Chemo-mechanical changes of low-pH concrete under deep geological repository-like conditions: coupled biotic and abiotic effects

Low-pH concrete (LPC, pH 10–11) represents a non-conventional cementitious material designed for environments where long-term durability and chemical compatibility with surrounding barriers are essential. Its potential use in underground construction, particularly as an engineered barrier material in deep geological repositories (DGR), remains uncertain because the biogeochemical stability of LPC under realistic exposure conditions is insufficiently understood. This study examined microbial colonization, mineral transformations, and mechanical performance of LPC after two years at air, groundwater, and bentonite interfaces. Amplicon sequencing and qPCR revealed interface-specific microbial succession: Streptomyces dominated in air, while Thiobacillus and Hydrogenophaga in groundwater and bentonite systems. Functional pathway analysis indicated progressive microbial adaptation from surface colonization to internal penetration, accompanied by biogenic carbonate and sulfate transformations. Mineralogical analyses confirmed transient formation of vaterite and gypsum. Mechanical testing further demonstrated interface-dependent durability: strongest deterioration at the bentonite interface, moderate degradation under groundwater exposure, and relative stability in air. The results highlight that microbial activity, coupled with environmental gradients, can significantly influence the mineralogy and mechanical performance of LPC. These findings advance the understanding of bio-mediated durability challenges in novel low-pH concretes and provide a framework for evaluating their long-term performance in underground construction applications, including nuclear waste repositories.