Intrinsic mechanical properties of calcium aluminate crystals via the linear comparison composite method coupled with nano-indentation

Monocalcium aluminate and monocalcium dialuminate are minerals with a monoclinic crystal structure, that result from the fusion of limestone and bauxite. They are involved in many applications including high alumina cement, advanced biomaterials, or cement-polymer composites such as macro-defect-free cement. Despite several theoretical and experimental studies, predicting the nonlinear behavior of cement-polymer composites remains a challenge. Our research goal is to connect the effective strength behavior to the micro- and nano-constituents for cement-polymer composites, using nonlinear micromechanics, computational mechanics and experimental nano-mechanics. To this end, we extend the linear comparison composite method to non-porous granular materials. The nonlinear strength upscaling scheme is integrated into a finite element model so as to represent the mechanical behavior of calcium aluminate-polyvinyl alcohol nanocomposites-or macro-defect-free cements- across multiple length scales, while taking into account the chemistry, internal friction and grain size. At the sub-micron level, grid nano-indentation tests are carried out. An inverse scheme is implemented to predict the behavior of the basic unit, mono-calcium aluminate and monocalcium di-aluminate crystals, at the molecular level. Meanwhile, a forward approach is selected to predict the uniaxial tensile and compressive strength at the macroscopic level. The theoretical predictions agree well with independent experiments reported in the scientific literature. Macro-defect-free cements owe their extraordinary mechanical properties to a high packing density of calcium aluminate crystals, a granular morphology and the lack of porosity. The inter-disciplinary framework built can be applied to yield fundamental insights into the behavior of a broad class of engineered nano-composites.