Development of geopolymer concrete mixes with ambient air curing

Mix design for geopolymer concrete is yet to evolve. Though geopolymer concrete has made the much required transition from laboratory to the field, due to inherent difficulties such as huge variation in the chemical composition of the raw material, the sensitivity of this composition and the chemicals involved in the fresh and hardened properties have led to most of the mix development through trials with the actual materials. The aim of the work reported in this paper was to design an optimum mix proportion for geopolymer concrete using fly ash and ground granulated blast furnace slag (GGBS) as an aluminosilicate source, which has good workability, slump retention, and compressive strength without special curing requirements. A total of 10 mixes were prepared with varying fly ash to GGBS ratios. Ambient curing for 28 days was adopted for all mixes; however, heat curing at 85°C for 24 hours, followed by ambient curing, was adopted for one mix alone. It was clear that with the increase in GGBS content, there was a reduction in setting time, and the mix set faster. However, with more GGBS, greater strength was achieved at a relatively milder alkali activator solution. When the GGBS content increased beyond a certain level, the slump retention time reduced to levels where the mixes were unfit for practical applications. However, it can be concluded that with a judicious choice of fly ash and GGBS proportions, a strength of as high as 60MPa can be achieved in geopolymer concrete without any special curing methods.