Microbial-induced calcite precipitation(MICP) is a newly emerged ground treatment technique in geotechnical engineering to treat the soil with poor engineering properties. However

differences on the particle size may occur in different types of soil

which will have influences on the properties of treated soil. In this study

three different ranges of particle size are adopted to be treated by MICP method; and then the effect of particle size on the strength of bio-cemented sand is investigated based on the unconfined compressive strength(referred to as strength hereafter)

porosity and calcite content of each sand specimen from macroscopic level

which are obtained via unconfined compression test

porosity measurement and acid treatment

respectively. Meanwhile

this effect is also analysed at the mesoscopic level according to the images of scanning electron microscope (SEM). The research results show that calcite crystals are precipitated on the surface of sand particles

as well as in particle-particle contacts. Calcite crystal clusters will be formed as the MICP process continues; meanwhile

its size increases with the superposition of calcite crystals. The pore space of sand with smaller particle size can be tightly filled and a higher proportion of effective calcite crystals can be obtained. Hence

the bio-cemented sand is able to present a higher strength due to the stronger structure established by the calcite crystal clusters.