Interactions between clay minerals, bacteria growth and urease activity on biocementation of soils

Biocementation is a low carbon footprint soil improvement technique alternative to the use of hydraulic binders such as cement, mainly applied in sandy soils due to the need for vital room for bacteria and easy access for nutrient transport to feed them. This treatment is not suitable for natural clayey soils, however pore clogging caused by the presence of clay in low percentages may not be a problem, and clay minerals can be beneficial to increase the area for bacteria to attach and therefore to precipitate larger quantities of biocement. In addition, clay minerals can also have a positive influence on bacteria activity itself, and this is the novelty of this investigation. The influence of kaolin and bentonite clays on biocementation using Sporosarcina pasteurii was measured by (i) the amount of calcium carbonate precipitated in sandy soil prepared with known percentages of the two clays (1%, 5%, 10% and 20% in dry sand mass), relating it with unconfined compression strength, and (ii) evaluating bacteria growth and urease activity in liquid media with 1% and 5% of each clay. The best enhancement of biocementation was found for kaolin percentage between 5% and 10% and for 5% of bentonite, considering as reference the case where no clay was added. These relatively small percentages were linked to physical aspects related with the room occupied by the clay minerals causing dispersion of the precipitate in the clay matrix, although the presence of clay in the fluids of soil voids could contribute positively for bacteria growth and long-term survival. These results confirm that biocementation treatment may be effective on soils with relatively small clay percentages and opens the possibility that clay can be used in controlled fixed amounts to improve the efficiency of this treatment in clean sands.