# Research of Enzyme-Induced Carbonate Precipitation on Strength Behavior of Reinforced Sand
## 4 Conclusions  
Soil reinforcement has long been a critical area of study in civil engineering, particularly for improving the mechanical properties of sandy soils, which are often prone to instability. Enzyme-Induced Carbonate Precipitation (EICP) has emerged as a promising eco-friendly soil stabilization method. However, the mechanisms underlying the strength improvement of EICP-reinforced sand remain poorly understood. In this paper, EICP is hypothesized to enhance the strength of sandy soil by bonding soil particles. The strength behavior and micro-mechanism of EICP-reinforced sand were investigated through a series of UCS, SEM and Micro-CT tests. The following key conclusions can be drawn:  
1. As the curing time increases, the strength of EICP-reinforced sand progressively increases, reaching a maximum at 7 days. After this period, no significant increase in strength is observed, indicating a plateau effect. The strength is a composite outcome of the inherent strength of the plain soil and the reinforcement provided by EICP, which is strongly influenced by the relative density (\(D_{\text{r}}\)) of the unreinforced sand and cementation solution concentration (\(C\)). The maximum reinforcing effect occurs when \(D_{\text{r}}\) is 0.7 and \(C\) is 1 mol/L. 2. At a microscale, strength improvement is attributed to the three tier modes resulting from precipitation, namely bonding, bridging, and encapsulating effects of the precipitation, which are influenced by specific crystal morphologies that enhance particle bonding. 3. EICP precipitation contributes to the disintegration of large pores and reduces the total pore volume within the soil. When cementation solution concentration exceeds 1.5 mol/L, small pores tend to be occupied and blocked by the crystals exhibiting an encapsulating effect, which results in a slight increase in strength. This trend substantiates the essence of the strength improvement achieved by the EICP technique, which involved binding particles together and increasing inter-particle friction. 4. As the cementation solution concentration increases, the proportion of connected pores decreases, and isolated pores become more prevalent. This degradation of pore connectivity hinders the formation of continuous microscopic failure surfaces, contributing to the observed strength enhancement. 5. Fractal dimension analysis shows that as cementation solution concentration increases from 0 to 1.5 mol/L, pore toughness and heterogeneity rise from 2.38 to 2.54, showing an increase of nearly 7%, indicating the formation of aggregated particles and higherinter-particle friction. This trend correlates with the observed increase in strength and offers further insight into the strengthening mechanisms. According to the conclusions above, the research hypothesis was proven correct through experiments and analysis. As a novel soil reinforcement technique that is low-carbon and environmentally friendly, EICP has been proven to enhance the strength of sandy soil in a short period of time, which endows it with the potential for application in engineering projects such as foundation reinforcement and embankment construction. However, the current studies on EICP-reinforced sand are still in the laboratory stage and focus mainly on short-term effects. Future work should involve large-scale model tests and investigate the long-term strength changes of EICP-reinforced sand under different environmental conditions. Methodology, J.W. ; Validation, X.L. ; Formal analysis, L.P. and X.L. ; Resources, J.W. ; Data curation, J.Z., S.L. and X.D. ; Writing--original draft, X.L. ; Writing--review and editing, J.W. ; Visualization, X.L. ; Supervision, L.P.; Funding acquisition, J.W. All authors have read and agreed to the published version of the manuscript. This research was funded by National Natural Science Foundation of China, grant number 42202309, 42172299 and Beijing Municipal Association for Science and Technology Golden Bridge Project Seed Project. Not applicable.