Hydration of MgO: Reaction Kinetics and pH Control on Brucite Crystal Morphology

The hydration of periclase [MgO] to yield brucite [Mg(OH)2] is of high relevance in many technical and man-made surroundings, regarding its volume expansive reaction and the development of distinct crystal morphologies. Although studies on the MgO–Mg(OH)2 conversion behavior are numerous, the decisive parameters controlling its kinetics as well as the size, the shape, and the orientation of individual brucite crystals remain poorly constrained. Therefore, a series of MgO hydration experiments were conducted to monitor and assess the influence of the surface area and the presence of secondary phases in MgO educts on (i) the chemical evolution of the reactive solution, (ii) the MgO–Mg(OH)2 conversion rate, and (iii) the development of the particle morphology of brucite. The reactive surface area of the MgO significantly stimulates the hydration and thus the conversion rate to yield brucite. In contrast, individual foreign components, such as CaO or MgCl2, within the MgO educts predefine the self-adjusting pH during progressing MgO hydration. The pH evolution regulates the charge distribution at the reacting MgO surface and thus has a key control on the morphology of the precipitating brucite crystals. Flaky versus columnar crystal shapes are formed preferentially below and above the point of zero charge of MgO and Mg(OH)2 (pHPZC ≈ 11 to 12), respectively, which has implications for tailored product engineering.