1:1 and 2:1 clay mixtures under controlled hydration: Experiment and molecular dynamics analysis

Clay minerals are among the most widespread minerals on Earth, and understanding their hydration and mechanical behavior, both individually and in combination, is of great practical interest. In this study, we combined compaction and uniaxial-compression experiments with finite-particle molecular dynamics simulations to clarify the hydro-mechanical response of 2:1 montmorillonite, 1:1 kaolinite and their mixtures. At the macroscopic scale, each soil exhibits an optimum water content at which dry density and compressive strength reach their respective maxima. Atomistic modeling reproduces these strength fluctuations and reveals the governing mechanisms: collapse of expandable interlayer voids in montmorillonite, rigid face-edge bonding in kaolinite, and mutual “wedge filling” in the blended system. Water at the plastic-limit content lubricates contacts and fills residual pores, whereas excess water separates clay platelets and enlarges porosity. The resulting stress-strain curves consistently rank strength as consolidated, plastic limit, fully saturated, with the largest gap observed for pure montmorillonite. The mixture shows moderate swelling yet retains an almost linear stiffness, indicating that adding 20%−40% kaolinite can optimise bentonite by increasing density, limiting expansion and enhancing long-term structural integrity.