Effect of additives on the compressive strength of geopolymerized fly ash

Geopolymer cements and geopolymer resins are newly advanced mineral binders that are used in order to reduce the carbon dioxide generation that accompanies cement production. The effect of additives on the compressive strength of geopolymerized class-F fly ash was investigated. Blast furnace slag, calcium hydroxide(Ca(OH)₂), and silica fume powders were added to fly ash. A geopolymeric reaction was initiated by adding a solution of water glass and sodium hydroxide(NaOH) to the powder mixtures. The compressive strength of pure fly ash cured at room temperature for 28 days was found to be as low as 291 kgf/cm^-2, which was not a suitable value for use in engineering materials. On the contrary, addition of 20 wt% and 40 wt% of blast furnace slag powders to fly ash increased the compressive strength to 458 kgf/cm^-2 and 750 kgf/cm^-2, respectively. 5 wt% addition of Ca(OH)₂ increased the compressive strength up to 640 kgf/cm^-2; further addition of Ca(OH)₂ further increased the compressive strength. When 2 wt% of silica fume was added, the compressive strength increased to 577 kgf/cm^-2; the maximum strength was obtained at 6 wt% addition of silica fume. It was confirmed that the addition of CaO and SiO₂ to the fly ash powders was effective at increasing the compressive strength of geopolymerized fly ash.