Stability of hydrotalcite (Mg-Al layered double hydroxide) in presence of different anions

MgO contained in cementitious materials is experimentally observed to precipitate as poorly crystalline hydrotalcite (Mg-Al LDH). However, the geochemical modelling of hydrotalcite is challenging due to the lack of consistent thermodynamic dataset for this phase. Hydrotalcites with Mg/Al = 2 were synthesised in the presence of NaHCO3, NaCl, Na2SO4, NaNO3, and NaOH. Mass balance, XRD and FT-IR indicated the incorporation of the anions in the interlayer together with some carbonate. The crystallinity of the phase increased depending on the anion: SO42− < Cl− < NO3− < OH− < CO32−. An in-situ increase of temperature monitored by XRD and TGA showed that the stability of the hydrotalcite structure with temperature also depended on the incorporated anion. The solubility products were calculated based on the solution analysis of samples re-equilibrated at different temperatures, while the entropy and heat capacity were obtained from the additivity method or the molar volume. A simple solid-solution model for hydrotalcite containing CO3, OH, SO4, Cl and NO3 is suggested.