Three-dimensional microstructural analysis of MICP cemented sand samples

Two sand samples with different particle D50s subjected to microbial induced calcite precipitation (MICP) treatments are studied. The medium and fine-grained sand samples exhibit similar peak and residual strengths in the untreated state, but yield upto 12 and 2.5 MPa compressive strengths respectively, after cementation with similar bond contents. The geometry of pore space, particle contacts and precipitated bond features are analyzed to explore the reasons behind the observed discrepancy. A method to delineate the continuous pore space into an interconnected network of bulky pore bodies and narrow pore throats is used to extract crucial pore space features like pore body and throat sizes, shapes, and orientation anisotropy, before and after precipitation. Geometrical tortuosity of the two materials is studied to provide a measure of the convolution of available pathways through the pore network. The geometry and spatial distribution of precipitated bonds and bond-grain contacts are also investigated. Analysis shows that the average bulk mass of bonds is not sufficient to determine the level of improvement to strength and stiffness of sands. Bonds exhibit distinctive geometries and spatial distribution patterns when MICP is applied to the different base materials. Similar amounts of treatment to fine and medium sand samples led to a smaller increase in contact area but a larger decrease in pore feature sizes, and a greater increase in tortuosity, for fine sands. This can be attributed to the initial packing and dry density of the samples along with initial volumes, sizes and orientation anisotropies of pore and contact features, which effect the formation of the final bond lattice. The spatial distribution of active bonds and its contribution in increasing the overall contact area of the precipitated microstructure is found to be the key factor translating to macroscopic strength parameters. This also has implications in efficiency and uniformity of treatment within the same sample. This study provides new insights into the evolution in the materials pore space architecture and contact network due to MICP-induced precipitation. The improved tools and understanding forms a new basis for formulating simulation models that incorporate notions of pore and contact mechanics to interpret macroscale phenomena.