Growth of brucite (Mg(OH)2) on portlandite (Ca(OH)2)

The potential protective effect of brucite (Mg(OH)2) grown on portlandite (Ca(OH)2) was analysed. Single crystals of portlandite were synthesized and exposed to Mg2+-containing solutions and the influence of the central parameters was analysed: (i) magnesium ion source (MgCl2 and MgSO4) (ii) initial Mg2+ concentration (0.01-1 mol/l), (iii) crystal surface to solution volume ratio, and (iv) duration of experiment. Solids and solutions were characterised by means of FTIR, ICP-OES, pH measurements, SEM and EPMA. Additionally, thermodynamic calculations were performed. The results showed that the morphology and thickness and thus the protective properties of the brucite layer were conditioned by the Mg2+ and Ca2+ concentration in the solution. Brucite layers grown during exposure to higher initial Mg2+ concentration were denser and thinner, offering better protection against further dissolution. In MgCl2 solutions, the precipitation of a compact brucite layer was accompanied by the formation of an empty cavity between brucite and portlandite. In MgSO4 solutions the space between brucite and portlandite was filled with gypsum as saturation conditions were reached providing extra protection but also leading to potential crack formation in the brucite layer.