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.

In the context of global warming, understanding the impact of thaw slump on soil hydrothermal processes and its responses to climate is essential for protecting engineering facilities in cold regions. This study aimed to investigate the effect of thaw slump development on active layer soil. We considered the early thaw slump development in the Tibetan Plateau as research object and conducted long-term monitoring of soil hydrothermal activity in the active layer of various parts of the landslide and the regional meteorology. The results showed that thaw slump development shortened the freezing and thawing time of the active layer, increased the freezing and thawing rates of the shallow soil (10-20 cm), and enhanced the heat exchange between the active layer soil and the atmosphere and the heat transfer between the soils. The heat-exchange efficiency of the active layer, from largest to smallest, was headwall > collapsed area > unaffected area (bottom of the slope) > unaffected area (top of the slope). Furthermore, thaw slump development lowered the water storage of the active layer prof ile and weakened the dynamic response of soil water to precipitation. The events of soil water responses and soil water increments were smaller in the landslide area than in the unaffected area. During a co-precipitation event, the overall soil water storage increment (SWSI) of the profile was significantly smaller in the landslide area than in the unaffected area (P < 0.05), with an SWSI of 2.04 mm in the headwall and 1.77 mm in the collapsed area. In addition, thaw slump development altered the mechanism of soil water transport driven by soil temperature changes, which affected soil water redistribution of profile. The study gives ecohydrology-related research in cold climates a scientific foundation, thereby guiding the construction and maintenance of infrastructure projects.

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.

Salinization of road base aggregates poses a critical challenge to the performance of coastal roads, as the intrusion of chlorine salts adversely affects the stability and durability of pavement structures. To investigate the cyclic behavior of salinized road base aggregates under controlled solution concentration, c, and crystallization degree, omega, a series of unsaturated cyclic tests were conducted with a large-scale triaxial apparatus. The results showed that variations in solution concentration had a negligible influence on the resilient modulus of road base aggregates, and no significant differences were observed in their shakedown behavior. However, the long-term deformational response of the aggregates was affected by the precipitation of crystalline salt. At low crystallization degrees, a significant increase in accumulated axial strain and a decrease in resilient modulus were observed with increasing omega. Once the crystallization degree exceeded a critical threshold (omega(c)), there was a reduction in accumulated strain and an increase in resilient modulus. The precipitation of crystalline salt also disrupted the shakedown behavior of road base aggregates. During the nascent stages of crystallization (omega < 0.33), the presence of fine crystalline powders and clusters in the saltwater mixture destabilized the soil skeleton, resulting in a transition from the plastic shakedown stage to the plastic creep stage. This poses potential risks to the long-term characteristics and durability of the road base courses.

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.

Recently, the biostimulation has received attention due to its sustained mineralization, environmental adaptability and lower cost. In the current study, a series of isotropic consolidated undrained triaxial shear (CU) tests were performed on biocemented soil treated through biostimulation approach to examine the effect of cementation levels on the undrained shear behaviors. The test results demonstrate that the biocementation generated by the biostimulation approach can improve the shear behaviors remarkably, with the observed changes in stress-strain relationship, pore water pressure, stress path, stiffness development, and strength parameters. The variations of the strength parameters, i.e., effective cohesion and effective critical state friction angle, with increasing cementation treatment cycles can be well fitted by an exponential function and a linear function, respectively, while the variation of the effective peak-state friction angle is relatively small. The increased shear strength, stiffness, effective cohesion, and strain softening phenomenon of biocemented soils are related to the densification, increased particle surface roughness, and raised interparticle bonding caused by biostimulation approach. The liquefaction index decreases with the increase in cementation treatment cycles, especially at lower initial mean effective stress (100 and 200 kPa), indicating that the biostimulation approach may be a viable method for anti-liquefaction of soil.

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.