Geo-environmental engineering is concerned with the contamination remediation, soil reinforcement, and ecological sustainability of problematic soil matrix, including landfilled soils, contaminated soils, sludges, and solid wastes. The emerging biomineralization technologies are gaining increasing attention as potential solutions to geo-environmental issues owing to their in-situ applicability, high efficiency, and environmental friendliness. Among them, the biologically induced phosphate precipitation (BIPP) method has an outstanding long-lasting amending effect and barely has secondary contamination from by-products. This study bridges the gap of a comprehensive overview of biophosphate application in the geo-environmental fields from the perspectives of heavy metal (HM) immobilization, soil reinforcement, and ecological reclamation. This multidisciplinary study can inspire the utilization of biophosphates as an integrated amendment for contaminated soil and waste piles, serving as a soil binder, fertilizer, and HM stabilizer all at once to promote reclamation, ecological sustainability, and carbon sinks. However, the research of biophosphates in geo-environmental technology is still in its infancy. Methods in balancing performances between the strength enhancement, HM stabilization, and vegetation of solid waste need in-depth innovation. Additionally, the long-term interaction mechanisms between these three functions remain unclear. Furthermore, lowering the cost of virgin materials and balancing the economic cost with the long-term environmental benefits of biophosphate technology are required for large-scale engineering practice.

The infiltration of groundwater and surface water is critical for the development of desiccation cracking of earthen site soils. It potentially leads to changes in the microscopic structures of pore water networks, posing a threat to the stability of earthen site soils. In this study, the effects of cementation solution concentrations on water holding capacity of earthen site soils and microscopic characteristics were investigated using the methods of enzyme-induced calcium carbonate precipitation (EICP), volumetric shrinkage and suction tests under wetting-drying loadings, and microscopic tests. The microscopic characteristics regarding pore structures were used to interpret the mechanism of desiccation cracking of EICP-treated earthen site soils from the microscopic perspective. The results show that the rate of drying shrinkage and sensitivity to drying and wetting conditions of earthen site soils are notably reduced by the method of EICP. The volumetric shrinkage curves appear three-stage evolving trends. The microscopic tests show that the treated earthen site soils feature bimodal distributions of pore structures. With this regard, the uniformities of pores are significantly improved, for which large and medium pores of untreated soils are filled by the method of EICP. Small and micro pores are compressed, which reduces the potential connectivity of pores inside earthen site soils. The research outcomes can provide fundamental knowledge for improving desiccation cracking of earthen site soils by the method of EICP.

This research examines the possibilities of utilizing fungi, namely Aspergillus oryzae, to increase sand's shear strength and durability. The experiments were conducted in a laboratory, where the samples were prepared as ideal cylindrical specimens for testing using the Unconfined Compression Test. Some factors that can improve the shear strength of the treated sample are being investigated, including inoculum dose, incubation method, incubation period, and supplementary nutrition (soybean flour or rice flour). The unconfined compressive strength test determines the shear strength of treated materials. Key results demonstrate that the outside incubation method yields better growth of fungi and produces higher shear strength. 10% or 15% of water content is suggested for the sample to be prepared. At 5% water content, the Fungi could not grow properly. Meanwhile, 10%–15% water content yields close shear strength of treated soil during the 28-day curing period, which is about 160 kPa. Moreover, additional soybean flour has a better effect than additional rice flour in the outside incubation method. Finally, soil treated with Aspergillus oryzae has a lower shear strength than soil treated with Rhizopus oligosporus and Rhizopus oryzae when soybean flour is added. With the 10% inoculum dosage, 5% soybean flour dosage, and 10% water content, the shear strength of Aspergillus oryzae sp. Fungi-treated soil could have about 20% lower shear strength than Rhizopus oligosporus and Rhizopus oryzae-treated soil. Moreover, when soybean flour is omitted, soil treated with Aspergillus oryzae performs better than soil treated with Rhizopus oligosporus and Rhizopus oryzae. This highlights Aspergillus oryzae's potential as a more effective alternative for soil stabilization. These findings provide valuable insights into optimizing fungal-based soil improvement strategies, supporting the advancement of sustainable and eco-friendly geotechnical solutions while reducing reliance on conventional chemical stabilizers.

Calcareous sand is a type of marine soil with the poor engineering performance and a high potential of liquefaction under seismic or wave action. To address this issue, microbially induced calcium carbonate precipitation (MICP), an eco-friendly bio-grouting technique, has been shown to effectively improve its liquefaction resistance. However, the particle characteristics of sand can influence both its cyclic behavior and effectiveness of MICP treatment. This study investigates the effects of particle size and gradation on the liquefaction resistance of MICP-treated calcareous sand through cyclic triaxial tests and microscopic scanning tests. The results indicate that increasing the median particle size (d₅₀) enhances the cyclic strength of untreated sand but reduces that of MICP-treated sand. In contrast, increases in the uniformity (C_u) coefficient lead to a reduction in cyclic strength for both untreated and MICP-treated sands, while change in the curvature (C_c) coefficient initially improve and then reduce the cyclic strength. SEM images reveal that the grading characteristics affect the precipitation of calcium carbonate, which in turn affects the cyclic behavior of MICP-treated sand. Additionally, the study discusses the stiffness degradation of MICP-treated calcareous sand under cyclic loading. These findings offer valuable experimental and theoretical insights for enhancing the liquefaction resistance of the MICP-treated calcareous sand with varying grading characteristics.

Interlayer gaps and shrinkage cracks have significant impact to negate the mechanical properties, especially durability, of 3D printed concrete, which registers a lasting challenge hindering practical engineering application of 3D concrete printing technology. To this gap, this study translates the microbial-induced calcium carbonate precipitation (MICP) for casted concrete into a novel microbial community self-healing agent with high calcium carbonate deposition performance for 3D printed concrete. Under microaerobic conditions, the mineralization performance of the microbial community is 3.63 times that of commonly used self-healing bacteria B. pasteurii. The microbial community structure demonstrated that Pseudogracilibacillus is the dominant genus in both aerobic and microaerobic conditions. Under micro-oxygen conditions, relative abundance ofunclassified_Bacillaceae_2 increases sharply to 32.08%–57.11%, registering the dominant genus to jointly promote mineralization. Addition of a microbial community self-healing agent will increase the flowability and setting time of 3D-printed concrete, and thus improving its cracking resistance. The total porosity, including interlayer gaps, is measured to be 0.72% at 1 day curing, which is one-third that of the control group. Compared to the counterparts in control group without the microbial community, the air voids and microcracks, mesopores, and macropores, decrease by 40%, 14%, and 72%, respectively. Among all the different crack depths observed, for cracks with an opening width of approximately 1 millimeter, the repairing area reachs 100% within 7 days of curing. Given the efficient metabolism of facultative bacteria under microaerobic conditions, the average depth of crack repairing is 30 mm, and the maximum depth can reach even 38 mm. It can be stated this research promises to resolve efficiently the long standing micro-defect problem of 3D-printed concrete.

An experimental investigation on the use of mussel shell powder (MSP) as a precursor for Alkali Activated Binders (AAB) is presented in this study. The results of this study opens up new perspectives on the production of biowaste-based binders (mussel shells) for the mechanical improvement of soils. Two alkaline solutions of different chemical composition were considered for this study, namely 12 molar sodium hydroxide solution (12 mol/L NaOH) and a mixture of NaOH and sodium silicate (Na₂SiO₃) solution. Mineralogical and microstructural changes of AAB were monitored over time by means of X-ray Diffraction (XRD), Thermogravimetric analysis (TGA), Scanning Electron Microscopy (SEM) and Mercury Intrusion Porosimetry (MIP). The alkaline environment induced by 12 mol/L NaOH promotes the dissolution of calcium carbonate and the precipitation of metastable gels, then transformed into crystalline carbonate hydrates (i.e., pirssonite and gaylussite). The availability of silica in the system, induced by the use of NaOH+Na₂SiO₃, favors also the development of a second class of reactions (i.e. pozzolanic reactions) responsible for the precipitation of silicate hydrated gels. As a consequence, the binder activated by 12 mol/L NaOH solution showed a reduction of Unconfined Compressive Strength (UCS) over time due to the transformation of metastable carbonate gel into crystals, whereas the binder activated with NaOH+Na₂SiO₃ showed a higher improvement of UCS for the precipitation of stable cementitious compounds. The carbon footprint of alkali-activated mussel shell binder was evaluated and compared with ordinary Portland cement. An insight into the mechanical effects induced by the Alkali Activated Mussel Shells Binders (AAMSB) on a dredged marine sediment was provided, highlighting the role of silicon availability in the alkaline activator on the effectiveness of the treatment.

Enzyme induced carbonate precipitation (EICP) is an emerging eco-friendly soil stabilization technique that produces calcium carbonate precipitation (CaCO₃) using a urease enzyme harvested from plants. The formation of CaCO₃ restricts the soil grain movement, affecting the original moisture variation-induced crack development. This study used indoor drying-wetting cycle simulation tests to examine crack development mechanism, swelling-shrinkage characteristics and lateral swelling pressure of expansive soil along with the related mechanism of EICP repairing and reinforcement effect on the cracks. The lateral swelling pressure of the samples was measured using the confined humidification expansion test setup, and a time history curve of its evolution during the humidification process was generated. The results have shown that, during the EICP crack-repairing and reinforcement process of cracked untreated expansive soil, the EICP solution rapidly infiltrated into the soil along the fracture channel, and finally formed a CaCO₃ solidification network. With the increase in the number of reinforcements after each drying-wetting cycle, the CaCO₃ gradually filled the internal cracks of the soil and enhanced the crack mitigation. The crack development in EICP repaired expansive soil sample could be divided into three stages: crack generation, crack shrinkage, and stability stages. Finally, the obtained data were compared and analyzed to explore the change law of lateral swelling pressure of EICP repaired cracks of the expansive soil, and a prediction model of the peak lateral swelling pressure of EICP-treated expansive soil at different depths was established.

The mechanical properties of clayey soils are intimately linked to Bound Water Content (BWC). Microbially Induced Calcium Precipitation (MICP), as an emerging technology for slope treatment and foundation engineering, has an unclear mechanism in the influence on BWC of soil. Therefore, quantitative analysis of the Thickness of the Bound Water Film (TBWF)-a direct microscale characterization of BWC-holds significant importance. To quantitatively analyze the influence of MICP technology on TBWF, this study proposes a TBWF prediction model based on soil mechanics theory and validates effectiveness through Atomic Force Microscopy (AFM) experiments. Taking Granite Residual Soil (GRS) as the research object, the study revealed that MICP technology significantly reduces TBWF: when the cementation solution concentration was 1.0 mol/L, TBWF decreased from 48.297 nm (untreated control) to 34.561 nm (1.0 mol/L treatment group), a reduction of 28.44%. Further investigations revealed that MICP treatment lowers the liquid limit moisture content of soil while increasing specific gravity, bound water density, and specific surface area. However, when the concentration exceeded 1.0 mol/L, TBWF rebounded due to suppressed urease activity. AFM experimental data showed high consistency with theoretical model predictions, verifying the model’s reliability. This study provides microscopic mechanism support for the application of MICP technology in geotechnical engineering fields such as landslide prevention and slope reinforcement, establishes a new method for quantitative analysis of the bound water film, and holds important significance for improving the effectiveness of geological disaster prevention and control.