Molecular and dissociative adsorption of a single water molecule on a β‐dicalcium silicate (100) surface explored by a DFT approach

Abstract

The adsorption of water on a C2S surface initiates belite to hydrate. In the present work, the adsorption behavior of single water molecule on a β‐C2S (100) surface is explored using density functional theory (DFT) due to the lack of alternative approaches for direct observation. Four possible calcium atom sites on the β‐C2S (100) surface slab are considered in our calculations. The results show that water can adsorb on the 2 five‐coordinated calcium sites only via molecular adsorption with adsorption energies of 0.59 and 0.85 eV, respectively, but can dissociate on the other 2 six‐coordinated calcium sites with higher adsorption energies of 0.96 and 0.99 eV, respectively. The energy barriers to the dissociative adsorption of water at the Ca(III) site(0.10 eV) is much lower than that at the Ca(IV) site, indicating that water prefers to adsorb and dissociate on Ca(III) sites. The dissociative adsorption of water causes more obvious surface calcium shifts and Si–O bond length increases than molecular adsorption. The dissociative adsorption of a water molecule changes the electron distribution, and the overlap between Ca 2p and O 2s orbitals leads to new Ca–O bond formations.

Understanding Cement Hydration of Cemented Paste Backfill: DFT Study of Water Adsorption on Tricalcium Silicate (111) Surface

“…The weakest adsorption of water is observed at AC5 with an adsorption energy of −0.93 eV. As investigated by Zhang et al [37], the adsorption energy of a water molecule on a β-dicalcium silicate (100) surface ranges from 0.59 eV to 0.99 eV. Therefore, the adsorption of water molecules on the M3-C 3 S (111) surface is much more favorable in energy compared to the β-dicalcium silicate (100) surface.…”

“…As shown, the adsorption energies for dissociative adsorptions, namely, AC3 and AC4, are −2.28 eV and −2.48 eV respectively. These two adsorption energies are much higher than the remaining adsorption energies in Table 3, indicating that dissociative adsorption is energetically favored compared to molecular adsorption [37]. Among the molecular adsorptions, the highest molecular adsorption energy is −1.81 eV for AC6, which means that the molecular adsorption of water under such configuration is strongest.…”

“…During cement hydration, water molecules typically react with cement surfaces by forming two different bonds, the bonds between O atoms in water (O w ) and surface Ca atoms and the bonds between H atoms and surface O atoms. As the bond length of O w -Ca is approximately twice that of H-O, the surface Ca atoms are often selected as the adsorption sites [37].…”

“…In this paper, a higher E ads represents an E ads with a more negative value. According to Equation (1), a higher E ads indicates stronger binding between the water molecule and the M3-C 3 S (111) surface, implying that such an adsorption is more favorable from a thermochemical point of view [37,[39][40][41][42].…”

“…Secondly, the current study focused on the adsorption of a single water molecule and the adsorption configuration with different water coverage was not investigated. The authors note that the adsorption of a single water molecule is always a good starting point, which has been widely used in the literature [27,37,54]. Finally, the influence of surface defects, such as vacancy and impurity, on water adsorption was not investigated.…”