Textural Modeling and Multiscale Mechanical Evolution Mechanisms of Calcium Aluminate Silicate Hydrate

Calcium aluminate silicate hydrate (CASH) is a principal binding phase in Al-doped cement systems, with the upper limits of macroscopic cementitious performance ultimately determined by its intrinsic nanoscale mechanical properties. Understanding the multiscale structure-property relationships is essential for guiding the design of improved CASH performance.The unclear submicron structure of CASH as a critical bridge between nanoscale behavior and macroscopic properties hinders the understanding of the evolution of multiscale mechanical properties. This study was to develop a CASH texture model from single crystals to polycrystals and to reveal the impacts of coupled Ca/Si and Al/Si ratios and water states on its physical and mechanical properties.

An upscaling algorithm was developed to remap coarse-grained models to all-atom models, overcoming barriers in constructing submicron textural structures, bridging experimental observations and molecular simulations, and providing some insights into CASH. This approach could enable the exploration of emergent cement phenomena in scales up to 100 nanometers with millions of atoms, offering a realistic representation of CASH pore structure, packing density, water content and state, as well as surface roughness. The resulting submicron-sized CASH was composed of randomly packed nanostructural units, reproducing structural features that were consistent with experimental observations. Molecular dynamics simulations were employed to reveal the multiscale structural evolution, the coupled effects of Ca/Si ratio and Al/Si ratio as well as the influence of multiple water states on the physical and mechanical properties of CASH.

The results indicate that a structural homogenization serves as a fundamental mechanism driving the transition of CASH from nanoscale anisotropy to submicron isotropy, with a disordered packing mitigating the interlayer fragility of single crystal CASH. When the Al/Si ratio increases from 0.05 to 0.10, Al–O bonds facilitate silicate chain polymerization, resulting in an enhancement of 15.40% in tensile strength. The submicron-sized CASH structure achieves an optimal mechanical performance at a Ca/Si ratio of 1.50 and an Al/Si ratio of 0.10. Further increasing the Ca/Si ratio leads to a reduction in strength due to a decrease in bridging Si–O bonds, whereas increasing the Al/Si ratio exerts an limited influence on the chain connectivity and framework stability. The cohesion of CASH is governed by intergranular contacts, and adsorbed water in the grain boundary regions is released as free water under humid conditions, significantly weakening cohesion and reducing strength by 40.80%.

In this study, an upscaling algorithm was developed to overcome barriers in constructing CASH textural models. Molecular dynamics simulations were employed to elucidate the multiscale structural evolution, Ca/Si and Al/Si coupling, and water governing the physical and mechanical properties of CASH. The findings could advance the atomic scale understanding of the submicron polycrystalline structure and mechanical behavior of CASH, paving a way for the design of more sustainable and durable cementitious composites.