Loading-rate-dependent effects of colloidal nanosilica on the mechanical properties of cement composites

Abstract

Colloidal nanosilica (CNS) has shown great potential in the fabrication of high-performance cement composites by improving their quasi-static mechanical properties. However, the effects of CNS under higher strain rates (i.e. dynamic loads) have been seldom explored. In this study, we thoroughly investigated the loading-rate-dependent effects of CNS on the mechanical performance of Portland cement paste. A Split Hopkinson Pressure Bar (SHPB) equipped with a high-speed camera was utilised to conduct dynamic testing under two different strain rates, namely ≈40 (1/s) and ≈120 (1/s), to analyse dynamic compressive strength, modulus of elasticity, failure strain, energy absorption, and fragment size distribution of the composites. Although CNS significantly improved the quasi-static mechanical properties, it had detrimental effects on the dynamic strength, stiffness, energy absorption, and fragmentation behaviour of the cement composites. These negative effects were attributed to the rapid flocculation of CNS upon mixing with the alkaline environment of cement, which led to the formation of a weak and porous hydration phase in the hardened cement paste, as corroborated by microscopy analysis. These findings demonstrate the importance of dynamic characterisations to fully understand the mechanical effects of nanomaterials because quasi-static investigations might not reveal their potential side effects.