Understanding the precondition of microbially induced corrosion from surface acidification of alkali-activated slag pastes

Microbially induced corrosion (MIC) has been recognized as a major cause of performance degradation and service life reduction of concrete sewer system. However, the mechanisms for early-stage corrosion and bacterial attachment are not fully understood yet. Aiming at the correlation between surface acidification and the following MIC process, this study investigated the early-stage corrosion of alkali-activated slag (AAS) pastes under organic acid attack (acetic and propionic acids at pH 6 and 4) considering the effect of precursor, alkali content and modulus of activator. The changes in mechanical property, appearance, and microstructure of AAS were analyzed through three-point bending strength test, visual observation, solution and surface pH monitoring, combined with microscale characterizations. The results demonstrate that two organic acids effectively regulate the surface pH of AAS to a range conducive for bacterial attachment. Specifically, pH 4 solutions reduced the surface pH to 4~5 within 35 d, which triggered severe decalcification, hydration product decomposition and pore structure coarsening, leading to a mechanical strength loss of over 64%. In contrast, pH 6 solutions decreased the surface pH to 8~9, but it had a slight impact on the appearance, strength, and microstructure of AAS. At the same pH value, acetic acid solution exhibited slightly stronger corrosivity than that of propionic acid. The findings provide insights into the corrosion process before bacterial attachment and the precondition of MIC on AAS, which facilitates more precisely conducting accelerated simulation tests for large-scale MIC.