Low-carbon calcium sulphoaluminate cements synthesized from industrial wastes and by-products

Compared to Portland cements, calcium sulphoaluminate (CSA) cements are energy saving and low-CO2 hydraulic binders, due to the decrease of synthesis temperature and kiln thermal input associated with the reduced limestone requirement and carbon dioxide generation. Further enhancements in terms of environmentally friendly features can be achieved if some industrial wastes are used as partial or total substitutes for the natural materials (limestone, bauxite, gypsum) involved in the CSA cement manufacture. This paper deals with a laboratory investigation of synthetic CSA clinkers generated in an electric oven at 1150°, 1200°, 1250° and 1300°C from raw mixtures containing (i) bottom ash derived from the fluidized bed combustion (FBC) of coal (essentially a source of noncarbonated lime and calcium sulphate) and (ii) anodization mud (AM) or alumina powder (AP), Al2O3 rich by-products originated by the production of anodized aluminium elements and the secondary aluminium manufacture, respectively. Six bauxite-free ternary blends, containing 10-35% limestone plus 0-31% natural gypsum or 0-40% FBC bottom ash plus 0-69% AM or 0-50% AP, and four binary blends, consisting only of 37-60% FBC bottom ash and 52-63% AM or 40-45% AP, were explored. They were generally more suitable than a reference mixture composed by 42% bauxite, 33% gypsum and 25% limestone. XRD analysis showed a complete conversion of reactants and a very good selectivity towards C A S 4 3 , the main CSA cement component.