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.