Hot sulfuric acid-resistance of fly-ash-based geopolymer paste product due to the precipitation of natroalunite crystals

In the final exhaust chimney of oil refinery plants, new refractory materials based on geopolymers that have high resistivity to hot sulfuric acid solution, and thus long life-times, are in high demand. In order to investigate the resistance to hot sulfuric acid, a hardened cylindrical fly-ash-based geopolymer paste product (15-mm diameter, 40-mm height) with an average compressive strength of 24 MPa was placed into a Teflon vessel filled with 1.0-M sulfuric acid solution. After heating at 60-220 °C for a fixed duration, the microstructure and compressive strength characteristics were investigated. The geopolymer paste product had no damage after treatment with hot sulfuric acid at various temperatures, although hardened ordinary Portland cement bodies swelled and surface parts broke away after exposure to the same sulfuric acid solution. After the geopolymer paste product was immersed in sulfuric acid at 180 °C for 192 h, the average compressive strength was 25 MPa. With hot sulfuric acid solution treatment, natroalunite (NaAl3(SO4)2(OH)6) formed inside the geopolymer product as a novel crystal phase, which led to an increase in compressive strength by the accompanying densification. The fly-ash-based geopolymer paste product is a candidate material for chimney refractories of oil refinery plants.