  From soft polymeric gels to hardened cement paste, amorphous solids under
constant load exhibit a pronounced time-dependent deformation called creep. The
microscopic mechanism of such a phenomenon is poorly understood in amorphous
materials and constitutes an even greater challenge in densely packed and
chemically reactive 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 hydration is taking place. Performing nano-indentation tests and
porosity measurements on a large collection of samples at various stages of
hydration, we show that the creep response of hydrating cement paste is mainly
controlled by the inter-particle distance and results from slippage between
(C-S-H) nanoparticles. Our findings provide a unique insight into the
microscopic mechanism underpinning the creep response in aging granular
materials, thus paving the way for the design of concrete with improved creep
resistance.
