Microbially-Induced Carbonate Precipitation in Low pH Cement; Potential for Self-Healing in Radioactive Waste...

In a geological disposal facility for radioactive waste, groundwater interacts with engineered cement barriers used for waste encapsulation, as a backfill material, and for construction purposes. This study investigates microbial interactions with low-pH cement (pH 10.5 and 11) in synthetic groundwater under anoxic conditions. Microcosm experiments used alkaliphilic microbes obtained from a high-pH lime kiln site. These were incubated with low-pH cement tablets in synthetic Oxford clay porewater with varying levels of electron donors (lactate or hydrogen) or electron acceptors (nitrate). In higher-carbon systems supplemented with lactate, reduction of nitrate was noted over six months, decreasing pH (10.0 to 8.8), presumably due to carbonic acid production. Calcium concentrations also dropped (9.2 to 4.7 mM). SEM-EDS and μ-XCT analyses confirmed that calcium carbonate precipitates had healed cracks and reduced porosity in the cement tablets. However, in the low-carbon system, microbial activity was minimal, leading to smaller pH changes (from 10.0 to 9.4) and greater leaching of Ca²⁺ and Mg²⁺ from the cement. This indicates that less carbonate was produced, and the reduction in pore size over six months was smaller compared to the high-carbon system. 16S rRNA gene sequencing revealed an increased abundance of alkaliphilic nitrate-reducing bacteria, including Anaerobacillus species, which were enriched in high-carbon treatments. Overall, findings suggest that in low-carbon environments, cement will gradually leach cations. By contrast, elevated organic carbon levels (e.g., from cementitious waste) can stimulate microbial activity, enhancing calcite precipitation, crack healing, and pore-clogging, which could improve cement barrier performance and limit contaminant migration.