mass. An annual increase of 50% in the production of Portland cement (PC) is expected by 2050. Considering that the production of one ton of Portland cement emits an almost equivalent amount of carbon dioxide (CO2), the search for sustainable materials has renewed interest in the supersulfated cement (SSC), since it is constituted by blast furnace slag (up to 90%), calcium sulphate (10-20%) and a low amount of alkaline activator (up to 5%). However, despite presenting advantages such as reduced environmental impacts caused by exploration and heating of its raw materials, good durability in chemically aggressive environments and low heat of hydration, research is still needed regarding its long-term physical and chemical characteristics. It is known that its main hydration products are ettringite (Aft) and hydrated calcium silicate (C–S–H). Ettringite is responsible for conferring resistance to CSS at early ages and matters regarding its instability throughout the curing process are little explored in the literature. Thus, this study aimed to investigate the influence of CSS's curing process on its formation. For this purpose, CSS pastes were produced, which were subjected to four different curing processes. The hydration process was followed by pH measurements, mechanical resistance tests, X-ray diffraction (XRD) and thermogravimetry (TG/DTG), from 7 to 92 days. The best mechanical strength results were obtained at 58 days for all systems evaluated, with a considerable drop in strength at 92 days. The hydration products were unstable at 58 and 92 days in all the curing processes investigated, with formation of hemihydrate at the expense of ettringite.