Soybean urease-induced calcium carbonate precipitation (SICP) is an innovative and eco-friendly approach with demonstrated potential for mitigating soil liquefaction. However, the specific impacts of the concentrations of soybean urease and salt solutions require further elucidation. The research examines how the two compositions influence calcium carbonate formation. Dynamic characteristics of one-cycle SICP-treated clean and silty sand were analyzed based on cyclic triaxial tests. It was revealed that SICP-treated specimens of both liquefied sand and silty sand exhibit reduced accumulation of excess pore pressure and diminished strain growth under cyclic loading, thereby delaying liquefaction failure. Although higher concentrations of both soybean urease and salt solution can enhance liquefaction resistance, salt solution concentration has a more pronounced effect on improving liquefaction resistance due to the more production of calcium carbonate. Scanning electron microscopy observations confirmed the presence of calcium carbonate crystals at the interfaces between sand particles and between sand and fine particles. These crystals effectively bond the loose sand and fine particles into a cohesive matrix, reinforcing soil structure. A direct linear correlation was established between the liquefaction resistance improvement and precipitated calcium carbonate content. Notably, the one-cycle SICP treatment method adopted in this study demonstrates a better biocementation effect compared to cement mortar or multi-cycle MICP-treated sand under the same content of cementitious materials. These findings provide valuable insights for optimizing SICP treatments, aiming to reduce the risk of soil liquefaction in potential field applications.

Enhancing the structural stability of Pisha sandstone soil is an important measure to manage local soil erosion. However, Pisha sandstone soil is a challenging research hotspot because of its poor permeability, strong soil filtration effect, and inability to be effectively permeated by treatment solutions. In this study, by adjusting the soil water content to improve the spatial structure of the soil body and by conducting unconfined compressive strength and calcium ion conversion rate tests, we investigated the effect of spatial distribution differences in microbial-induced calcium carbonate deposition on the mechanical properties of Pisha sandstone-improved soil in terms of the amounts of clay dissolved and calcium carbonate produced. The results demonstrate that improving the soil particle structure promotes the uniform distribution of calcium carbonate crystals in the sand. After microbial-induced carbonate precipitation (MICP) treatment, the bacteria adsorbed onto the surface of the Pisha sandstone particles and formed dense calcium carbonate crystals at the contact points of the particles, which effectively enhanced the structural stability of the sand particles, thereby improving the mechanical properties of the microbial-cured soils. The failure mode of the specimen evolved from bottom shear failure to overall tensile failure. In addition, the release of structural water molecules in the clay minerals promoted the surface diffusion of calcium ions and accelerated the nucleation and crystal growth of the mineralization products. In general, the rational use of soil structural properties and the synergistic mineralization of MICP and clay minerals provide a new method for erosion control in Pisha sandstone areas.

Recycled aggregates (RA) from construction and demolition waste have many shortcomings such as high porosity and low strength due to adhered mortar and defects inside. If the defects (micropores and microcracks) of RA were repaired, the quality of RA could be improved greatly and its application could be further enlarged. Our previous study has proposed a new modification method, enzyme-induced carbonate precipitation (EICP), to repair the internal defects of RA. In this study, the efforts were focused on the optimization of the EICP treatment. It was found that the two-step immersion method, consisting of preimmersing in CO(NH2)2-Ca(NO3)2 solution for 24 h, then adding urease solution at once with single treatment duration of 5 days and cycling two treatments, was the optimal treatment. Compared with the untreated RA, the water absorption and crush value of treated recycled concrete aggregates (T-CA) were decreased by 7.01% and 9.91%, respectively, and 21.59% and 14.40% for treated recycled mixed aggregates (T-MA), respectively. By use of the optimized EICP-treated RA, the compressive strength of concrete increased by 6.05% (T-CA concrete) and 9.23% (T-MA concrete), and the water absorption of concrete decrease by 11.46% (T-CA concrete) and 18.62% (T-MA concrete). This indicates that the optimized EICP treatment could reduce the porosity and improve the strength of aggregates, thus enhancing the mechanical properties and impermeability of recycled concrete.

Tufa, a loose and porous calcium carbonate deposit, is vulnerable to weathering, which can heighten the risk of geological hazards. This study investigated the potential of microbial-induced calcite precipitation (MICP) to stabilize weathered tufa by isolating urease-producing bacteria from Jiuzhaigou, Sichuan Province. Two strains with the highest urease activity, identified as Stenotrophomonas sp. (U1) and Lysinibacillus boronitolerans (U2), were selected for mixed cultures (Mc). The physiological characteristics and calcification capacity of the strains (U1, U2, and Mc), along with the mechanical properties of treated tufa columns (SCU-1, SCU-2, and SCM), were analyzed. The findings revealed that these strains effectively induced the formation of CaCO3. Mc demonstrated strong growth dynamics (OD600 = 3.9 +/- 0.1) and urease activity (865 +/- 17 U/ml), leading to enhanced CaCO3 production. Furthermore, MICP significantly improved the compressive and shear strength of the weathered tufa, with the SCM sample showing superior results compared to SCU-1 and SCU-2. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses confirmed that Mc produced a greater quantity of CaCO3 in the crystalline form of calcite. Overall, the results indicate that MICP represents a promising environmental protection technology that can effectively enhance the engineering properties of weathered tufa.

Microbially Induced Magnesium Carbonate Precipitation (MIMP) technology provides an innovative method for solidifying and stabilizing heavy metal-contaminated soils. However, the mechanical strength and microstructure of the soil following remediation require further investigation. This study evaluates the mechanical properties of zinc-contaminated soil solidified using MIMP technology under varying zinc ion concentrations, cementing solution concentrations, and curing times. Unconfined compressive strength tests, carbonate production tests, and microscopic analyses are employed to assess microstructural changes. The results indicate that MIMP enhances the unconfined compressive strength of red clay, with significantly higher strength observed in samples without zinc contamination than those with zinc contamination. The maximum unconfined compressive strength is achieved at a cementing solution concentration of 1.25 mol/L and a curing time of 15 days, conditions under which the production of magnesium carbonate also peaks. As the zinc ion concentration increases, the unconfined compressive strength of the samples gradually decrease, accompanied by a reduction in magnesium carbonate production. With longer curing times, the unconfined compressive strength increases while the amount of magnesium carbonate rises and stabilizes. Microscopic analysis reveals that MIMP treatment fills internal pores, reducing their number and enhancing the bonding strength between soil particles. The primary mineral composition consists predominantly of hydromagnesite and magnesium carbonate.

Cementations bind sand/soil particles via physical and chemical interactions to form composite solids with macroscopic mechanical properties. While conventional cementation processes (e.g., silicate cement production, phosphate adhesive synthesis, and lime calcination) remain energy-intensive, bio-cementation based on ureolytic microbially induced carbonate precipitation (UMICP) has emerged as an environmentally sustainable alternative. This microbial-mediated approach demonstrates comparable engineering performance to traditional methods while significantly reducing carbon footprint, positioning it as a promising green technology for construction applications. Nevertheless, three critical challenges hinder its practical implementation: (1) suboptimal cementation efficiency, (2) uneven particle consolidation, and (3) ammonia byproduct emissions during ureolysis. To address these limitations, strategic intervention in the UMICP process through polymer integration has shown particular promise. This review systematically examines polymer-assisted UMICP (P-UMICP) technology, focusing on three key enhancement mechanisms: First, functional polymers boost microbial mineralization efficacy through multifunctional roles, namely microbial encapsulation for improved survivability, calcium carbonate nucleation site provision, and intercrystalline bonding via nanoscale mortar effects. Second, polymeric matrices enable homogeneous microbial distribution within cementitious media, facilitating uniform bio-consolidation throughout treated specimens. Third, selected polymer architectures demonstrate ammonium adsorption capabilities through ion-exchange mechanisms, effectively mitigating ammonia volatilization during urea hydrolysis. Current applications of P-UMICP span diverse engineering domains, including but not limited to crack repair, bio-brick fabrication, recycled brick aggregates utilization, soil stabilization, and coastal erosion protection. The synergistic combination of microbial cementation with polymeric materials overcomes the inherent limitations of pure UMICP systems and opens new possibilities for developing next-generation sustainable construction materials.

In this article, the mechanical properties and frost resistance of soil solidification rock (SSR) recycled coarse aggregate concrete (RCAC) prepared by using SSR as a total replacement for ordinary silicate cement were investigated, based on which bio-mineralisation was used to improve the properties of recycled aggregate (RCA) in SSR RCAC as a means of improving the performance of SSR RCAC. The results showed that the mineralisation modification by Bacillus pasteurii enhanced the apparent density of RCA by 3.5%, reduced the water absorption by 20.4% and decreased the crushing value by 17.6%. SSR RCAC prepared using mineralised RCA increased its compressive and flexural strengths by 91.2% and 33.3%, respectively, at the age of 28 days, and maintained 93.5% relative dynamic elastic modulus after 225FTCs, with a 100% enhancement in frost durability factor compared with the untreated group. Although the slow early hydration of SSR resulted in low initial concrete strength, the combination of biomineralisation enhanced the early compressive strength growth by about 140%. It increased the post-freeze-thaw compressive strength residual to 67%. The SSR RCAC proposed in this study provides a solution with both environmental benefits and engineering applicability for infrastructure such as roads and bridges in seasonal permafrost regions.

The disposal of tailings in a safe and environmentally friendly manner has always been a challenging issue. The microbially induced carbonate precipitation (MICP) technique is used to stabilise tailings sands. MICP is an innovative soil stabilisation technology. However, its field application in tailings sands is limited due to the poor adaptability of non-native urease-producing bacteria (UPB) in different natural environments. In this study, the ultraviolet (UV) mutagenesis technology was used to improve the performance of indigenous UPB, sourced from a hot and humid area of China. Mechanical property tests and microscopic inspections were conducted to assess the feasibility and the effectiveness of the technology. The roles played by the UV-induced UPB in the processes of nucleation and crystal growth were revealed by scanning electron microscopy imaging. The impacts of elements contained in the tailings sands on the morphology of calcium carbonate crystals were studied with Raman spectroscopy and energy-dispersive X-ray spectroscopy. The precipitation pattern of calcium carbonate and the strength enhancement mechanism of bio-cemented tailings were analysed in detail. The stabilisation method of tailings sands described in this paper provides a new cost-effective approach to mitigating the environmental issues and safety risks associated with the storage of tailings.

This study investigated the effectiveness of enzyme-induced carbonate precipitation (EICP) technology in remediating Pb- and Zn-contaminated sand. The research focused on the immobilization of heavy metals and the enhancement of sand strength. Experimental results demonstrated that urease activity increased linearly with enzyme concentration, stabilizing at 100 g/L with an activity of 18 mmol/min, and reached a peak at a pH of 8. Temperature variations also positively impacted urease activity, and effective remediation levels were achieved at standard room temperature. The EICP method effectively transformed heavy metal ions from a mobile exchangeable state to a stable carbonate-bound state, and removal rates exceeded 80% for Zn2+ and 90% for Pb2+ after three treatment cycles. Furthermore, the technology significantly improved the unconfined compressive strength of contaminated sand, increasing Pb-contaminated sand strength to 0.57 MPa and Zn-contaminated sand strength to 0.439 MPa. These findings highlight the potential of EICP technology as a viable solution for the remediation of heavy metal-contaminated sand, offering both immobilization of contaminants and enhancement of sand mechanical properties.

Microbially induced carbonate precipitation (MICP) is an eco-friendly technique for weak soil reinforcement. In this study, Sporosacina pasteurii was used to strengthen silty sand after multigradient domestication in an artificial seawater environment. The efficiency of MICP was investigated by carrying out a series of macroscopic and microscopic tests on biocemented silty sand specimens. It was found that the salt ions in seawater impacted bacterial activity. The best activity of the bacterial solution in the seawater environment was achieved after five-gradient domestication, which was approximately 8% lower than that in the deionized water environment. The significant effects of domesticated bacteria on silty sand reinforcement were demonstrated by the content of precipitated carbonate and the unconfined compressive strength (UCS) of the treated specimens. The seawater positively impacted the MICP procedure due to the roles of calcium and magnesium ions, indicated by the X-ray diffraction spectra. The scanning electron microscopy (SEM) results showed that carbonate precipitations distributed primarily on the surfaces and near the contact points of the soil particles, contributing to the soil strength. The cementation solution concentration and injection rate significantly influenced the content and distribution of carbonate precipitations and UCS of the biocemented silty sand, and the values corresponding to good reinforcement efficiency were 1.0 mol/L and 1.0 mL/min, respectively. The results of consolidated undrained triaxial tests showed that the mechanical properties of treated specimens were influenced by biocementation cycles. It was found that the stress-strain behavior of biocemented samples changed from strain hardening to strain softening when the number of reinforcement cycles increased. The peak strength of silty sand was increased by 1.9-3 times after 5 times MICP treatment. The effect of biocementation cycles on the shear strength parameters could be represented by relating the effective friction angle and effective cohesion of biocemented silty sand to the carbonate content.