Deconstructing water sorption isotherms in cement pastes by lattice density functional theory simulations

Abstract

The moisture content in cement pastes influences their mechanical properties and durability. However, the complex, multiscale nature of cement pastes makes it challenging to isolate the contributions of each scale to their macroscopic water sorption isotherms. In particular, the contribution of the calcium-silicate-hydrate gel (the binding phase of cement pastes) remains only partially understood. Here, we introduce a density functional theory lattice model describing water sorption in calciumsilicate-hydrate, which properly reproduces experimental water sorption isotherms in cement pastes. Based on this model, we deconstruct the contribution of each pore scale (interlayer spacing, gel pores, and capillary pores) to the total sorption isotherm.We show that, when the relative humidity is below 80%, the calcium-silicate-hydrate gel accounts for more than 90% of the moisture content adsorbed in cement pastes. In turn, we find that the contribution of the interlayer space within the calcium-silicatehydrate grains is governed by the competition between the rate of interlayer space opening and the increasing propensity for water to fill larger pores upon increasing relative humidity. Overall, our results highlight the key role played by the calciumsilicate-hydrate in governing the sorption isotherms of cement pastes.

Effects of elevated temperature and water re-curing on fracture process of hybrid fiber reinforced concretes

“…3(a). The average water evaporation ratio at 400 • C reached 8.30% and 6.78% for Mix P and Mix S, accounting for 67.8% and 59.5% of that at 800 • C. Capillary water evaporated easily when the temperature increased from 20 • C to 400 • C. The inter-layer water between the layers of the C-S-H [52,53] evaporated when the temperature exceeded 200 • C. Some composites, like calcium hydroxide and aggregates, experienced decomposition and the chemical water in them evaporated, leading to the further increase of the water evaporation. The melting and evaporation of the polypropylene fiber when the temperature exceeded 170 • C provided extra paths for water evaporation, leading to the higher water evaporation ratio in Mix P than that of Mix S.…”

“…Capillary water, inter-layer water, and chemical combined water are the three kinds of water in concrete [35,[50][51][52]. Under the high temperature environment, water evaporates from inside to the outside, leading to physical (pore and microstructure) and chemical changes (dehydration of the C-S-H).…”

Effect of single water adsorption on the bond order of calcium silicates and its implication for hydration variation

“…9,10 The hydration of dicalcium silicate (Ca 2 SiO 4 , C 2 S, 15−30 wt%) and tricalcium silicate (Ca 3 SiO 5 , C 3 S, 50−70 wt%), which are the main components of Portland cement, directly influences the development of material strength. [11][12][13][14][15][16] The difference in the hydration reactivities of C 2 S and C 3 S has been the subject of many investigations. [17][18][19][20] Atomic simulations have demonstrated higher accuracy than experimental observation and have been widely used in the study of cement.…”

“…Due to the complexity of cement systems, the mechanism of cement hydration has not been reached an agreement yet 9,10 . The hydration of dicalcium silicate (Ca 2 SiO 4 , C 2 S, 15−30 wt%) and tricalcium silicate (Ca 3 SiO 5 , C 3 S, 50−70 wt%), which are the main components of Portland cement, directly influences the development of material strength 11–16 . The difference in the hydration reactivities of C 2 S and C 3 S has been the subject of many investigations 17–20 .…”