Power-law creep in C–S–H emerging from subdiffusive dynamics

Abstract

Power-laws have long been used to describe creep in cementitious systems, from C-S-H to concrete, yet their mechanistic origin remains unclear. Diffusion, layer sliding, and permeation are often invoked as microscopic drivers of creep in C-S-H, but it is uncertain whether these represent distinct mechanisms or different expressions of a single underlying process.This work explores the interrelation of these processes and their connection to creep kinetics using theoretical considerations informed by molecular simulations. C-S-H systems with slit pores spanning interlayer and gel pores are simulated via equilibrium molecular dynamics to get self-diffusion coefficients of confined water, and non-equilibrium Couette-like simulations to probe shear behavior. The theory provided shows that power-law creep emerges naturally from subdifusive dynamics. Power-law exponents and creep shear modulus can be obtained from (subdiffusive) mean squared displacements obtained from molecular simulations.Subdiffusion in C-S-H interlayers is shown to be effectively asymptotic rather than transient. Confinement induces an exponential increase in water viscosity and enhanced subdiffusion, suggesting a universal behavior in nanolayered adsorbing materials. Water relaxation emerges as the fundamental mechanism, having dielectric relaxation as an associated manifestation. Diffusion, shear behavior, viscosity, dielectric relaxation, and creep thus appear as macroscopic expressions of the same structural relaxation processes.Finally, it is shown that the mechanistic picture provided might explain not only power-law creep but also logarithmic creep: at short to long times, subdiffusion-controlled motion of interlayer water drives power-law creep, whereas at very long times, activated rearrangements (after exhaustion of low activation-energy barrier processes) dominate logarithmic creep.