Long-term behaviour of hydraulic binders based on calcium sulfoaluminate and calcium sulfosilicate

Abstract

Hydration, physico-mechanical and carbonation studies have been carried out on the ternary system of calcium sulfoaluminate (C 4A 3S̄), calcium sulfosilicate (C 5S 2S̄) and anhydrite (CS̄), synthesised by single firing at 1200 °C. The raw materials used were natural materials, viz. limestone, bauxite and clay, and also industrial process; wastes and by-products, viz. phosphogypsum, fly ash and blast furnace slag. These clinkers containing calcium sulfoaluminate and calcium sulfosilicate can be produced at about 1200 °C i.e. at temperatures 200–250 °C lower than OPC clinker. The fired clinker is relatively soft and friable, it is easy to grind, and consequently the whole process results in considerable saving of energy. The ground clinker hydrates rapidly and has good cementitious properties. Good early (1 day) paste strengths of 35–50 MPa, and 28-day strengths of 58–76 MPa were obtained, when the materials were tested in a wet state. Dry testing gave 50–80 MPa strengths after 1-day curing, and up to 103 MPa after 28 days curing for the composition containing fly ash. All the compositions had very good dimensional stabilities, similar to OPC pastes. The dried pastes had good resistance to atmospheric carbonation to 1 year, although the ettringite component of the specimens tended to carbonate. However, even after extensive accelerated carbonation, about 67% of the original mechanical strength of the specimens was still retained. Apart from energy saving, the use of industrial process wastes assists in the reduction of the cost of the raw materials, the clinkering process, and the cement produced. Large-scale utilisation of industrial process wastes and by-products also contributes to the reduced exploitation of natural resources. The decreased generation of CO 2 during firing can contribute to the reduction of the greenhouse effect.