Ziyu Wang

Island reef road construction faces a complex marine service environment characterized by high salinity and high humidity. Meanwhile, rapid construction and prompt subgrade repair are urgently required, creating a strong demand for novel calcareous-sand-based stabilization materials that combine excellent mechanical performance with resistance to seawater erosion. To this end, this study developed an early-strength cemented calcareous-sand reinforcement material for road base construction. Sulfoaluminate cement (SAC) and ferrite-aluminate cement (FAC), both featuring rapid setting/early strength development and superior corrosion resistance, were used to cement calcareous sand (CS) and to investigate its mechanical and microstructural characteristics under different water environments. Unconfined compressive strength tests (UCS) showed that SC-CS and FC-CS could meet subgrade requirements at 1 d and 7 d, with SC-CS and FC-CS reaching 3.12 MPa and 3.44 MPa at 1 d, and 3.26 MPa and 3.67 MPa at 7 d, respectively, under seawater SS conditions. Seawater mixing and immersion were found to promote the early strength and stiffness development of both SC-CS and FC-CS, with a more pronounced effect observed for FC-CS. Based on experimental results, a damage model for the stabilized specimens was established with a fitting accuracy of R² > 0.97. This constitutive model accurately describes the stress–strain relationship of the material and quantitatively characterizes its damage evolution. Microscopic XRD and SEM analyses indicated that the main hydration product in freshwater-cured specimens was ettringite, and the interparticle connection of CS was dominated by bridging through rod-like ettringite. In contrast, under seawater conditions, the ettringite content decreased, while hydrotalcite and calcium aluminate hydrate increased, forming massive and lamellar bridging products. Compared with SC-CS, the bridging structure in FC-CS was denser. Moreover, the compactness of the bridging structure not only affected its mechanical properties but also governed the movement mode of CS particles, thereby influencing the damage evolution and failure mode of the specimens. The findings provide theoretical support for the construction needs of island road.

Combining the Microbial-Induced Calcium Carbonate Precipitation (MICP) technique with plants to reinforce calcareous sand slopes on reef islands is expected to achieve both reinforcement and economic benefits. In this study, MICP was combined with Festuca arundinacea (MICP-FA) for calcareous sand reinforcement. Based on water retention and scanning electron microscopy (SEM) tests, the water retention performance and mechanism of MICP-reinforced calcareous sand under different cementation solution concentrations and cementation cycles were analyzed. The growth adaptability of Festuca arundinacea was evaluated under different bacteria solution concentrations, cementation solution concentrations and cementation cycles. The engineering applicability of MICP-FA-reinforced calcareous sand was evaluated by wind erosion tests, and the synergistic reinforcement mechanism was analyzed. The results show that with the increase in the cementation solution concentration and cementation cycles, more calcium carbonate filled and adhered to the calcareous sand particles, significantly improving the water retention performance. MICP-FA can enhance the wind erosion resistance of calcareous sand. This synergistic mechanism lies in the surface cementation effect of MICP and the deep anchoring effect of plant roots. This study provides theoretical basis and technical guidance for engineering applications in calcareous sand areas.

Microbially Induced Calcium Carbonate Precipitation (MICP) technology has garnered significant attention in geotechnical engineering and environmental remediation due to its environmentally friendly and cost-effective advantages. However, the current MICP technology faces challenges in practical engineering applications, particularly the prolonged cementation time, which makes it difficult to meet the requirements for coastal slope reinforcement. Therefore, this study designed novel cultivation conditions for Sporosarcina pasteurii by regulating external nitrogen source concentration and evaluated its environmental adaptability by measuring OD600, urease activity, and bacterial length. By monitoring the changes in Ca²⁺ concentration, pH, and precipitation rate over time during the mineralization process, rapid cementation under MICP conditions was achieved. The engineering applicability of this approach in slope reinforcement was comprehensively assessed through simulated on-site scouring and penetration tests. The reinforcement mechanism and the microstructure of the cementation under novel cultivation conditions were analyzed using scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), and X-ray diffraction (XRD). The results indicated that the activity of Sporosarcina pasteurii in the modified NH₄-YE medium significantly improved in freshwater environments, and the MICP mineralization reaction was rapid, completing within 4 h. The primary crystal form of the generated precipitate was rhombohedral calcite, which formed a tightly bonded microstructure with calcareous sand, achieving a maximum reinforcement strength of 13.61 MPa. The penetration strength increased by at least 20%, and the precipitation rate improved by at least 2-fold. The scouring morphology remained essentially unchanged within 6 h. The findings of this study provide foundational and theoretical data for the application of MICP reinforcement technology to coastal calcareous sand models.

Calcareous sand, a critical construction material in reef engineering and building foundations, possesses unique internal microstructures and inherent mechanical properties. Given these characteristics, it is essential to thoroughly evaluate its strength under various loading conditions to ensure its reliability in building applications. This study examines the strength, deformation, and failure characteristics of calcareous sand through consolidated drained shear failure tests using a GDS stress path triaxial apparatus. The effects of shear rate, particle gradation, and compactness are systematically investigated to assess their impact on structural stability in building foundations and load-bearing applications. The results indicate that at low confining pressures, calcareous sand exhibits strain softening, whereas at higher confining pressures, strain hardening is observed. For samples with the same gradation, both peak deviatoric stress and failure strain increase linearly with confining pressure. The volume strain evolution during shear follows three stages: shear shrinkage, shear dilatancy, and stabilization. At low confining pressures, dilatancy is favored, while high confining pressures promote shrinkage. Additionally, under constant confining pressure, peak strength increases and failure strain decreases linearly with compactness. Increasing the loading rate from 0.01 to 0.1 mm/min results in a slight increase in the friction angle, with minimal impact on cohesion. Particle gradation plays a significant role in determining the shear strength of calcareous sand, as its effects vary depending on the combination of compactness and gradation. These findings provide valuable insights for the design and construction of stable building foundations, roadbeds, and other load-bearing structures in reef engineering and coastal developments, where calcareous sand is widely used.

Coastal erosion is increasing worldwide due to the increasing frequency of extreme natural phenomena and excessive human exploitation. In this study, a small model experiment was conducted to investigate the solidification effects of three enzyme sources—soybean urease, freshwater Bacillus pasteurella, and seawater domesticated Bacillus pasteurella—on coastal sediments and their impacts in a seawater environment. The solidifying effect of different enzyme sources was determined by measuring the mechanical properties and corrosion resistance of the cured specimen model. The influence of solidified seawater in a seawater environment was obtained by measuring the changes in the pH value, calcium ion concentration, and ammonia nitrogen content of solidified seawater. The results show that different enzyme sources have a certain strengthening effect on coastal sediments. The mechanical properties of coastal sediments can be enhanced by increasing the amount of enzyme solution or level of solidification and can effectively resist simulated flow erosion. Comparing the reinforcement effects of different enzyme sources, it can be seen. It was observed that Bacillus pasteurella acclimated in seawater had better reinforcement effects than Bacillus pasteurella fresh water, and Bacillus pasteurella fresh water had better reinforcement effects than soybean urease. In the seawater measurement tests, the solidification of coastal sediments using different enzyme sources led to a decrease in the seawater pH value, and the acidification of seawater dissolved the generated calcium carbonate, increased the concentration of calcium ions in seawater, and produced ammonia nitrogen as a byproduct in the seawater. It was observed that, compared with the other two enzyme source solutions, the seawater-domesticated Bacillus pasteurella can better adapt to the high-salt environment of seawater, microbial metabolism is not inhibited, urea decomposition ability is improved, and calcium carbonate production is higher, which can effectively improve the engineering characteristics of coastal sediments and play a positive role in coastal protection and development.

To improve the mechanical properties of calcareous sand, it is proposed that microbial induced calcium carbonate precipitation (MICP) technology be used. A series of solidification tests were conducted in natural seawater and freshwater environments. The standard stress path static triaxial apparatus was used to conduct shear tests on calcareous sand and solids under varying reinforcement conditions. The composite power-exponential (CPE) model is proposed to describe the stress–strain relationship curve of the solid, and the method for determining model parameters is presented. The experimental results showed that the strength of calcareous sand with solids increased with the increase in number of reinforcement times for both test environments. Owing to the high salinity of seawater, which inhibits the activity of urease in bacterial solutions, the reinforcement strength in the seawater environment was generally lower than that in the freshwater environment. The compactness had an evident effect on the strength of the added solids. With the increase in compactness, the strength of the sample also increased, but the rate of increase was reduced. The simulation results showed that the established constitutive model can accurately describe the stress–strain relationship of microbial-reinforced calcareous sand and verified the applicability of the model.