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

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 degrees C and then 28 days at 20 degrees 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.