Abiotic carbonate precipitation has garnered significant interest as a mechanism for mineral trapping of carbon dioxide (CO2) in geologic carbon storage, as a natural diagenetic process frequently occurring in marine environments, and as an engineering approach for soil improvement. This study explored pore-scale precipitation of calcium carbonate (CaCO3) and its effect on the permeability of porous media, using X-ray computed microtomography (CMT). In a column experiment, CaCO3 was precipitated in a sand pack from a supersaturated CaCO3 solution, while porosity, pore volume fraction of carbonate, and permeability were being monitored and X-ray CMT images were being acquired. Permeability reduction by similar to 99.94% was observed when precipitated carbonate occupied similar to 46-47% of pore volume. The X-ray CMT images showed that carbonate crystals were initially nucleated onto sand grain surfaces, which facilitated subsequent precipitation, indicating a predominantly grain-coating behavior. The scanning electron microscopy revealed the carbonate crystals of similar to 1-20 mu m in size and the presence of internal pores in the carbonate layers at the submicrometer scale. Variations in carbonate layer thickness and geometric tortuosity, and preferential carbonate precipitation behavior with local clogging were examined through morphological analysis and phase segmentation. Particularly, the pore-scale precipitation pattern and hence the pore geometry were found to evolve with continued precipitation from a grain-coating behavior, through a pore-filling behavior, and finally into a dramatic pore-throat-clogging behavior. Our results provide unique experiment data for predictive modeling of long-term CO2 transport and provide new insights into the changes in physical and transport properties during CO2 mineral trapping.