Effects of nano-silica on the strength development of geopolymer cured at room temperature

• Effects of nano-silica on strength and microstructure of geopolymers were studied.
• Early-age strength of ambient-cured fly ash geopolymers increased by nano-silica.
• Dense microstructure with better interlocking morphology was resulted by nano-silica.
• Use of OPC or GGBFS with nano-silica and fly ash yielded CSH or CASH and NASH.

Use of nano-silica is gaining wider attention due to its significant effect on the microstructural and mechanical properties of Portland cement based binders. Effects of the incorporation of nano-silica in fly ash based geopolymer binders have been investigated in this study. Low-calcium fly ash was used as the principal source of aluminosilicate and it was blended with either blast furnace slag or Portland cement at small percentages in order to accelerate the curing at room temperature. Nano-silica was used at a rate up to 3% of the total binder in order to understand its effect on the strength and microstructural development. The experimental results show that the strength and microstructural properties could be further developed with inclusion of nano-silica in geopolymer mixes. Compressive strength increased with the increase of nano-silica content up to 2%. Scanning electron microscopy (SEM) images showed denser microstructures with well-connected interlocking morphology for the optimum nano-silica dosage. The strength increase is contributed by densification of the microstructure with the addition of nano-silica.