Pore-scale precipitation pattern and grain-scale cementation strength by microbially induced calcium carbonate precipitation (MICP)

¹ Korea advanced institute of Science and Technology, Department of Civil and Environmental Engineering, 291 Daehakro 291, Dajeon, South Korea
² University of California, Civil and Environmental Engineering, One Shields Avenue, Davis, CA, USA

^* Corresponding author: t.kwon@kaist.ac.kr

Abstract

Microbial induced calcium carbonate precipitation (MICP) is widely investigated as a sustainable method of soil improvement. The pore-scale precipitation habit of calcium carbonate (CaCO₃) and the grain-scale mechanical strength of the cementing bonds play a significant role in determining the mechanical response of MICP-treated soils. This study presents the pore-scale precipitation patterns in MICP-treated sands and the grain-scale cementation strength of two beads cemented by MICP. X-ray computed microtomography imaging of MICP-treated sands with different grain sizes reveal that the surface area, number of contacts, and flow-induced shear detachment as well as bacterial cell loci have a profound effect on the pore-scale precipitation. With different grain size, the precipitation can show either a contact-cementing pattern or a surface-coating habit. Particularly, evolution of the precipitation pattern is observed for large-sized grains from the surface-coating mode to the contact-cementing mode with an increase in CaCO₃ content. The grain-scale mechanical strength tests on MICP-bonded bead pairs demonstrate that the failure with MICP bonding occurs in three modes: debonding, internal, and mixed failure modes. Amongst, the debonding failure mode has the greatest strength and the internal failure mode shows the lowest strength. The tensile strength is greater than the shear strength in all modes; particularly in debonding failure mode, the tensile strength of ~35 kPa and shear strength of ~13 kPa. The presented results advance our insight into pore-scale and grain-scale behavior of MICP-treated sands, which can be further extended to develop DEM models and transport models.