## 3 Results and Discussion
### Potential for Beach Sand Solidification by MICP Treatment
#### 3.3.2 Effect of Mg\({}^{2+}\) Addition on Microbial Induced Carbonate Precipitation  
Not much investigation have been carried out so far to investigate the effect of magnesium on the microbial inspired carbonate precipitation. Nevertheless, magnesium carbonate possesses similar cementing ability as calcium carbonate, which can significantly affect the desired outcomes of MICP treatment. Figure 10 illustrates the results of MICP test tube tests incorporating Mg\({}^{2+}\). Although Mg\({}^{2+}\)/Ca\({}^{2+}\) molar ratio was raised from 0 to 1, no remarkable change in the amount of precipitate was observed. However, the amount of carbonate precipitate slightly increased up to 0.43 Mg\({}^{2+}\)/Ca\({}^{2+}\) ratio and then decreased as Mg\({}^{2+}\) was further raised until ratio became 1. Therefore, it was deduced that a Mg\({}^{2+}\)/Ca\({}^{2+}\) ratio approximately equaled to 0.5 is the optimum condition in this case. Similarly, in a study by [PERSON] et al. [38] demonstrated that magnesium carbonate has a lower productive rate than calcium carbonate resulting much lower magnesium carbonate precipitation than that of calcium carbonate in any specific condition. Further, [PERSON] et al. [65] claimed that the total amount of precipitation decreases with an increase in Mg\({}^{2+}\)/Ca\({}^{2+}\) ratio from 0 to 2.3. This may be attributed to the higher solubility of magnesium carbonate which is almost one order of magnitude higher than its calcium counterpart and/or the possibility of partial hydrolysis of magnesium carbonate to generate magnesium hydroxide [38]. The size and morphology of the precipitated crystals has serious implications on the success of MICP treatment. Therefore, the microstructure and mineralogy of the precipitates from the aforementioned experimental cases were determined. In the absence of Mg\({}^{2+}\), SEM imagery showed that the precipitate was mainly composed of entirely spherical crystals, some as large as 30-45 um in diameter and agglomerations of relatively smaller crystals (Figure 11a). The XRD spectra revealed that the mineralogical composition was exclusively vaterite (Figure 12a), which is a metastable polymorph of CaCO\({}_{3}\). It is unstable, rapidly transforms into calcite (or aragonite) in aqueous solutions under room temperature, and forms necessarily in the presence of high supersaturation conditions [66]. The urea hydrolysis and resultant local alkalinity rise along with bacterial cells acting as nucleation sites might have developed a high supersaturation  
Figure 10: Variation of microbial induced carbonate precipitation under different Mg\({}^{2+}\)/Ca\({}^{2+}\) molar ratios. in the medium, giving rise to massive vaterite crystallization here. On the other hand, extracellular polymeric substances (EPS) play an important role in polymorph selection [67] and vaterite crystals have been abundantly reported with a high amount of accumulated EPS [68], which is possible in this case too. Moreover, vaterite generation is also preferred with the introduction of various organic matter to the aqueous system [69]. As the Mg\({}^{2+}\)/Ca\({}^{2+}\) molar ratio was stepped up to 0.25, the calcite (and magnesium calcite) peaks emerged at the expense of the proportion of vaterite peaks (Figure 12b). The corresponding change in SEM images was the smaller spherical crystals bursting and becoming fibrous (Figure 11b), as reported by [PERSON] et al. [65]. When the Mg\({}^{2+}\)/Ca\({}^{2+}\) ratio is 0.43, aragonite was also spotted in the XRD spectra (Figure 12c). This is in consistent with the SEM images where the emergence of needle shaped crystals can be clearly seen (Figure 11c). Magnesium inducing the formation of needle/thorn shaped aragonite rather than calcite and vaterite in this manner, either abiotically or biotically, is a very well documented phenomenon in the literature [70; 71; 72; 73; 74].