Influence of pore-scale heterogeneity on the precipitation patterns in Microbially Induced Calcite Precipitation (MICP)

The principle of tailoring material properties to improve the mechanical behaviour of soils through compaction or cement grouting dates to the 60s. The increasing trends of urbanization worldwide require new solutions for the development of resilient and sustainable infrastructures. During the past decades, a new field has emerged, pushing the boundaries of ground improvement towards new frontiers. This is no other than the field of biogeotechnologies, which is commonly referred to the biomediated soil improvement technology Microbially Induced Calcite Precipitation (MICP) or biocementation. MICP harnesses microbial activity that facilitates the formation of calcium carbonate precipitates that fill the pores and can improve the mechanical properties of the soil. Despite significant steps towards the characterization and application of MICP, less is known about the effect of the available pore network of various base materials on the MICP process. This is considered to be fundamental towards understanding and addressing precipitation inhomogeneities, which result from flushing porous media with bacterial suspensions and calcifying solutions to induce the formation of biocementing binders.The present thesis focuses on the effect of pore-scale heterogeneity on the precipitation patterns and deposition in MICP. Observations at the pore-scale were coupled to macroscale observations of chemical reaction efficiency and permeability. This multi-scale approach was achieved with the use of experiments in meter-long microfluidics and time-lapse microscopy. By means of this novel experimental setup that encompassed periodic pressure measurements, and an image processing algorithm that was developed in the context of this thesis, the spatiotemporal evolution of MICP along distance of the treated medium and the resulting permeability change were investigated. By applying the same MICP injection strategy in triplicates in the two microfluidic replica of homogeneous and heterogeneous porous media of same porosity, the results revealed that despite a similar bacterial distribution across the whole chip, the chemical reaction efficiency was higher in the heterogeneous than the homogeneous porous medium. This higher chemical reaction efficiency in the heterogeneous porous medium stems from a combination of higher number and slightly higher average size of crystals, as well as a higher precipitation rate in the heterogeneous than the homogeneous porous medium.Through a second set of experiments, the effect of pore network heterogeneity on the deposition of calcite during MICP in glass bead columns that comprised of three distinctive granulometries of increasing gradation, namely a uniform, poorly-graded and well-graded one was investigated. The analysis was conducted by combining bench-scale experiments with chemi-cal monitoring, micro-computed tomography and numerical simulation of absolute permeability using the reconstructed 3D volumes. Collectively, the results suggested distinctive precipitation patterns in porous media that were subjected to the same MICP treatment due to their intrinsic structure.