# Research of Enzyme-Induced Carbonate Precipitation on Strength Behavior of Reinforced Sand
## 1 Introduction  
With rapid urbanization and infrastructure expansion globally, construction in geotechnically challenging environments--such as desertified regions requiring sand stabilization--has become imperative. Typically, the sandy soil prevalent in these areas requires reinforcing through methods like tamping [1, 2] or chemical grouting [3, 4] before being used as construction materials. However, these traditional soil stabilization techniques are associated with considerable energy consumption and a substantial carbon footprint during both material production and utilization phases, which may pose risks of environmental harm and ecosystem disruption [5, 6]. Consequently, bio-cementation has emerged as a sustainable alternative, garnering attention for its eco-friendliness and high efficiency in calcium carbonate yield [7, 8, 9, 10, 11]. Among bio-cementation approaches, Microbially Induced Carbonate Precipitation (MICP) and Enzyme-Induced Carbonate Precipitation (EICP) are widely studied. MICP relies on bacteria (e.g., _Sporosarcina pasteurii_) to hydrolyze urea,producing carbonate ions for CaCO\({}_{3}\) precipitation. In contrast, EICP directly utilizes plant-derived or commercial urease (Figure 1), eliminating bio-safety concerns associated with bacterial use. Moreover, urease's smaller molecular size enhances its mobility in soil pores, potentially improving treatment uniformity [12; 13].  
Recent research has demonstrated that the Microbiology Induced Carbonate Precipitation (MICP) significantly enhances the mechanical properties of sandy soils, including improving strength [14; 15], reducing permeability by blocking pores [16; 17], and mitigating wind erosion [18; 19]. Conversely, the impact of the EICP technique on reinforcing sandy soils has received comparatively less attention. The available studies predominantly indicate that EICP effectively increases the strength of sandy soils [20; 21; 22; 23; 24]. However, the imperfections in the research still limit the application of this technology in geotechnical engineering. For instance, investigations into the micro-mechanisms of EICP-reinforced sand have primarily used scanning electron microscopy (SEM), which does not fully capture the three-dimensional effects of precipitate aggregation and distribution within the soil [21; 25]. Furthermore, the complexity of pore structures can vary even with the same amount of EICP precipitation, affecting the soil's properties. However, quantitative descriptions of pore structure in EICP-reinforced soils and their relationships with soil strength (e.g., fractal dimension) are rare in the literature.  
To address these gaps, this paper investigates the strength behavior and micro-mechanisms of sand reinforced by EICP, evaluating the effects of varying densities and cementation solution concentrations. The paper also provides a comparative analysis of the EICP reinforcement mechanism using SEM, three-dimensional micro-computed tomography (CT) scans, and fractal dimension analysis, aiming to clarify the strength response and underlying mechanisms of EICP-reinforced sandy soil for future engineering applications.