C–S–H gel densification: The impact of the nanoscale on self-desiccation and sorption isotherms

Abstract

The relationship between humidity and water content in a hydrating cement paste is largely controlled by the nanostructure of the C-S-H gel. Current hydration models do not describe this nanostructure, thus sorption isotherms and self-desiccation are given as constitutive inputs instead of being predicted from microstructural evolution. To address this limitation, this work combines a C-S-H gel description from nanoscale simulations with evolving capillary pore size distributions from a simple hydration model. Results show that a progressive densification of the C-S-H gel must be considered in order to explain the self-desiccation of low-alkali pastes. The impact of C-S-H densification on the evolution of microstructure and sorption isotherms is then discussed, including the effect of water-tocement ratio, cement powder fineness, and curing temperature. Overall, this work identifies an area where nanoscale simulations can integrate larger-scale models of cement hydration and poromechanics.Eq. 1 assumes perfect wetting and hemispherical liquid-vapour interfaces. γ = 0.073 Nm −1 is the liquid-vapour surface tension of water, M V w = 18.02 · 10 −6 m 3 mol −1 is the molar volume of water, R is the gas constant, and T is the temperature in Kelvin degrees. At room temperature, T = 298 K, the Kelvin equation predicts that iRH starts to decrease appreciably only when submicrometre pores get desaturated (see Fig. 1.d): these pores are mostly within the hydration product, in particular the porous calcium-silicate-hydrate (C-S-H) gel. The desaturation of C-S-H gel pores requires that the larger capillary pores are desaturated first, and this must occur early during hydration, in order to explain onsets of self-desiccation already during the 3

Inferring pore connectivity from sorption hysteresis in multiscale porous media

“…Our calculated f values indicate the importance of finite size percolation effects in different materials. The model is simplified with certain assumptions as discussed above, nevertheless it addresses the currently missing link between 3D simulations of colloidal systems and macroscopic continuum models of vapor sorption, and provides a pathway to overcome the current empirical treatment of sorption isotherms in continuum poromechanical models [83,84,85,86].…”

Modeling of microstructural effects on the creep of hardened cement paste using an experimentally informed lattice model

“…Therefore, the identified creep moduli of C–S–H phases are actually higher than the pure C–S–H phase. In addition, if C–S–H phases experience significant heating, drying, or aging, for example, hydration‐induced microstructure modification or silicate polymerization at C–S–H level (Bažant et al., 1997; Di Luzio & Cusatis, 2013; Do et al., 2016; Giorla & Dunant, 2018; Granger & Bažant, 1995; Masoero, Cusatis, & Di Luzio, 2018), the creep functions for C–S–H must vary relative to the temperature, humidity, or age. Therefore, in this context, the nonaging (time‐invariant) viscoelasticity of C–S–H phase for the case of no temperature and moisture change is identified by the current model.…”

Influence of Pore Size and Fatigue Loading on NaCl Transport Properties in C-S-H Nanopores: A Molecular Dynamics Simulation

“…Minet [24] has experimentally determined that the pore width of C-S-H ranges from 0.5 nm to 10 nm. Based on the results obtained by Enrico Masoero [25], the predominant pore diameter of C-S-H is 3 nm when the hydration degree reaches 80%. Chloride ion transport causes deterioration of durability of RC structures during service.…”