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.

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.

Enzyme-induced carbonate precipitation (EICP) is an appealing bio-cementation technology for soil improvement in geotechnical engineering. This study investigated the bio-reinforcement efficacy of sword bean crude urease (SWCU)-mediated EICP and the enhancement effect of various additives on it. A set of sand column specimens with different bio-cementation levels were prepared. Magnesium chloride, sucrose, xanthan gum, sisal fiber, calcite seeds, and skim milk powder were adopted for comparison. Bio-reinforcement efficacy was evaluated by mechanical properties. SWCU possessed a similar to 127% higher specific activity than entry-level commercial urease while saving over 2000 times the enzyme cost. All specimens treated with SWCU-mediated EICP presented excellent moldability and uniformity for one-time treatment. UCS increased exponentially with bio-cementation level due to the uniformly growing CaCO3 content and crystal size. UCS of similar to 1.8 MPa was achieved in a single treatment using 60 g/L SWCU and 3.0 M urea-CaCl2. SWCU exhibited a superior bio-reinforcement efficiency over soybean crude urease, commercial urease, and bacterial urease, since higher soil strength was achieved at lower CaCO3 content. Magnesium chloride showed the most significant enhancement effect, implying an extensive application prospect of SWCU-mediated EICP in seawater environments. The absence of wet strength, markedly elevated dry strength, and notably higher stiffness and hardness at low stress (load) phase indicated that xanthan gum would be more suitable for windbreak and sand fixation in arid/semi-arid environments. Sisal fiber could also effectively improve soil mechanical properties; however, the labor and time costs caused by its premixing with soil should be considered additionally in practical applications.

Enzyme-induced carbonate precipitation (EICP) has emerged as an environment-friendly solution for soil improvement. As a composite material, it is challenging to determine the micromechanical properties of EICP-reinforced sand using common macromechanical tests. In this work, a systematic study was conducted to determine the micromechanical properties of EICP-reinforced sand. The development of the micromechanical properties obtained from indentations along the route of sand particle-CaCO3-sand particle was examined. The width of the interfacial transition zone (ITZ) in EICP-reinforced sand was investigated. The effect of the reaction environment on ductility (i.e., the ratio of elastic modulus over hardness) of CaCO3 was investigated. The experimental results have identified that the width of ITZ in EICP-reinforced sand ranges from 0 to 180 mu m, which is significantly influenced by the crystal crystallinity or crystal morphology of CaCO3. The presence of porous media (i.e., sand particles) leads to the decrease in impurity content in the crystal formation environment, resulting in the lower ductility of CaCO3 accordingly. The mean value of fracture toughness of CaCO3 precipitation was identified to be the lowest one among sand particles, CaCO3 precipitation, and sand particles-CaCO3 interface. The lowest fracture toughness of CaCO3 indicating the failure of biocementation is derived from the CaCO3-CaCO3 breakage.

Enzyme-induced carbonate precipitation (EICP) has emerged as an innovative soil stabilization technology to precipitate CaCO3 by catalyzing urea decomposition. Although extensive efforts have been made to increase the calcium carbonate content (CCC) formed in the EICP process for the better biocementation effect, the cementability and micromechanical properties of CaCO3 are rarely known. A study of the cementitious characteristics and micromechanical properties of CaCO3 precipitates with different mixing percentages of crystal morphology is essential for soil improvement. In the present study, ultrasonic oscillation tests and nanoindentation tests were performed to investigate the cementability and micromechanical properties of CaCO3 precipitate. The results show that the cementability and micromechanical properties of CaCO3 precipitate are related to the composition of the crystal morphology. A high content of calcite is beneficial to improve the adhesion of calcium carbonate precipitate. Calcite has better mechanical properties (elastic modulus, hardness and ductility) than vaterite, and the presence of vaterite can significantly affect the measured value of mechanical properties in nanoindentation tests. The ductility of CaCO3 precipitate induced by crude soybean urease (CSU) is higher than that of CaCO3 precipitate induced by commercially available pure enzyme, suggesting that commercially available pure enzyme can be replaced by CSU for cost-effective field-scale engineering applications. This work can provide insight into optimizing the properties of CaCO3 precipitate from the micro-scale. (c) 2024 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

Enzyme-induced carbonate precipitation (EICP) is an attractive bio-geotechnical technique for soil improvement. As promising alternatives to commercial ureases, legume ureases crudely extracted from primary agricultural products can provide remarkable cost savings. This study investigated the bio-cementation effect of legume ureases with different protein contents on pore-scale, mechanical, and hydraulic properties of EICP-treated sand and revealed the causes, mechanisms, and effects of the bio-clogging induced by high protein level-legume urease. Urease centrifugal liquids of sword bean (JU), pigeon pea (PU), and soybean (SU) were prepared at equal activity of 10 mM/min for sand bio-cementation. Mechanical properties were analyzed based on CaCO3 content and soil strength. Pore-features were revealed by mercury intrusion porosimetry and scanning electron microscopy, and permeability was measured to evaluate the hydraulic properties. Results showed that JU and PU with lower protein content were more effective in multi-cycle EICP-treatments, since denser bio-cemented sands with higher strengths were obtained while being vertically uniform in CaCO3 distribution and pore structure. Conversely, the high protein level of SU induced uneven bio-cementation and surface bio-clogging, resulting in bad mechanical properties, such as low strength and a destruction pattern of bottom collapse. Bio-clogging virtually eliminated the effectiveness of subsequent EICP-treatments. SU exhibited an advantage over JU and PU in reducing soil permeability, as a dramatically lower permeability was achieved at a lower treatment cycle. Comprehensive analysis concluded that the high protein level, salting-out, different precipitation rate between protein and CaCO3, and limited soil filtration capacity were the key reasons for bio-clogging induced by SU.

In this study, enzyme-induced carbonate precipitation (EICP) combined with chitosan curing technology was used to improve the mechanical properties of standard sand, and the curing effect of EICP combined with different chitosan contents was studied by macroscopic tests, such as the unconfined compressive strength test, direct shear test, and calcium carbonate content test, and microscopic tests, such as scanning electron microscope (SEM) and nuclear magnetic resonance (NMR). The results show that compared with the pure EICP treatment, the unconfined compressive strength, shear strength, and calcium carbonate content of the sand treated by EICP combined with chitosan were significantly improved, and increased first and then decreased with the increase of chitosan content, reaching the maximum value when the content is 1.5%. The calcium carbonate content is positively correlated with the strength, indicating that calcium carbonate crystals can effectively play a role in filling and cementation. After the incorporation of chitosan, the shape of calcium carbonate crystals is still mainly spherical, but the number and volume become larger. At the same time, the incorporation of chitosan can greatly reduce the proportion of large pores and medium pores, significantly increasing the proportion of small pores, which greatly improves the pore structure.

Subgrade stability is a key factor that influences the long-term performance of the pavement structure under repeated traffic loading. Enzyme-induced carbonate precipitation (EICP) has been recently explored as a bioinspired solution to improve the mechanical properties of sandy soils. This study evaluates the potential of EICP to cement silica sand as a treated subgrade in pavement applications. The study investigated the mechanical performance of EICP-treated sand compacted at varying initial degrees of saturation and treatment cycles through repeated loading triaxial (RLT) and unconfined compressive strength (UCS). The results show that specimens having a lower initial EICP degree of saturation facilitated efficient carbonate precipitation bridging between sand grains, resulting in higher resilient modulus and UCS. Furthermore, surface percolation of EICP cementing solution resulted in a further increase in carbonate content and treated soil UCS and resilient modulus (Mr). A maximum UCS and Mr values of about 700 kPa and 165 MPa, respectively, were achieved, which implies the possibility of using EICP-treated sand as a treated subgrade layer or subbase to reduce the base layer thickness or improve the pavement structure performance. The Scanning electron microscopy (SEM) images further explained the influence of the carbonate crystal shape and morphology on the treated soil strength and modulus. Finally, simulations executed using AASHTOWare Pavement ME Design software show the potential of utilizing EICP for improving subgrade for pavement structures, with a significant reduction in base layer thickness while maintaining the same rutting and fatigue performance. Hence, demonstrating the potential of EICP in enhancing subgrade properties for pavement structures.