Effect of temperature on the strength development of mortar mixes with GGBS and fly ash

The concrete mixes used were of 28-day mean strengths of 50 and 30 MPa and also had partial Portland cement (PC) replacement with ground granulated blast-furnace slag (GGBS) and fly ash (FA). These mixes were the ones used in a UK based project which involved casting of blocks, walls and slabs. The strength development of “equivalent” mortar mixes was determined in the laboratory for curing temperatures of 10, 20, 30, 40 and 50 °C. High curing temperatures have a beneficial effect on the early age strength but a detrimental effect on the long term strength. GGBS has been shown to be more sensitive to high curing temperatures than PC and FA and this is reflected in its higher “apparent” activation energy. The accuracy of strength estimates obtained from maturity functions was examined. The temperature dependence of the Nurse-Saul function, i.e. the concrete strength gain rate varies linearly with temperature, was not sufficient to account for the improvement in early age strengths resulting from high curing temperatures. The Arrhenius based function, on the other hand, overestimated them because of the detrimental effect of high curing temperature on strength starting from a very early age. Both functions overestimate long term strengths as neither accounts for the detrimental effect of high curing temperatures on the ultimate compressive strength.