Cementitious composites of pulverised fuel ash and blast furnace slag activated by sodium silicate: effect of Na2O concentration and modulus

Abstract Pastes of pulverised fly ash (PFA) and blast furnace slag (BFS) in proportions of 100–0, 75–25, 50–50, 25–75 and 0–100 (wt-%) were chemically activated using sodium silicate with modulus (SiO2/Na2O) of 0, 0·75, 1, 1·5 and 2; the %Na2O was added at 4, 6 and 8% relative to the binder weight. The pastes were cured for 24 h at 75°C and then 28 days at 20°C. For 100%PFA pastes, the higher %Na2O, the better the strength; 4%Na2O was ineffective, whereas the highest strength of 25 MPa was reached using modulus 1. For 100%BFS pastes, the highest strengths were for 4%Na2O (80–85 MPa); the optimum modulus was 1·5. The addition of BFS to PFA rich pastes was favourable; for 50% blends, the strengths were between the above mentioned figures and the best modulus was 1–1·5 (45–48 MPa). The lower glass content of PFA (70%) compared with the BFS (>90%), together with the porous nature of the PFA, were considered to have reduced the strength in PFA blends.