Production of low-CO2 cements using abundant bauxite overburden “Belterra Clay”

Abstract

Eco-friendly binders are among the opportunities to reduce the approximately 8% global CO2 emissions generated by the cement industry, with calcium-sulphoaluminate (CSA) -based cements standing as a potential low-CO2 alternative to the world most used ordinary Portland cements (OPC). Belterra Clay (BTC), an abundant low-cost alumina-rich clay overburden on the huge bauxite reserves of the Brazilian Amazon was tested to produce CSA-based binders. Successive Designs of Experiments were used to prepare mixtures containing BTC, CaCO3, and gypsum, seeking the maximum BTC and minimal CaCO3 consumption under the lowest possible temperature to maximize CSA's main phase ye'elimite, Ca4Al6(SO4)O12, in the clinkers. X-ray powder diffraction accompanied by Rietveld phase quantifications, X-ray fluorescence, and scanning electron microscopy were used to characterize the obtained products and guide the experimental work. CSA-belite and CSA-ternesite clinkers were successfully sintered at 1250 °C using up to 42% of BTC. The hydration of selected clinkers was investigated by isoperibolic heat flow calorimetry after blending with gypsum at 95:5 and 90:10 mass ratios. The main hydrations of the clinkers were faster (within 24 h) when using 5% of gypsum. Ettringite was the main hydrated phase, followed by kuzelite, straetlingite, and hemicarboaluminate. Produced mortars reached up to 40 Mpa after 28 days of curing, a strength development comparable to that of an OPC (46 Mpa) tested under the same conditions. BTC, currently a drawback during bauxite mining and rehabilitation of mined areas, was successfully used to produce clinkers that generate ca. 30% less CO2 during calcination when compared to OPC, with further CO2 and energy savings expected because of the required lower sintering temperature.