Effect of slag on the improvement of setting time and compressive strength of low reactive volcanic ash geopolymers synthetized at room temperature

This paper presents an investigation on the effect of ground granulated blast furnace slag on the geopolymerization of low reactive volcanic ash. Volcanic ash was blended up to 50 wt% with slag at 10% intervals. The fresh geopolymer samples were cured at 25 and 60 °C for 3, 7 and 28 days. XRD, FTIR, TG and SEM were used for phases analysis. The results outlined that only 10 wt% of slag was enough to reduce the initial setting time of the geopolymer from more than 7 days to couple of hours (6.7 h). At 25 °C, the 28 days compressive strength increased with the addition of slag in the system until an optimum value of about 85 MPA. This strength development was suggested to arise from a synergetic formation of C-A-S-H/N-A-S-H gel. At 60 °C, curing for periods longer than 7 days was not beneficial for strength development. These results are of interest for the valorization of low reactive volcanic ashes in the development of structural geopolymers, with related environmental and socioeconomic benefits. •Geopolymers from low reactive volcanic ash (VA) and slag were prepared.•Only 10% of slag was enough to reduce the setting time from a week to 7 h at 25 °C.•A compressive strength of 85 MPA was obtained at 25 °C for sample made of 50% VA.•At 60 °C, curing for periods longer than 7 days was not beneficial for strength.•Some synergetic effects were observed in volcanic ash/slag geopolymer system.