Filler Efficiency Versus Pozzolanic Efficiency: Effects on Hydration Kinetics and Compressive Strength

The efficiency of a quartz-based filler (andesite sand powder) compared to fly ash a pozzolan as a partial replacement for cement has been studied. Both andesite sand powder and fly ash are silica rich material, however, their reactivity and mechanism of reaction by which they influence the properties of cementitious systems are examined. Both andesite sand powder and fly ash were used to partially replace Portland cement at 15% and 30% levels using two w/b ratios of 0.4 and 0.6. The heat of hydration was monitored with an isothermal calorimeter using paste samples while 100 mm concrete cubes were cast to determine the effect on compressive strength. The compressive strength recorded for concrete containing andesite sand powder at 15% content, w/b ratio of 0.4 and 7 days of curing is 63MPa compared to 58MPa for concrete containing fly ash at the same content, w/b ratio and curing age. At 28days of curing, concrete containing fly ash has a strength of 71MPa compared to 70MPa for concrete containing andesite sand powder. The results indicate that the sand being a filler has an early age contribution to strength development with the strength at 7 days being higher than that of fly ash which has a later age contribution to strength development due to its pozzolanic reaction in addition to hydration reaction of Portland cement.