Molecular insights into the shear behavior of interstratified illite- smectite clays: Effects of hydration and illitization

WB Niu and CF Zhao and HM Alaoui and ZX Yang and PY Hicher, GEOMECHANICS FOR ENERGY AND THE ENVIRONMENT, 44, 100759 (2025).

DOI: 10.1016/j.gete.2025.100759

Clayey geomaterials rarely occur in a pure mineralogical state in nature and are more commonly found as mixed-layer clays, such as interstratified illite-smectite. These clays consist of varying proportions of illite and smectite layers, which significantly affect their mechanical properties. Under variable mechanical conditions, the shear behavior of mixed-layer clays exhibits considerable complexity, underscoring the need for in-depth investigations. This paper presents a molecular-scale study on the behavior of interstratified illite-smectite minerals, simulating a geotechnical shear setup at the molecular level. Multiple molecular models were constructed to explore the effects of water content and illite layer proportions, effectively replicating stages of the illitization process. The results reveal that the mixed- layer clays exhibit clear stick-slip behavior during shear simulation. Models with low illite content demonstrated relatively similar shear characteristics, while higher illite content led to a significant reduction in nanoscale cohesion and a slight increase in friction coefficient. Pure illite exhibited the highest shear strength among the studied materials, with a friction coefficient and cohesion of 0.111 and 0.172 GPa, respectively. Furthermore, the illitization process was observed to progressively enhance the shear modulus, ranging from 0.63 GPa to 26.81 GPa under various hydrostatic pressures. A statistical analysis was also performed to further examine the stick-slip behavior of mixed-layer clays. These findings provide essential insights into the nanoscale mechanical properties of mixed-layer clay minerals, contributing to a deeper understanding of geomaterial stability in critical applications.

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