This study aims to evaluate the performance of low calcium fly ash based geopolymer (FA-GPm) and alkali-activated slag based mortars (AASm) in an aggressive sewer environment. Specimens were extracted from field exposure, after 6 and 12 months. Visual, chemical and physical analyses were performed to evaluate the deterioration. Surface pH was evaluated to identify the microbial induced corrosion (MIC) stage in each specimen. The neutralization depth of the specimens was measured by using the phenolphthalein indicator. Extensive microstructural analyses were carried out to assess the extent and type of deterioration of gel matrix by using techniques such as scanning electron microscopy (SEM), energy dispersive X-Ray (EDX), X-Ray diffraction (XRD) and Raman spectroscopy. Results showed greater depth of neutralization and surface pH reduction in FA-GPm as compared to AASm. Further investigation on penetration of sulphur confirmed much higher diffusion in FA-GPm as compared to AASm. XRD and SEM with EDX analysis showed the crystallization of thenardite and gypsum within the matrix of FA-GPm and AAS mortar, respectively, which was also, confirmed by Raman spectroscopy results. Extend of overall matrix deterioration was much higher in FA-GPm as a result of thenardite crystallization as compared to AASm after 12 months of exposure
