Role of Mg Impurity in the Water Adsorption over Low-Index Surfaces of Calcium Silicates: A DFT-D Study

Calcium silicates are the most predominant phases in ordinary Portland cement, inside which magnesium is one of the momentous impurities. In this work, using the first-principles density functional theory (DFT), the impurity formation energy (E_for) of Mg substituting Ca was calculated. The adsorption energy (E_ad) and configuration of the single water molecule over Mg-doped β-dicalcium silicate (β-C₂S) and M3-tricalcium silicate (M3-C₃S) surfaces were investigated. The obtained Mg-doped results were compared with the pristine results to reveal the impact of Mg doping. The results show that the E_for was positive for all but one of the calcium silicates surfaces (ranged from −0.02 eV to 1.58 eV), indicating the Mg substituting for Ca was not energetically favorable. The E_ad of a water molecule on Mg-doped β-C₂S surfaces ranged from –0.598 eV to −1.249 eV with the molecular adsorption being the energetically favorable form. In contrast, the E_ad on M3-C₃S surfaces ranged from −0.699 eV to −4.008 eV and the more energetically favorable adsorption on M3-C₃S surfaces was dissociative adsorption. The influence of Mg doping was important since it affected the reactivity of surface Ca/Mg sites, the E_ad of the single water adsorption, as well as the adsorption configuration compared with the water adsorption on pristine surfaces. View Full-Text