Synthesis and properties of a belite–calcium sulfoaluminate cement obtained using only waste materials

Belite–calcium sulfoaluminate (BCSA) clinkers are usually obtained at temperatures lower than 1350°C by burning a raw meal composed of limestone, clay, natural gypsum and bauxite. The use of limestone quarry waste, dredged sediments, water potabilisation sludge and flue gas desulfurisation gypsum as complete substitutes for natural raw materials was investigated. Two BCSA clinkers based on natural raw materials and on waste materials, respectively, were synthesised in a laboratory electric furnace. Two BCSA cements were prepared by blending the clinkers with natural anhydrite at the same calcium sulfate/ye'elimite ratio, and their hydration behaviour was investigated. The BCSA cement made from the waste materials showed a faster early reaction than the one prepared from natural raw materials. At later ages, both cements developed a similar hydrate assemblage, consisting mainly of ettringite, AFm phases and strätlingite. Also the physical properties, namely compressive strength, porosity and volume stability, were rather similar, with the exception that late compressive strength (after 56 days) was significantly higher for the cement made from waste materials. The results show that it is possible to manufacture BCSA clinkers entirely from local waste materials.