Effect of citric acid on calcium sulfoaluminate cements synthesised from industrial wastes at low temperature

A study of the effects of citric acid on the mechanical properties of calcium sulfoaluminate (C₄A₃S̄) cements in hydraulic pastes and mortars was performed. Clinker made up of C₄A₃S̄ was obtained from the calcination of a mixture of fly ash, aluminium dross and fluorogypsum at 1100°C. The C₄A₃S̄ cement was prepared by mixing the clinker with certain percentages of commercial hemihydrate. Hydraulic pastes and mortars were prepared with 0·05% citric acid relative to the total weight of the cement in order to slow down the reactions and also to act as a structural modifier. The initial and final setting times of the pastes were up to 189 and 336 min, respectively. The compressive strength was evaluated in specimens cured at room temperature, reaching, in some mortar formulations, more than 80 MPa at 360 d with high initial development. Hydration reactions were significantly affected by both the citric acid and the increase in temperature, as confirmed by isothermal conduction calorimetry. The main hydration product was ettringite, which promoted the formation of dense and compact microstructures.