This study investigated the impact of optimum dosages of nano-calcium carbonate (nano-CaCO3) and nanosilica on the engineering behavior of black cotton soil. The desired percentage of nano-addition, 2%, for both nanomaterials, was determined by analyzing the plasticity-compaction characteristics and the relative strength index values of treated samples. The study unveiled that the entire clay microstructure was transformed into a nanocrystalline matrix after treatment. The deviatoric strength enhancement with confining pressure and curing period was significant after treating the soil with either nano-CaCO3 or nanosilica. The nanosilica treatment was found to be more effective in improving the California bearing ratio (CBR) strength of black cotton soil samples compared with nano-CaCO3 stabilization. The addition of nanomaterials induced the formation of nanocrystalline hydrate gels and silica gel, resulting in an increased resistance to volumetric deformation under compressive stresses. The hydraulic conductivity of nano-treated samples dropped due to the highly tortuous networks between pores in the nano-crystalline structure. The experimental results were substantiated by analyzing the microstructure of nano-treated soils using X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform infrared (FTIR) techniques.

Background This study investigated the effect of B. Subtilis bacteria on the properties of cement mortar. This was done by using soil samples from Sharkia, Egypt, to isolate 48 bacterial strains, after which they were cultured using the Johnson method and various media. Bacteria were then added to the cement mortar in amounts of 5% and 10% by weight to evaluate their effect on the mechanical and chemical properties of the modified mortar. Results The study examined the compressive and flexural strength of the modified mortar over time, as well as its microscopic properties and chemical composition after 28 days. The results indicated that bacterial additions of 5% and 10% increased the compressive strength of the mortar after 28 and 56 days compared to the control. A 5% bacteria concentration resulted in significant improvements in strength, showing the best concentration for increasing mortar strength. The addition of 5% bacteria significantly enhanced the early flexure strength, while the 10% showed superior long-term strength after 56 days. Scanning electron microscopy (SEM) revealed high CaCO3 deposits in the bacterial samples, indicating microbial-induced calcite precipitation that filled the small cracks and increased strength. Fourier-transform infrared spectroscopy (FTIR) confirmed the presence of hydroxyl, carbonate, and silicate groups, with bacterial samples having a higher carbonate content, indicating an increase in calcium carbonate formation and microstructure. Conclusions The ideal bacterial concentration was 5% as it improved the compressive and flexural strength while also promoting a more flexible microstructure. This study supports the employment of microorganisms in the production of more durable and environmentally friendly building materials, enhancing the sustainability of building practices.

The variable characteristics of weathered marl cause engineering problems, particularly in geotechnics, and require indepth studies to design structures. Where these characteristics are poor, lime is used to stabilize the soil. Experimental research is being carried out on various Tizi-Ouzou marls composed of different percentages of CaCO3. The aim is to study their behavior in the presence of quicklime and its impact on the evolution of their geotechnical characteristics to provide effective and economical solutions for stabilization. These marls are mixed with increasing percentages of lime and subjected to a series of tests in which cyclic shear is essential for simulating dynamic effects. The results obtained confirm the improvement in their geotechnical characteristics. On the one hand, interstitial pressures and cyclic deformations have decreased, thus avoiding the risk of liquefaction, subsidence, or settlement. On the other hand, cyclic stresses and resistances have increased, resulting in better resistance of these stabilized marls to dynamic stresses. Finally, the number of cycles required to reach failure has increased, thus reducing the risk of pavement damage. These results depend primarily on the percentage of CaCO3 in the marl.

Enzymatically induced carbonate precipitation (EICP) is an emerging and eco-friendly technology, which is considered a green alternative to traditional cement in soil stabilization. When stabilizing soil use one-phase grouting method, the activity of urease is often adjusted, leads to changes in the composition and cementitious characteristics of CaCO3. Previous studies primarily focused on the mechanical properties of solidified soil samples, while the production and cementitious characteristics of CaCO3 influenced by the control of urease activity is rarely discussed. This study investigated production and cementitious characteristics of CaCO3 under different reaction environment (pH adjustment, temperature adjustment, and addition of inhibitors), during which the urease activity tests, pH tests, CaCO3 production tests and ultrasonic oscillation tests are conducted. Meanwhile, the morphological characteristics and mineral composition of CaCO3 are revealed through Scanning Electron Microscope-Energy Dispersive Spectrometer (SEM-EDS) tests and X-ray Diffraction (XRD) test. The results demonstrate that all three one-phase grouting methods can delay the production of CaCO3 at early-stage of EICP, while the pH should be maintained above 4 to prevent significant urease deactivation. The CaCO3 generated in EICP mainly consists of calcite and vaterite, and the size of CaCO3 increases with the urease activity increased. The cementitious characteristics of CaCO3 is mainly determined by the percentage composition of calcite and vaterite, where higher vaterite content results in weaker cementitious characteristics. This study provides insights for evaluating the cementitious characteristics of CaCO3, which is beneficial for guiding the promotion and application of one-phase grouting method.

Microbially induced carbonate precipitation (MICP) utilizing a urease active bioslurry is an ecofriendly method that can improve soil strength. However, the micromechanisms, such as ion diffusion, production rate of CaCO3, porosity, and permeability of pile reinforced by bioslurry, require further investigation. In this study, both biopile model tests and a coupled fluid-flow, solute transport and biochemical reactive model were conducted to analyze the mechanical property and biocementation mechanism of pile formed by urease active bioslurry. Results showed that the simulated CaCO3 content along the biopile length after 120 h grouting was close to test results. The UCS of the biopile decreased from 3.44 MPa to 0.88 MPa and the CaCO3 content decreased from 13.5% to 9.1% with increasing depth. The largest reduction in CaCO3 content was observed in the middle part of the biopile as the CaCO3 crystals in the upper part hindered the downward transport of the cementation solution. The morphology of CaCO3 crystals was influenced by cementation solution concentration, as evidenced by the predominance of spherical vaterite crystals in the upper part of the biopile and rhomboidal calcite crystals in the middle and lower parts. During the grouting process, the concentration of calcium ions and urea decreased, while the ammonium ion levels increased with depth due to the utilization of calcium ions and urea for CaCO3 precipitation and ammonium ion production. The production rate of CaCO3 first increased rapidly to reach a peak value and then decreased. The porosity and permeability demonstrated both linear and nonlinear decreasing trends as the CaCO3 concentration increased. The largest reduction in porosity and permeability, reaching 20% and 58% in the biopile top.

Maintaining concrete structures by using bacterial agents to repair microcracks is a promising strategy for maximizing their lifespan. A non-ureolytic and alkali -tolerant B6 strain, newly isolated from paddy soil, was tested for microbiologically induced calcium carbonate precipitation (MICP) performance along with its antibacterial activity for pathogen removals. Whole genome and bioinformatic analyses showed that our B6 strain could be designated as Bacillus altitudinis with its 16S rDNA and average nucleotide identity. Field emission scanning electron microscopy (FE -SEM) and X-ray diffractometry (XRD) revealed that the B6 strain could form vaterite and produce extracellular polymeric substances, thereby contributing to excellent biofilm formation, even under high pH conditions. The MICP in the B6 cells, inoculated on cracked mortars, could repair microcracks (0.3 mm) within 14 days. The presence of vaterite and survivability by the B6 cells inside the healed area was verified using energy -dispersive X-ray spectroscopy and FE -SEM. Antibacterial activity was determined using the supernatant of the B6 cells grown in rich media at pH 8 and showed that the cell -free extract could kill Grampositive bacteria, including Staphylococcus aureus and Enterococcus faecalis, by damaging their cellular membranes. Thermostable thiocillin, a putatively secreted antibacterial compound, was identified using biosynthetic gene cluster analyses for secondary metabolites and chemical analysis using reversed -phase high-performance liquid chromatography (RP-HPLC/UV) and ultra -performance liquid chromatography (UPLC)-mass spectrometry (MS). Our data demonstrated that the MICP and antibacterial activities of the B6 strain could be promising components for repairing microcracks and controlling the pathogen contamination of concrete.