| Summary: | 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 (<i>E</i><sub>for</sub>) of Mg substituting Ca was calculated. The adsorption energy (<i>E</i><sub>ad</sub>) and configuration of the single water molecule over Mg-doped β-dicalcium silicate (β-C<sub>2</sub>S) and M3-tricalcium silicate (M3-C<sub>3</sub>S) 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 <i>E</i><sub>for</sub> 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<sub>ad</sub> of a water molecule on Mg-doped β-C<sub>2</sub>S surfaces ranged from –0.598 eV to −1.249 eV with the molecular adsorption being the energetically favorable form. In contrast, the E<sub>ad</sub> on M3-C<sub>3</sub>S surfaces ranged from −0.699 eV to −4.008 eV and the more energetically favorable adsorption on M3-C<sub>3</sub>S surfaces was dissociative adsorption. The influence of Mg doping was important since it affected the reactivity of surface Ca/Mg sites, the E<sub>ad</sub> of the single water adsorption, as well as the adsorption configuration compared with the water adsorption on pristine surfaces.
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