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

The study of mixed Al³⁺/Fe³⁺ 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-NO₃ hydrates containing varying proportions of trivalent Al³⁺ and Fe³⁺ cations were synthesized and characterized. The AFm-NO₃ phase constitutes a continuous solid solution between the two end-members Ca₂Al(OH)₆·NO₃·2H₂O and Ca₂Fe(OH)₆·NO₃·2H₂O. The obtained Ca₂Al_1-xFe_x(OH)₆·NO₃·yH₂O 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-NO₃ stability was examined in the presence of other anions, demonstrating the low stability of NO₃⁻ with respect to Cl⁻ and CO₃²⁻ anions.