Sintered and unsintered pressed fly ash geopolymer: A comprehensive study on structural transformation in nitric and sulfuric acid

Acidic attacks contribute to the degradation of cementitious materials, diminishing the structural service life and increasing the requirement for maintenance of the structure. To address the limited understanding of the impact of the sintering process on the acid resistance of pressed geopolymer, an investigation and comparison of the acid resistance of room-cured (RPG) and sintered (SPG) pressed geopolymer was performed. Specimens were immersed in 3 % and 8 % nitric (HNO3) and sulfuric (H2SO4) acids for 7 and 28 days. Despite the higher sorptivity, SPG demonstrated better mechanical strength retention than that of RPG. Specifically, the compressive strength of SPG after 3 % of HNO3 immersion for 28 days increased (+14.2 %), surpassing the control specimen, while RPG experienced a 14.5 % strength drop. The strength increment in SPG was due to the further geopolymerization during acid immersion. In RPG, a new crystalline phase of NaNO3 was observed after immersion in 8 % HNO3 for 28 days. In contrast, SPG showed no evidence of NaNO3 formation, indicating lower reactivity with HNO3 compared to RPG. Additionally, both RPG and SPG exhibited gypsum formation after immersion in H2SO4. The presence of gypsum induced crack formation in RPG, whereas SPG, with its intensive cross-linked structure, effectively compensated for gypsum expansion, preventing crack formation. This finding is crucial for practical applications where exposure to aggressive acids is a concern, as it provides a method to enhance the acid resistance of geopolymer structures.