Ca-Al layered double hydroxides (LDHs) from the rehydration of calcined katoite: Chloride binding capacity and cementitious properties

In this work, calcium‑aluminum layered double hydroxides (Ca-Al LDHs) were obtained through the rehydration of calcined katoite, a novel approach to produce a fast-reactive binder able to incorporate chloride ions for applications in cementitious materials. The katoite precursor was first synthesized via a mechanochemical method and then calcined at 400 °C. In-situ techniques were employed to investigate the rehydration reaction, phase formation, and the resulting material properties, with a focus on chloride binding and cementitious properties. Hydrocalumite (Ca-Al LDH) was identified as the main product from the rehydration of calcined katoite in the chloride solution, incorporating chloride ions and water while releasing heat immediately. The LDH phase constituted up to 69 wt% of the rehydrated product after 24 h of reaction, and an estimation of the proportion of Cl ions pointed out to ${\chi }_{2\mathrm{Cl}}=2\mathrm{Cl}/\mathrm{Ca}=0.70$, with possible partial incorporation of carbonate ions. The kinetics of the rehydration reaction, structural parameters and possible mechanisms of formation of the LDHs were assessed. Cementitious properties were observed from the rehydrated paste, which exhibited a high yield stress (273 Pa) even with a liquid volume of 75 %. The combination of rapid reactivity, significative chloride and water binding, and appealing fresh-state and hardening behavior indicate the potential of calcined katoite as a fast-reactive binder, able to be further explored as supplementary cementitious materials (SCMs) and for durability purposes.