Microscopic phase identification of cement-stabilized clay by nanoindentation and statistical analytics

Abstract

The mechanical properties of cement-stabilized clay (CSC), are significantly inferior to that of concrete and mortar due to the presence of clay minerals. To reveal the interaction between clinker's functional constituent and clay minerals within the CSC matrix to promote the performance of stabilized clay, large data sets cross-scale nanoindentation coupled with thermogravimetric analyses were involved. Two commercially available clays (i.e., kaolin and bentonite, whose dominant clay minerals are kaolinite and montmorillonite, respectively) stabilized by two key functional constituents of cement clinker, i.e., tricalcium silicate (C3S) and tricalcium aluminate (C3A), were investigated to unravel the primary hydration of the cement clinker, pozzolanic reactions with clay minerals, and the microstructure's evolution. Large data sets of elastic modulus and hardness derived from the nanoindentation tests were statistically investigated by two deconvolution methods: probability density function and cumulative distribution function. The results indicated that calcium silicate hydrates (C-S-H) and calcium aluminate hydrates (C-A-H) were the major reaction products for C3S-stabilized and C3A-stabilized clays. However, during the secondary pozzolanic reaction process, the dissolved colloids from the clay minerals and the hydrated products would react and accumulate at the interface between the primary cementitious paste and clay particles to form a new soft, porous phase. The deconvolution and thermogravimetric analysis also indicate that more pozzolanic products and high-density (HD) porous phases were detected in bentonite than in kaolin, suggesting that the pozzolanic reaction in montmorillonite is much more pronounced than that in kaolinite. This understanding can clarify the interaction between clay minerals, hydration products, and distinct CSC for traditional cementitious materials.