Comparative analysis of silt, kaolinite, and montmorillonite particle effects on bio-cementation in sandy soils

Efficiency of microbiologically induced carbonate precipitation (MICP), an eco-friendly ground improvement method, relies heavily on soil composition, particularly the type and percentage of fine particles. Past research has predominantly focused on pure sands, with limited studies on the effects of a single type of fine, mainly silt or kaolinite clay, in spherical particle sands. Consequently, there remains a lack of fundamental knowledge regarding how different fines impact MICP differently. This study addresses this gap through a novel comparative analysis of the impacts of silt, kaolinite, and montmorillonite on MICP in two sands with spherical and angular particles. Laboratory column tests were performed on sands containing 0%, 2.5%, and 7.5% fines. All fines (up to 7.5%) improved the unconfined compressive strength (UCS) of MICP-treated samples. Silt achieved the most uniform cementation and highest UCS increase (∼7 times). Kaolinite was the least effective, causing the highest bio-cement heterogeneity. The mode of cementation—whether contact or surface—was primarily governed by the host sand properties when fine content was low (≤2.5%), but was influenced by fines’ characteristics in addition to characteristics of the host sand at higher fines content. Findings reveal that a greater cement content does not guarantee higher strength in MICP-treated sandy soils, highlighting the critical role of fines properties and other governing factors in strength development.