Calcium silicate hydrates containing sodium [C–(N)–S–H], and sodium aluminosilicate hydrates [N–A–S–H] are the dominant reaction products that are formed following reaction between a solid aluminosilicate precursor (eg, slags, fly ash, metakaolin) and an alkaline activation agent (eg NaOH) in the presence of water. To gain insights into the thermochemical properties of such compounds, C–(N)–S–H and N–A–S–H gels were synthesized with compositions: 0.8≤Ca/Si≤1.2 for the former, and 0.25≤Al/Si≤0.50 (atomic units) for the latter. The gels were characterized using thermogravimetric analysis (TGA), scanning electron microscopy with energy‐dispersive X‐ray microanalysis (SEM‐EDS), and X‐ray diffraction (XRD). The solubility products (KS0) of the gels were established at 25°C and 50°C. Self‐consistent solubility data of this nature are key inputs required for calculation of mass and volume balances in alkali‐activated binders (AABs), and to determine the impacts of the precursor chemistry on the hydrated phase distributions; in which, C–(N)–S–H and N–A–S–H compounds dominate the hydrated phase assemblages.

Synthetic hydrotalcites were produced by a co‐precipitation method. The hydrotalcites are represented by the general formula [MII(1‐x)MIII(x)(OH)2][An−]x/n·zH2O, where MII is a divalent cation (eg, Mg2+or Ca2+), MIII is a trivalent cation (eg, Al3+) and An− is the interlayer anion. Herein, MII = Mg, and MIII = Al such that [Mg/Al] = [2, 3] (atomic units) and An−, represents intercalant species including: OH−, SO42− and CO32− anions. The thermochemical data of each compound including their solubility constants (Kso), density and molar volume were quantified at T = 25 ± 0.5°C, and P = 1 bar. The solubilities of the synthetic hydrotalcites, irrespective of their divalent‐trivalent cation partitioning ratio, scaled as CO32− < SO42− < OH−; in order of decreasing solubility. The type of anion, very slightly, affected the solubility with less than ±1 log unit of variation for [Mg/Al] = 2, and ±2 log units of variation for [Mg/Al] = 3. The solubilities of these phases were strongly correlated with that of gibbsite (Al(OH)3); such that activity of the [AlO2−] species was solubility determining with increasing pH. The tabulated thermodynamic data were used to construct solid‐solution models for phases encompassing both cation distribution ratios and to calculate stable phase equilibria relevant to alkali‐activated slag (AAS) systems for diverse activator compositions.