Microbially induced calcite precipitation has garnered significant attention in recent years as a promising and environmentally friendly process for addressing sand stabilization and soil consolidation. While its potential to stabilize sand dunes and mitigate soil liquefaction is well-documented, the applicability of MICP in the consolidation of stone materials has not been thoroughly investigated. Tuffs, which are silicate-laden pyroclasts, have historical significance as hybrid materials for cement, emphasizing their importance in cultural heritage. In previous MICP applications, there has been a lack of exploration regarding the incorporation of urease-producing bacteria (UPB) into Tuffs. Consequently, the potential of MICP in substrates such as Tuffs has been largely overlooked. The primary objective of this study is to assess the feasibility of incorporating UPB into Tuffs as a consolidating agent to facilitate calcium carbonate precipitation. We aim to investigate the microstructure of the resulting polymorphs and the previously unknown mechanism of increased resistance to W-D cycles in the treated samples at the nanoscale. In this study, we compared the cementation response of Tuff when subjected to two different UPBs by evaluating changes in key parameters, including capillary water absorption (CWA), cyclic tests such as wet-dry and salt attack, and uniaxial compressive strength. Furthermore, the characteristics of crust formation were examined using FESEM, and an in-depth analysis of the microstructure of the deposited bacterial CaCO3 was conducted. The mechanism of increased W-D resistance in Tuffs reinforced by MICP has not been previously addressed by researchers. Our study demonstrated that the increase in resistance to W-D cycles was more pronounced in samples treated with Bacillus pasteurii. . Further FESEM analysis showed the presence of Bacillus pasteurii spores in the nanopores of the (W-D) exposed samples. N2 adsorption analysis revealed that nanopore alteration occurred only in samples treated with Bacillus pasteurii. . Additionally, Tuffs are prone to disjoining pressure, which occurs in nanopores. Based on these observations, we postulate that the increased resistance to W-D cycles in the samples treated with Bacillus pasteurii was caused by a decrease in disjoining pressure due to the recrystallization of amorphous calcium carbonate (ACC) in the nanopores of the studied Tuff.

Microbial geoengineering technology, as a new eco-friendly rock and soil improvement and reinforcement technology, has a wide application prospect. However, this technology still has many deficiencies and is difficult to achieve efficient curing, which has become the bottleneck of large-scale field application. This paper reviews the research status, hot spots, difficulties and future development direction microbial induced calcium carbonate precipitation (MICP) technology. The principle of solidification and the physical and mechanical properties of improved rock and soil are systematically summarized. The solidification efficiency is mainly affected by the reactant itself and the external environment. At present, the MICP technology has been preliminarily applied in the fields of soil solidification, crack repair, anti-seepage treatment, pollution repair and microbial cement. However, the technology is currently mainly limited to the laboratory level due to the difficulty of homogeneous mineralization, uneconomical reactants, short microbial activity period and large environmental interference, incidental toxicity of metabolites and poor field application. Future directions include improving the uniformity of mineralization by improving grouting methods, improving urease persistence by improving urease activity, and improving the adaptability of bacteria to the environment by optimizing bacterial species. Finally, the authors point out the economic advantages of combining soybean peptone, soybean meal and cottonseed as carbon source with phosphogypsum as calcium source to induce CaCO3.