Nanoindentation characterization of clay minerals and clay-based hybrid bio-geomaterials

Layered hydrous aluminosilicates are key constituent minerals in rocks, soils, and other parts of the Earth crust. Understanding mechanical properties of these aluminosilicates is crucial for seismic study, stability of parent rocks and geomaterials, and for geophysical subsurface exploration. Recently, growing prospects of clay based nanocomposites have renewed further interest in understanding the fundamental elastic and plastic properties of hydrous aluminosilicates. Their distinct, nanoscale layered crystal structure is known to result in anisotropic responses to loading, however, owing to their tiny sizes; it is a significant challenge to determine the anisotropic properties. There is a little data available in the literature on the elastic and plastic properties of clays and clay-based geomaterials. This Ph.D. research work has undertaken the novel approach to study mechanical properties of geomaterials using a pioneering nanomechanical testing method, nanoindentation, to probe the elastic and plastic properties at nano/micro scale. Genesis of these nanomechanical properties are analyzed in the light of microstructures, interatomic bonds, mineralogical and chemical composition. A nanoindentation study of muscovite mica revealed highly anisotropic behavior. Both elastic and plastic anisotropy exist for indentation normal and perpendicular to the basal plane. Nano-mechanical behavior of the well-ordered, nanocrytalline clay minerals is, mainly, governed by generation and storage of dislocations, formation and annihilation of kink bands and intermittent kink bands. Interpretations of indentation test based on continuum mechanics and gradient plasticity models, therefore, give satisfactory result. On the other hand, chemical, morphological, microstructural and structural characterization of a hybrid clay-lime-starch bio nanocomposite material revealed highly heterogeneous, amorphous and multi-phase matrix. Statistical analysis of the massive volume of data obtained using grid indentation technique revealed that this matrix is composed of five distinct mechanical phases. Thus, this study proposes that slow and weak pozzolanic reaction between clay and lime produces cementitious binder network consisting of C-S-H phase. Interaction of the biopolymer (sticky rice) with clay and C-S-H resulted in intercalated nanocomposites that have superior mechanical and barrier properties. Clay, sand, and silt particles act as active fillers in this composite, whereby forming a dense and compact mass, which renders durability and toughness to this hybrid composite