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Microbially induced calcite precipitation (MICP) is used as a reinforcement technique in non-cohesive soils. Sporosarcina pasteurii bacteria induce the precipitation of calcite crystals in the pores, which bond grains together and turn sand into a cohesive medium [a,b]. One of the challenges associated with industrial development of the technique is the characterisation of the material durability, and in particular the prediction of how the mechanical behaviour of the biocemented media evolves in acidic environments.
To understand better the interactions between transport, chemistry and mechanics, X-ray tomography dissolution experiments were performed at two different scales, at synchrotron SOLEIL (pixel size 1.3 µm) and Ghent University Center for X-ray tomography (pixel size 7 µm) to evaluate the fluid-mineral and mineral-mineral interfaces. Dissolution flow-through experiments on two-grain biocemented sand samples and on biocemented granular columns were performed under different flow and pH conditions in order to understand the evolution of calcite distribution in space and time, and to evaluate the changes in the contact surface area that creates the cohesion between grains.
Calcite is shown to rapidly dissolve, with a dissolution rate increasing at high flow rate and low pH. The rate of the fluid-mineral interface displacement is quantified and depends on the coupling between chemical reactions and transport close to the fluid-mineral interface. Comparison with dissolution of single crystals of calcite shows different dissolution rate distributions. In particular, the geometry the two-grain samples favors a delayed dissolution of the contacts at the expense of calcite crystals that grow freely at the sand surface. At the column-scale, preferential dissolution pathways develop (Figure 2) at high flow rate and low pH whereas the dissolution front is flat at low flow rate and pH close to neutral. The evolution of the cohesive contact area will be used as input parameter in a Discrete Element Model developed at 3SR [c] in order to predict the evolution of the strength of the material.
References:
[a] J. T. DeJong, B. M. Mortensen, B. C. Martinez, and D. C. Nelson, “Bio-mediated soil improvement,” Ecological Engineering, vol. 36, no. 2, pp. 197–210, 2010.
[b] La Bella M., Sarkis M., Geindreau C., Emeriault F., Fang H., Wright J.P., Noiriel C. and Naillon A. (2025) Insights on the textural and crystallographic properties of calcite obtained through MICP using advanced synchrotron diffraction imaging. Journal of Applied Crystallography 58, 1728-1741, doi: 10.1107/S1600576725007010C.
[c] M. Sarkis, M. Abbas, A. Naillon, F. Emeriault, C. Geindreau, and A. Esnault-Filet, “D.E.M. modeling of biocemented sand: Influence of the cohesive contact surface area distribution and the percentage of cohesive contacts,” Computers and Geotechnics, vol. 149, p. 1048
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