Effect of Ggbfs and micro-silica on mechanical properties, shrinkage and microstructure of alkali-activated fly ash mortar

Performance of drying shrinkage, flow rate, mechanical properties, and microstructure of three materials - alkali-activated fly ash (FA); ground granulated blast-furnaced slag (GGBFS); and un-densified micro-silica (M) are investigated. Mixtures used herein are referred to as AAM - alkali activated materials - of four types according to composition: AAM-IV, AAM-V, AAM-VI and AAM-VII (note: types 1 through 3 were investigated in previous research) with corresponding mixture FA, GGBFS and M ratios of 47.5/47.5/5, 45/45/10, 42.5/42.5/15, 40/40/20 by weight percentage. The AAM samples were air-cured under a sealed condition for 6 days followed by unsealed curing up to the test at 18-20° C and a low relative humidity of 30-50%. The samples AAM-IV through VII were composed of a progressive decrease of fly ash and GGBFS and an increase in M content. Results show that flow rate and compressive strength increased from AAM-IV to AAM-VI; contrarily, it decreased in AAM-VII. Both flexural strength - a positive aspect - and drying shrinkage - negative as it leads to cracking - increased in all samples. Incorporation of both GGBFS and M in alkali-activated fly ash mortar was found to improve performance compared to that incorporated solely of GGBFS in alkali-activated fly ash. The incorporation of M of certain values improves strength and flowability of AAM; conversely, it results in a higher drying shrinkage value, which leads to increased cracking. SEM and XRD results confirm these results. Unreacted particles of AAM-VI and AAM-VII appear to act as a 'micro-aggregate,' resulting in increased compressive and flexural strength.