LONG-TERM BEHAVIOR OF HYDRAULIC BINDERS BASED ON CALCIUM SULFOALUMINATE AND CALCIUM SULFOSILICATE

Hydration, physico-mechanical and carbonation studies have been carrie d out on the ternary system of calcium sulfoaluminate (C(4)A(3) (S) ov er bar), calcium sulfosilicate (C5S2 (S) over bar) and anhydrite (C (S ) over bar), synthesised by single firing at 1200 degrees C. The raw m aterials used were natural materials, viz. limestone, bauxite and clay , and also industrial process wastes and by-products, viz. phosphogyps um, fly ash and blast furnace slag. These clinkers containing calcium sulfoaluminate and calcium sulfosilicate can be produced at about 1200 degrees C, i.e. at temperatures 200-250 degrees C lower than OPC clin ker. The fired clinker is relatively soft and friable, it is easy to g rind, and consequently the whole process results in considerable savin g of energy. The ground clinker hydrates rapidly and has good cementit ious 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 t ested 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 co ntaining fly ash. All the compositions had very good dimensional stabi lities, 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 ac celerated carbonation, about 67% of the original mechanical strength o f the specimens was still retained. Apart from energy saving, the use of industrial process wastes assists in the reduction of the cost of t he raw materials, the clinkering process, and the cement produced. Lar ge-scale utilisation of industrial process wastes and by-products also contributes to the reduced exploitation of natural resources. The dec reased generation of CO2 during firing can contribute to the reduction of the greenhouse effect.