Phase transformation and microstructure of in-situ concrete after 20-year exposure to harsh mining environment: A case study

The phase transformations and microstructural properties of a drilled cylinder specimen from a C40 concrete dam used for mining wastewater disposal were evaluated in this study, covering a span of up to 20 years. Various analysis methods, including PT, ICP-MS, XRD, FTIR, TGA, MIP, and SEM-EDS, were employed to assess the performance of powder samples obtained through layer-by-layer grinding. The results obtained from XRD, FTIR, and TGA analyses indicated that the presence of calcite in all ground powder samples, suggesting that carbonation can occur in all layers, even including the inner layers. Moreover, the MIP and SEM-EDS results illustrated that the microstructural properties of the concrete could be influenced by phase transformations induced by various ions present in the mining wastewater, resulting in increased porosity and looser interfacial transition zones (ITZs) in the sample closer to the outer layer. The PT results confirmed the penetration of chloride into the concrete, although the relative chloride content decreased with the increasing depth of the layers. It was observed through TGA analysis that a portion of the chloride ions could attribute to the formation of Friedel’s salts, derived from ettringite or calcium aluminate monosulfate. Additionally, SEM-EDS examination revealed that some chloride ions could bind to the C-S-H gels. The presence of different ions, particularly magnesium ions in the mining wastewater could lead to decalcification of the hydrated gels. These research findings provide significant support for the safety assessment of the concrete structure exposed to mining environments.