An in-depth investigation of AFm-NO3 hydrates: Fe to Al substitution, hydration levels and anionic exchange capabilities

The study of mixed Al 3+ /Fe 3+ nitrate hydrates is gaining increasing interest with the emergence of new hydraulic binders and Supplementary Cementitious Materials (SCMs) containing significant amounts of iron and aluminum oxides that can be activated by nitrate addition. Layered AFm-NO 3 hydrates containing varying proportions of trivalent Al 3+ and Fe 3+ cations were synthesized and characterized. The AFm-NO 3 phase constitutes a continuous solid solution between the two end-members Ca 2 Al(OH) 6 ·NO 3 ·2H 2 O and Ca 2 Fe(OH) 6 ·NO 3 ·2H 2 O. The obtained Ca 2 Al 1- x Fe x (OH) 6 ·NO 3 · y H 2 O solid solution proves stable for four hydration states, with a significant impact on the interlayer distance: y = 5 (at high relative humidity with an interlayer distance of 10.4 Å), y = 2 (at ambient conditions with an interlayer distance of 8.6 Å), y = 1 (above 60 °C with an interlayer distance of 7.4 Å) and y = 0 (anhydrous above 175 °C with an interlayer distance of 8.0 Å). In fact, thermal variations reveal some differences between the two Al-rich and Fe-rich sides. Finally, AFm-NO 3 stability was examined in the presence of other anions, demonstrating the low stability of NO 3 − with respect to Cl − and CO 3 2− anions. • The study demonstrates for the first time the existence of a complete solid solution in the AFm-NO 3 · phase. • The different hydration levels were determined with respect to humidity, temperature, and Fe 3+ /Al 3+ ratio. • The exchange of nitrate anions from the interlayer was clearly demonstrated for Cl - and CO 3 2- .