Calcium sulfoaluminate cement pastes from industrial wastes: effect of hemihydrate content

In this work, the effects of the variation of hemihydrate and the water/cement ratio were evaluated on the compressive strength and reaction mechanisms in pastes of calcium sulfoaluminate cement (CSA cement) prepared from industrial wastes as fluorgypsum, aluminum dross and fly ash. Based on stoichiometric calculations and the chemical compositions of the raw materials, CSA clinker, with low content of belite was synthesized at 1250 degrees C for 4 h. In general, compressive strength of similar to 40 MPa were reached after 360 days of curing: strength increased as water/cement reduced. Ettringite and gypsum were the main hydration products forming dense microstructures of intertwined crystals; while gehlenite and spinel formed in the CSA clinker did not present any hydraulic behavior. The increment in the hemihydrate provoked lower setting times by an enhanced rate of ettringite formation; however, in some cases it was detected a decrease in strength after 90 days due to the delayed formation of this phase. It was observed the formation of two main exothermic manifestations by isothermal calorimetric tests corresponding to gypsum and ettringite formation; however, an overlap of both processes was noted as the temperature was increased.