DFT-D study of single water adsorption on low-index surfaces of calcium silicate phases in cement

doi:10.1016/j.apsusc.2020.146255

Applied Surface Science

Volume 518, 15 July 2020, 146255

https://doi.org/10.1016/j.apsusc.2020.146255 Get rights and content

Highlights

•
Single water adsorption was modelled on low-index surfaces of two types of silicates.
•
The β-C₂S and M3-C₃S phases were considered.
•
Molecular adsorption was energetically favoured on β-C₂S surfaces.
•
Dissociative adsorption was energetically favoured on M3-C₃S surfaces.
•
The hydration of silicates is related to the mechanical strength of cement.

Abstract

The hydration of calcium silicate phases dominates the application of cement clinkers and has important practical implications. The reaction mechanisms between water and calcium silicate phases are complex and remain poorly understood at the atomic level. Herein, the single water adsorption on all low-index surfaces of β-C₂S and M3-C₃S was investigated using DFT-D calculations. The surface energy was calculated and the influence of surface cleavage discussed. Both dissociative and molecular adsorptions were investigated. Molecular adsorption was energetically favoured on β-C₂S surfaces while dissociative adsorption was energetically favoured on M3-C₃S surfaces. A Wulff construction was used to describe the equilibrium morphology with and without an adsorbed water molecule for the β-C₂S phase. Water adsorption promoted the solid dissolution by weakening the bond among surface atoms. Electron transfer was observed mainly from the surface atoms to water atoms during the adsorption. These findings provide a novel insight into the adsorption of a water molecule on different calcium silicate surfaces using the same level of theory, which includes a representation of van der Waals forces, therefore, laying the foundation for better understanding the hydration mechanism.

Graphical abstract

Introduction

The wide application of cement-based materials depends on cement hydration for strength development [1]. Cement hydration provides hydrates to bind unreactive solid particles, and calcium-silicate-hydrates (C-S-H) are the main binding phase in ordinary Portland cement [2]. As the nomenclature indicates, C-S-H rely on the reaction of water and calcium silicate phases in cement. For ordinary Portland cement, calcium silicate presents in the form of dicalcium silicate (Ca₂SiO₄, C₂S) and tricalcium silicate (Ca₃SiO₅, C₃S). In a typical cement clinker, C₃S can constitute up to 50–70 wt% and C₂S can constitute up to 15–30 wt% [3]. Investigating the reaction between water and calcium silicate is thus essential for an understanding of the cement hydration process [4].

The majority of the literature focuses on the hydration heat, microstructure and mineralogical characterization to investigate the hydration of calcium silicate. Significant advances have been achieved through these techniques and hypotheses have been proposed to explain experimental observations. To date there has been no consensus on the process of hydration of calcium silicate. For example, two main hypotheses, namely the protective membrane theory and the dissolution theory, have been suggested to explain the origin of the induction period of C₃S [5]. Both hypotheses have advantages and disadvantages, even as new experimental data continues to be collected [4]. The atomic structure of calcium silicate controls the hydration process, which is hard to explain using phenomenological experiments [6]. This calls for an atomic-level study of the hydration mechanisms of calcium silicate.

The hydration process of calcium silicate involves many steps, the initial step being the adsorption of water molecules on calcium silicate surfaces [7], [8], [9], [10]. The dissolution of calcium silicate into water follows, leading to an increase of ion concentration, such as Ca²⁺ and OH⁻ [11]. The ion concentration would increase until it reaches thermodynamic equilibrium. Hydrates start to precipitate with the continuous dissolution of cement. Investigating the water adsorption on calcium silicate surfaces is thus the first step for the atomic study of cement hydration.

First-principles calculation is the standard technique to explore water adsorption at the atomic level. In the past decade, studies have been carried out to investigate water adsorption on calcium silicate surfaces using first-principles calculations. The adsorption of a single water molecule on the β-C₂S (1 0 0) and a few of M3-C₃S low-index surfaces has been investigated by Zhang et al. [12], [13]. Both molecular and dissociative adsorptions were observed and the corresponding adsorption configurations/energies were presented. Qi et al. [14] performed a more detailed study on the adsorption of a water molecule on M3-C₃S (1 1 1) surface. Interestingly, they concluded that such water adsorption weakened the bonding among surface M3-C₃S atoms with bulk atoms, thus promoting the dissolution of M3-C₃S. However, there have been no studies about the water adsorption on all low-index surfaces of calcium silicate, especially with more accurate first-principle methods like the dipole and van der Waals corrections.

The main contributions of this study are fourfold: (1) The single water adsorption on all low-index surfaces of calcium silicate was investigated. The β-C₂S (P21/C symmetry group) and M3-C₃S (Cm symmetry group) were studied since they are the most frequently observed morphologies during industrial applications [15], [16], [17]. (2) Both molecular and dissociative adsorptions were investigated using the improved DFT methodology, which includes increased energy cutoff and diploe and van der Waals corrections. (3) The surface energies for low-index surfaces, with and without adsorbed water are calculated. Moreover, the surface cleavage and adsorption configuration were discussed in details. (4) For the β-C₂S phase, a Wulff construction was simulated to express the equilibrium morphology with and without an adsorbed water molecule on the surface. The authors believe such first-principles calculations could shed light on the interaction mechanisms between water and calcium silicate surfaces. Moreover, these results could be of fundamental significance once the atomic level can be linked to microstructure using statistical mechanics.

Section snippets

Computational details

First-principles calculations were performed within density functional theory (DFT), using the projector augmented-wave (PAW) method using the VASP code [18], [19], [20]. The exchange-correlation potential was approximated within the generalised gradient approximation (GGA) using the PBE functional [21] and the PBE_sol functional [22]. The 3p⁶4s², 3s²3p², 2s²2p⁴, and 1s¹ were treated as valence electrons for Ca, Si, O and H, respectively. The energy cutoff and the energy tolerance were chosen to

β-C₂S

Table 2 shows the comparison between the calculated and experimental lattice parameters of β-C₂S. It can be seen that there was a good agreement among current calculations, experimental measurements and previous calculations. The calculation using the PBE functional was closer to experimental values than the calculation using the PBE_sol functional in the current study.

M3-C₃s

Table 3 shows the comparison between the calculated and experimental lattice parameters of M3-C₃S. Overall, the calculation

Conclusions

A systematical DFT-D study was conducted to investigate the single water adsorption on low-index surfaces of β-C₂S and M3-C₃S. Based on the above results, the following conclusions can be drawn:

1.
PBE functional was more suitable than PBE_sol functional for the unit cell calculation of β-C₂S and M3-C₃S. The current unit cell calculation outperformed previous calculations in the literature due to more accurate DFT settings, such as larger energy cutoff and increased slab size.
2.
The influence of

CRediT authorship contribution statement

Chongchong Qi: Conceptualization, Investigation, Writing - original draft, Writing - review & editing, Funding acquisition. Dino Spagnoli: Conceptualization, Writing - original draft, Writing - review & editing. Andy Fourie: Conceptualization, Writing - original draft, Funding acquisition.

Acknowledgement

The first author is supported by the China Scholarship Council (grant number: 201606420046). This work was supported by resources provided by the Pawsey Supercomputing Centre with funding from the Australian Government and the Government of Western Australia. DS would like to thank Prof Stephen C. Parker for the very useful discussions on Wulff construction of the M3-C₃S phase.

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Full Length ArticleDFT-D study of single water adsorption on low-index surfaces of calcium silicate phases in cement

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Computational details

β-C2S

M3-C3s

Conclusions

CRediT authorship contribution statement

Acknowledgement

Miner. Eng.

Cem. Concr. Res.

Cem. Concr. Res.

Miner. Eng.

Constr. Build. Mater.

Appl. Surf. Sci.

Cem. Concr. Res.

Comput. Mater. Sci.

Appl. Surf. Sci.

Appl. Surf. Sci.

Appl. Surf. Sci.

Appl. Surf. Sci.

Cem. Concr. Res.

J. Catal.

Appl. Surf. Sci.

Nanoscale origins of creep in calcium silicate hydrates

Nat. Commun.

Cement chemistry

Hydrationbehavior of C2S and C2AS nanomaterials, synthetized by sol–gel method

J. Therm. Anal. Calorimetry

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J. Phys. Chem. C

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Minerals

Understanding and controlling the reactivity of the calcium silicate phases from first principles

Chem. Mater.

Full Length Article
DFT-D study of single water adsorption on low-index surfaces of calcium silicate phases in cement

β-C₂S

M3-C₃s