## 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
The change of the crystals from spherical to a needle-like shape is a result of the inhibition of crystal growth in the a and b axes while allowing the same along the c axis in the presence of magnesium [65]. Substitution of Mg\({}^{2+}\) until the Mg\({}^{2+}\)/Ca\({}^{2+}\) ratio became 1.0 increased the intensity of aragonite peaks further showing a mixture of other peaks, including calcite and magnesium calcite (Figure 12d). Subsequently, vaterite peaks disappeared. The corresponding change in the SEM micrographs is the substantial appearance of the needle shaped crystals fanning out into the space in all directions (Figure 11d). These observations clearly revealed that although increments of Mg\({}^{2+}\) did not affect the quantity of bio-induced precipitation to a great deal, the morphology of the crystals was far more sensitive to them. Figure 11: Scanning electron microscopy (SEM) images of microbial induced precipitate by _Sporosarcina_ sp. with (**a**) Mg\({}^{2+}\)/Ca\({}^{2+}\) = 0.00; (**b**) Mg\({}^{2+}\)/Ca\({}^{2+}\) = 0.25; (**c**) Mg\({}^{2+}\)/Ca\({}^{2+}\) = 0.43; (**d**) Mg\({}^{2+}\)/Ca\({}^{2+}\) = 1.00. The precipitation results alone are not adequate to predict the effectiveness of MICP for beach sand solidification in the presence of magnesium. Therefore, syringe sand solidification tests were conducted and estimated UCS are displayed in Figure 13.  
Overall, it can be seen that the strength improvement by MICP treatment is considerably impaired in the presence of magnesium, and specimens without Mg\({}^{2+}\) show the highest strength gain. Evidently, the specimens treated with an Mg\({}^{2+}\)/Ca\({}^{2+}\) ratio of 0.43 exhibit better strength than those with an Mg\({}^{2+}\)/Ca\({}^{2+}\) ratio of 0.25. This can be partly supported by the results of precipitation tests where the highest amount of precipitate was obtained for the Mg\({}^{2+}\)/Ca\({}^{2+}\) = 0.43 condition. SEM microscopic observations were conducted in order to ascertain the effect of crystal morphology on strength enhancement. It is clear that the precipitated crystal morphology changed from rhombohedral to spherical and then to needle shape when the Mg\({}^{2+}\)/Ca\({}^{2+}\) ratio gradually advanced from 0 to 1 (Figure 14). In the absence of Mg\({}^{2+}\), the crystal morphology was exclusively rhombohedral, the typical crystalline form of calcite (Figure 14a). The grain surfaces were fully covered with crystals  
Figure 12: X-ray diffraction (XRD) spectra of the microbial induced precipitate by _Sporosarcina_ sp. with (a) Mg\({}^{2+}\)/Ca\({}^{2+}\) = 0.00; (b) Mg\({}^{2+}\)/Ca\({}^{2+}\) = 0.25; (c) Mg\({}^{2+}\)/Ca\({}^{2+}\) = 0.43; (d) Mg\({}^{2+}\)/Ca\({}^{2+}\) = 1.00. Figure 13: Variation of local strength of MICP treated sand for different Mg\({}^{2+}\)/Ca\({}^{2+}\) molar ratios. of average size 10 \(\upmu\)m and matrix was supported by relatively larger crystals (Figure 14b). The crystals were very well interweaved and crosslinked at the grain contact points thereby effectively bonding the loose sand particles together. When Mg\({}^{2+}\)/Ca\({}^{2+}\) ratio was 0.25 only, the crystals showed slight alterations from rhombohedral form whereas contact cementing and matrix supporting seemed not as significant as in previous case (Figure 14c). As Mg\({}^{2+}\)/Ca\({}^{2+}\) ratio was increased further to 0.43, crystals became spherical and accumulated well together at contact points (Figure 14d). Fibrous needle shaped crystals bundled together were also abundantly seen on grain surfaces as well as on contact cements. This pattern of crystal morphology resulted in a stronger bond between grains thus yielding compressive strengths closer to that without Mg\({}^{2+}\). Stepping up the Mg\({}^{2+}\)/Ca\({}^{2}\) ratio from 0.43 to 1.00 drastically changed the crystal morphology of the bioccent. Grains were slightly cladded by small crystals while pore spaces were completely occupied by needle shaped crystals, some as long as 100-200 \(\upmu\)m (Figure 14e,f).