Morphology prediction of portlandite: Atomistic simulations and experimental research

Calcium hydroxide (CH), as a building material, can be useful for enhancing constructional efficiency, delaying steel corrosions and so on. Especially, its morphology can be significantly modified by admixtures, which could be a way of optimizing microstructures and improving mechanical properties of cement paste. In this paper, atomistic simulation methods and experimental work were conducted to study the morphology of portlandite under different conditions. First-principles calculations were employed to study the electronic structures and surface energies of low-index surfaces of CH with or without adsorbents. The reactions between a bulk of water and portlandite surfaces were studied by molecular dynamics simulations at 300 K. The crystal morphology according to Wulff construction changed from a hexagonal platelet in the vacuum to a facetted tetrakaidekahedron in the water. The analysis of electronic structures indicated that the Ca and O atoms on the top layer of surfaces were easily under nucleophilic and electrophilic attacks by the surroundings during the growth of CH, respectively. Subsequently, the morphology of the obtained CH crystals by coprecipitation experiments (CaCl₂ + NaOH) follow the rule of growth mechanism by simulations, which may help in the screening of admixtures with different functional groups to control the crystal morphologies.