Strength Properties of Slag/Fly Ash Blends Activated with Sodium Metasilicate and Sodium Hydroxide plus Silica Fume

This study presents the strength properties of alkali activated Ground Granulated Blast Furnace Slag (GGBFS)/Fly Ash (FA) blend mortars prepared with sodium meta-silicate and, sodium hydroxide-silica fume combination mixture. The GGBFS/FA ratios were arranged at (100/0, 80/20, 60/40, 40/60, 20/80, 0/100). Water to GGBFS+FA ratio was kept constant at 0.5. Na concentration ratio to GGBFS+FA was kept constant at 6%. These ratios were in mass basis. Three different mixture parameters were used. In the first series of mixture, only sodium meta-silicate was used as activator. In second and third series, sodium hydroxide and silica fume combination was used as activator. Sodium hydroxide and silica fume was mixed with water and used directly in preparation of second series of mortar mixtures. For preparation of third series of mortar mixture, sodium hydroxide and silica fume were mixed continuously with water for three days to allow dissolution of silica fume in sodium hydroxide solution. At first, 3-days compressive strengths of all alkali activated mortar mixture were measured. After that due to the very low compressive strength of mortars made with high volume fly ash content, only 100/0 and 80/20 slag/fly ash ratios were investigated for 7, 14 and 28 days compressive and 28 days flexural strength. The results of the 3 days tests show that decreasing the slag/fly ash ratio decreases distinguishably the compressive strength of the mortars. The mortars produced with the mixture of sodium hydroxide and silica fume combination as activator showed satisfactory results when compared with those activated with sodium meta-silicate.