Creep in reactive colloidal gels: a nanomechanical study of cement hydrates

From soft polymeric gels to hardened cement paste, amorphous solids under constant load exhibit pronounced time-dependent deformations called creep. The microscopic mechanism of such a phenomenon is poorly understood and constitutes a significant challenge in densely packed and chemically aging granular systems. Both features are prominently present in hydrating cement pastes composed of calcium silicate hydrate (C-S-H) nanoparticles, whose packing density increases as a function of time, while cement's hydration is taking place. Here we show that the creep response of hydrating cement paste allows for a unique insight into the effect of packing onto creep in general. Our extensive study based on nano-indentation testing and porosity investigations shows that the creep response primarily results from slippage between the nanoparticles and is controlled by the inter-particle distance. Our results might lead to the design of concretes with significantly reduced creep and in turn strongly reduced environmental impact.