Strengthening mechanisms in cement-stabilized kaolinite revealed by cross-scale nanoindentation

The macroscale mechanical properties of cement-stabilized soil have been widely reported, but the microscale behavior remains largely unexplored. This paper presents the first attempt to reveal the microscale strengthening mechanisms in a cement-stabilized kaolinite via big data cross-scale nanoindentation coupled with X-ray diffraction, scanning electron macroscopy, and upscaling analyses. A high-purity kaolinite clay was treated by 15 wt% Portland cement, and the 28-day-cured specimens were investigated to unravel the primary hydration of the cement, secondary reactions between cement and kaolinite, and the resulting constituents and microstructure of the solidified composite. Massive depth-dependent Young's modulus data, obtained by statistical nanoindentation with continuous stiffness measurements, were processed by three different deconvolution techniques including probability density function, cumulative distribution function, and Gaussian mixture modeling. Results from compositional, microstructural, and micromechanical analyses show that two mechanically distinct phases exist in the final composite: a relatively homogeneous fine-grained matrix consisting of solidified cement hydrates-kaolinite mixture, and relatively stronger, coarse-grained inclusions as fillers that are made of pure, cement hydrate aggregates randomly distributed within the fine-grained matrix. Furthermore, the cement hydrates function as two distinct roles in strengthening the kaolinite: (1) bonding the platy kaolinite particles at the edge surfaces and (2) forming the stronger constituents as aggregates embedded in the matrix. Upscaling analysis based on four micromechanical models further validates the above nanoindentation results and inclusion-matrix microstructure. Such an improved understanding of the strengthening mechanisms is expected to shed light on the practical applications of cement stabilization for soft clays.