Models of Aged Magnesium–Silicate–Hydrate Cements Based on the Lizardite and Talc Crystals: A Periodic DFT-GIPAW Investigation

Classical MD simulations and periodic DFT-GIPAW calculations have been employed to investigate possible structural models of aged magnesium–silicate–hydrate (M–S–H) cements. Four models were built, two based, respectively, on the lizardite Mg3Si2O5(OH)4 and talc Mg3Si4O10(OH)2 crystals in which we have modified the Mg/Si ratio and distribution of Qn silicon species (n is the number of bridging oxygens for each quaternary silicon atom) according to previous experimental evidence. The other two models were still based on the lizardite and talc crystals, but the original Mg/Si ratios were retained. The latter two models were used to simulate MSH binders containing different proportions of talc- and lizardite-based amorphous phases as proposed in several experimental works. From these initial models, additional structures were generated by adding increasing amounts of water molecules to investigate its effect on the structure and on the 29Si and 25Mg MAS NMR spectra. Our results demonstrate that MSH phases with an Mg/Si ratio close to 1 can be described as a defective talc-based model or as the combination of talc and lizardite amorphous models with the former being the dominant phase. In both cases, the hydrated models provide the best agreement with NMR experiments.