Enhanced Nitric Acid Resistance of Slag-Fly Ash Based Geopolymers through Optimization of Mix Proportion and Alkali Content

Geopolymers, as sustainable construction materials, offer a considerable reduction in carbon dioxide emissions when compared to Portland cement. To date, comprehensive investigations examining how slag-fly ash based geopolymer (SFG) composites perform under nitric acid exposure remain limited, especially regarding the impact of blend ratios and alkali content on the corrosion mechanisms. Therefore, this research systematically examines how the slag/fly ash blend ratio and alkali content influence the nitric acid resistance of SFG mortars. The deterioration under acid attack was evaluated through visual inspection, corrosion depth measurements, and mechanical strength tests before and after exposure. The mechanisms of performance enhancement and degradation were further revealed by analyzing hydration/erosion products (XRD, DTG) and microstructure (MIP, SEM). The findings indicate that adding a moderate amount of FA (10%) significantly improves the nitric acid resistance of SFG mortars. This enhancement is mainly attributed to the formation of N-(A)-S-H induced by FA, which crosslinks with C-(A)-S-H gel generated from slag hydration to form a more acid-resistant C-(N)-A-S-H gel, effectively blocking nitric acid penetration pathways. Increasing alkali content (4%-8%) also improves resistance, as higher alkalinity facilitates the release of Si and Al species, promoting the development of acid-resistant gel phases. Conversely, excessive alkaline activator concentrations cause over-extraction of aluminum and silicon ions, which suppresses N-(A)-S-H and C-(N)-A-S-H gel development, consequently diminishing post-acid exposure strength retention. These findings offer both fundamental insights and practical recommendations for enhancing geopolymer durability in acidic environments, contributing to the engineering application of low-carbon binders in aggressive environments. • Slag-fly ash based geopolymer retains up to 94.4% strength after 90 d HNO 3 immersion. • FA induces C-(N)-A-S-H crosslinking, refining pores and impeding HNO 3 ingress. • Excess FA or alkali over-leaches Si/Al, weakening gel network and durability. • XRD, TG-DTG, MIP, SEM link microstructure evolution to improved acid resistance.