Mechanisms and Diagnostic Indicators of Biodeterioration in Mineral Coating–Substrate Systems: A Critical Review

Biodeterioration of mineral coating–substrate systems develops through interactions among microbial colonization, environmental exposure and the physicochemical properties of the coating, interface and substrate. Microbial presence, however, is still often taken as evidence of deterioration even when a causal link with material transformation or loss of performance has not been demonstrated. This critical review examines mechanisms and diagnostic indicators relevant to cement-, lime-, gypsum-, silicate- and polymer-modified mineral coatings. It distinguishes between susceptibility, microbial colonization, biological activity, chemical and mineralogical transformation, microstructural and moisture-transport alteration, and mechanical or functional deterioration. Reported mechanisms include acidification, mineral dissolution, ion redistribution, secondary mineral precipitation, pore opening or filling, moisture redistribution, cracking, cohesion loss and interfacial deterioration. These responses can be non-monotonic and depend strongly on material composition and exposure conditions. Most material-response indicators have limited biological specificity, since similar changes may result from carbonation, salts, moisture, pollutants or aging. Reliable attribution depends on convergence between microbial activity and spatially corresponding material transformation, with plausible abiotic mechanisms considered at the same time. Five evidentiary categories are distinguished: susceptibility, microbial colonization, active microbial process, probable biodeterioration and confirmed biodeterioration. On the basis of current evidence, universal numerical severity thresholds are not justified.