Equation of state and second-order elastic constants of portlandite Ca(OH)2 and brucite Mg(OH)2

Hydroxide minerals brucite Mg(OH) 2 and portlandite Ca(OH) 2 (space group P3 ¯ m1) are very important phases for several geological and industrial applications which often require the knowledge of the mechanical properties. In the present work, the equation of state (EoS) and the second-order elastic constants of the two minerals were calculated by ab initio quantum mechanical methods. The aims are extending the knowledge of their important applicative mechanical properties and providing a consistent relative dataset. In addition, the simple crystal-chemical composition and structure of Ca(OH) 2 and Mg(OH) 2 is ideal to simulate and characterize the effect of the proton disorder on the elastic properties, which could be useful for the comprehension of more complex hydrous minerals and synthetic phases. The third-order Birch–Murnaghan EoS parameters obtained in the present study were V 0 = 39.59(1) Å 3 , K 0 = 48.0(9) GPa and K′ = 9.1(3), and V 0 = 54.0(7) Å 3 , K 0 = 30.1(9) GPa and K′ = 8.5(4) for Mg(OH) 2 and Ca(OH) 2 , respectively. Axial compressibilities were found to be in ratio β(a):β(c) = 1.000:3.600 for brucite, and 1.000:3.777 for portlandite. The theoretical results agree with the general trend experimentally observed in the available literature, and further extend the knowledge of the mechanical properties of the two phases. The results could be very helpful for petro-geological investigations and for the synthesis and use of concrete nanocomposites and layered double hydroxides with tailored mechanical properties.