Sulfuric acid resistance of fly ash based geopolymer concrete

The results indicate that the inclusion of OPC (as fly ash replacement) improves the compressive strength of fly ash based geopolymer concrete specimens significantly whereas it did not have a similar effect on its resistance to sulfuric acid. The increase in compressive strength for the unexposed geopolymer concrete specimens was due to the additional calcium hydrated products which co-existed with alumina-silicate polymer structures. On the other hand, for the specimens exposed to sulfuric acid for 365 days, the inclusion of OPC at 10% showed the maximum retained compressive strength of around 52% of the strength value achieved for unexposed specimens at the same age. However, OPC inclusion beyond 10% decreases the ability of geopolymer concrete specimens to retain compressive strength. Maximum deterioration was observed when fly ash was replaced by OPC at 30%. This was due to the formation of additional calcium sulfate which increased with the increase in calcium products in the mixture. Microstructural changes were also observed for the exposed specimens at 365 days and confirmed the presence of sulfur compounds as a major cause for the deterioration.