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

Low-pH concrete (LPC) 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 of exposure to bentonite, air, and groundwater interfaces. Amplicon sequencing and qPCR revealed interface-specific microbial succession: Streptomyces dominated under air, while Hydrogenophaga and Thiobacillus prevailed 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 (XRD, SEM, EDS) confirmed transient formation of vaterite and gypsum and interface-specific redistribution of carbon and calcium. Mechanical testing further demonstrated interface-dependent durability: strongest deterioration at the bentonite interface, moderate degradation under groundwater exposure, and relative stability in air.