Competing Mechanisms of Ca/Si Ratio and Water Content on the Structure and Nanomechanical Properties of Amorphous C‐S‐H

ABSTRACT Calcium‐silicate‐hydrate (C‐S‐H) is the primary binding phase responsible for the overall performance of cement‐based materials. The calcium‐to‐silicon (Ca/Si) ratio plays a decisive role in governing the structural and mechanical properties of C‐S‐H; yet, it is challenging to precisely investigate the intrinsic mechanical behaviors due to their nanoscale size. In this work, amorphous C‐S‐H gels with varying Ca/Si ratios and water content were constructed using the reactive force field and explored their nanomechanical responses. The results indicated that as the Ca/Si ratio increases from 0.8 to 2.0, the bulk, shear, and elastic moduli rise from 28 to 46 GPa, 13 to 22 GPa, and 49 to 80 GPa, respectively, owing to the formation of denser atomic networks and stronger Ca‐silicate chain interactions. In contrast, the presence of water among C‐S‐H clusters weakens their stiffness, with the bulk, shear, and elastic moduli reduced by approximately 14%, 26%, and 19%, respectively. Moreover, the coupled effects of water content and Ca/Si ratios induce ductile‐to‐brittle transitions in shear behavior of C‐S‐H. These findings demonstrate that C‐S‐H stiffness increases with Ca/Si ratio but decreases with water content, revealing the competing effects of Ca/Si‐induced structural densification and water‐induced softening that govern its nanoscale mechanical behaviors.