# Microbial Precipitation of Calcium Carbonate for Crack Healing and Stabilization of Sandy Soils
## 1 Introduction  
Carbonate-forming microbes (CFMs) are capable of decomposing substrates in metabolic processes to produce carbonate, which then reacts with calcium ions to produce calcium carbonate (CaCO\({}_{3}\)) [1]. CaCO\({}_{3}\) precipitation via CFM activity is generally considered a form of induced mineralization because the type of mineral produced depends on the environmental conditions [2]. Microbiology induced calcite precipitation (MICP) usually occurs in open environments, and the process is often related to the microbial cell's surface structure and metabolic activity. Metabolisms that can locally increase carbonate saturation and promote MICP are known to rely on enzymes such as urease and carbonic anhydrase (CA). Ureolysis catalyzed by urease enables microorganisms to use urea as a nitrogen and carbon source [3]. CA promotes carbon transport into cells through CO\({}_{2}\)-HCO\({}_{3}\)\({}^{-}\) interconversion [4]. Both enzymes generate carbonate anions and increase the pH as a consequence of their activity [5]. In addition, previous studies have reported that microbial extracellular polymeric substances (EPS) can trap and bind calcium to promote CaCO\({}_{3}\) precipitation, thus affecting its morphology and mineralogy [6; 7; 8]. MMICP has been employed in a variety of environmental and geotechnical applications [9]. For example, MICP can contribute to the immobilization of contaminants in groundwater, including ionized heavy metals and radionuclides such as strontium [10; 11; 12]. In addition, divalent cations have been found to be substituted for Ca or co-precipitatedwith the CaCO\({}_{3}\) produced by microorganisms. For geotechnical applications, MICP can serve as an environmentally friendly, low-maintenance solution for stabilizing natural biological soils. In previous studies, it has been found that MICP can improve the compressive strength and durability of loose sandy soil [13; 14] and soft clay [15], as well as seal cracks in cement and concrete [16]. Additives, such as Portland cement, have been widely used for the improvement of soil, but recently, the demand for the use of natural additives and the development of eco-friendly processes such as MICP has increased. Natural phyllosilicates, also known as clay minerals, have also been proposed as natural additives for soil improvement to replace Portland cement due to their low costs, high reactivity, and large surface areas. Phyllosilicates are a class of silicates with a characteristic stratified structure formed by tetrahedral (T) and octahedral (O) sheets [17]. For example, montmorillonite (MMT) is a 2:1-type layered silicate consisting of packets of a dioctahedral 2:1 phyllosilicate consisting of two tetrahedral sheets and one octahedral sheet (T:O:T) with adjacent margins. These sheets retain a negative charge, which is neutralized by exchangeable cations such as Na\({}^{+}\) or Ca\({}^{2+}\) located in the interlayer spacing and on the surface [18]. Therefore, MMT has excellent adsorption capacity and high cation exchange and is known to induce biomineralization by interacting with bacteria [19]. In particular, it has been reported that MMT can effectively promote the growth of microorganisms by buffering the pH in biological processes and contribute to microbial nucleation, thus facilitating mineral formation [1; 20; 21]. [PERSON] et al. (2017) demonstrated that the addition of MMT to an MICP process for the stabilization of uranium tailings controls the crystal form of CaCO\({}_{3}\), balances the acidity and ions of the tailings, and reduces toxicity [1]. In addition, when 6% of MMT was added to the uranium tailings, the maximum strength of the cement body reached 2.18 MPa, representing an increase of 47.66% when compared with its absence. Recently, [PERSON] et al. (2021) reported the effects of the addition of Na-MMT to an MICP process on the precipitated minerals and soil mechanical properties [22]. The addition of Na-MMT accelerated the Ca\({}^{2+}\) ion reaction and led to higher levels of CaCO\({}_{3}\).