Thermochemical data and phase equilibria of halide (Cl-, Br-, I-) containing AFm and hydrotalcite compounds

Layered double hydroxide (LDH) phases that form during cement hydration can incorporate a variety of interlayer anions in their interlayer positions. Here, a range of phases of general formula [(M(1-x)M(x)III)-M-II(OH)(2)][A(n-)](x/n)<middle dot>zH(2)O were synthesized, where M-II = Mg2+ (hydrotalcite) or Ca2+ (AFm), M-III = Al3+ such that [M-II/Al] = 2 (Ca and Mg, atomic units) or 3 (Mg only), and A = Cl-, Br-, or I-. All the synthesized phases were characterized to assess their composition, density, and crystal structure. By approach from undersaturation, the solubility data of these compounds was measured at 5, 25, and 60 degrees C. This thermochemical data was used to successfully model their formation using thermodynamic modeling and to infer the fields of stability of these compounds for conditions of relevance to cementitious systems. It is seen that halide-containing hydrotalcite phases strongly compete with hydroxide-containing hydrotalcite, with the latter prevailing at high pH. In contrast, halide-containing AFm compounds are more stable compared with hydroxide-containing AFm compositions.