Effect of silica fume on fly ash based geopolymer mortar with recycled aggregates

Abstract

Geopolymer mortar is obtained by replacing cement by a cementatious material like fly ash, ground granulated blast furnace slag or silica fume with the addition of alkaline solution and silicate solution as activators for binding instead of water. In the current study, effect of silica fume on fly ash based geopolymer mortar using recycled aggregates is studied in two series. In the first series, only fine aggregates are replaced by recycled fine aggregates in various percentages and in the second series, along with replacement of M Sand by recycled fine aggregates in various percentages silica fume (10%) is added. In both the cases, compressive strength and tensile strengths are determined for 7 and 28 days where the samples are cured at 32 °C, i.e., ambient temperature. The 7-day compressive strength had an increasing and decreasing pattern for 0% silica fume. The tensile strength increased till 50% replacement by recycled aggregates and decreased with further increase in replacement of recycled aggregates. For 28 days, the compressive strength increased for control mix and decreased with further increase in replacement of recycled fine aggregates. The tensile strength shows the same pattern as in the 7-day strength. These patterns may be due to high water absorption in recycled aggregates which does not permit the liquid content to be available for geopolymerization. As the ambient temperature was preferred to cure the specimens, in the initial stages high temperature was not available for aiding geopolymerization which may also be a probable reason for the strength pattern. Addition of silica fume did not considerably increase the strength as expected initially. The mortar mix considered for the current study can be used as medium grade masonry mortar.