Properties of natural pozzolan-based geopolymer concrete: effects of natural pozzolan content, type of alkaline activator, and silicate/alkali ratio

This paper reports the effects of key factors on the properties of geopolymer concrete mixtures containing 100, 90, and 85 mass% of natural pozzolan (NP) as primary and 0, 10, and 15 mass% of Portland cement as secondary cementitious materials. Two types of alkaline activators (NaOH and KOH), and three different sodium silicate (SS)/(NaOH or KOH) ratios of 1, 2, and 4 (by mass) were used. NaOH performed better than KOH in the development of compressive strength. While the strength increased with increase of silicate/alkali hydroxide ratio, a decrease in the strength was observed at a higher NP content. Results of the tensile strength, modulus of elasticity, shrinkage, thermal resistance, water absorption, acid resistance and SEM analysis tests conducted on selected mixtures also confirmed the beneficial effects of using a lower NP content activated by NaOH. An optimum mixture of geopolymer concrete, having a NP content of 85% and SS/NaOH ratio of 4.0, is recommended for the construction of structural members requiring high-performance concrete.