## 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
Apparently, these thin crystals, individually fanning out in all directions over the void spaces, had a very low binding capacity compared to the rhombohedral/spherical crystals accumulated together at grain contacts. Further, thin needle shaped crystals can be easily subjected to destruction on application of compressive forces compared to the rhombohedral or spherical crystal aggregates. These results are in good agreement with that of [PERSON] et al. [73] who showed that the occurrence of needle cement in the presence of higher concentrations of magnesium deteriorates the compressive strength of the bio-mortar prepared by the common ureolytic bacterium _Sporosarcina pasteurii_. However, the majority of the studies done to address this aspect has revealed that magnesium addition enables the bioccent to have a higher strength, lower porosity, and better resistance to acids compared to control samples in absence of  
Figure 14: SEM images of variation in crystal morphology with different Mg\({}^{2+}\)/Ca\({}^{2+}\) molar ratios: (a,b) Mg\({}^{2+}\)/Ca\({}^{2+}\) = 0.00 and its closer view; (c) Mg\({}^{2+}\)/Ca\({}^{2+}\) = 0.25; (d) Mg\({}^{2+}\)/Ca\({}^{2+}\) = 0.43; (e,f); Mg\({}^{2+}\)/Ca\({}^{2+}\) = 1.00 and its closer view. magnesium [19; 38; 65; 70; 75; 76]. The general understanding from the above studies is that the inherent properties of aragonite, such as higher specific gravity and Mohs hardness than those of calcite and morphology which provides a denser, compact arrangement, can be the possible reason for better results. The exact reasoning to this discrepancy in results are not fully discovered yet and need more investigation. Another interesting observation that should be noted here is the local strength variation along the sand column. Unlike the results presented under the Section 3.3.1, the estimated compressive strength increases with sand column depth. This can be attributed to the differences in particle size gradation of the two sands (sand 1 and sand 2) used. \"Sand 1\" is a well graded sand with a considerable amount of fines percentage while \"sand 2\" is a comparatively coarser uniformly graded sand. As the sand becomes finer and well graded, the sand becomes densely packed, void spaces become narrower and fewer, and the sand offers more particle-to-particle contact. The movement of bacteria through the sand column is obviously through the pore throats between grains either by self-propelled movement or by passive diffusion, and the geometric compatibility between the size of bacteria and that of the pore throat is a crucial factor [12]. In a dense particle arrangement like in sand 1, bacteria may be filtered out and cell concentration may reduce from top to bottom, whereas in a loose particle arrangement like in sand 2, bacteria may be easily flushed out with daily dose of cementation solution and retained at the bottom [44; 50]. This can lead to a better cementation on top in sand 1 and on bottom for sand 2. This dictates that distribution of bacteria is a critical factor in the process. Many researchers that have attempted to exploit the effect of grain size and gradation have presented similar results [58; 77; 78; 79]. However, [PERSON] et al. [58] stated that availability of oxygen plays the governing role rather than particle size distribution, thus efficacy of MICP is reduced with depth irrespective of the grain sizes. The pertinent reason for contrasting results may be that the _Sporoscrina_ sp. strain used in this study has a greater tolerance to oxygen depletion than the _Bacillus megaterium_ strain used in the aforementioned study, as the column length is only in centimeter scale.