**Green Synthesis of Hydrocalumite (CaAl-OH-LDH) from Ca(OH) 2 and Al(OH) 3 and the Parameters That Influence Its Formation and Speciation**

#### **Bianca R. Gevers \* and Frederick J.W.J. Labuschagné**

Institute of Applied Materials, Department of Chemical Engineering, University of Pretoria, Pretoria 0002, South Africa; johan.labuschagne@up.ac.za **\***

 Correspondence: bianca.gevers@tuks.co.za

Received: 14 July 2020; Accepted: 1 August 2020; Published: 3 August 2020

**Abstract:** Hydrocalumite is a layered double hydroxide (LDH) that is finding increased application in numerous scientific fields. Typically, this material is produced through environmentally polluting methods such as co-precipitation, sol-gel synthesis and urea-hydrolysis. Here, the hydrothermal green (environmentally friendly) synthesis of hydrocalumite (CaAl-OH) from Ca(OH)2 and Al(OH)3 in water and the parameters that influence its formation are discussed. The parameters investigated include the reaction temperature, reaction time, molar calcium-to-aluminium ratio, the morphology/crystallinity of reactants used, mixing and the water-to-solids ratio. Hydrocalumite formation was favoured in all experiments, making up between approximately 50% and 85% of the final crystalline phases obtained. Factors that were found to encourage higher hydrocalumite purity include a low water-to-solids ratio, an increase in the reaction time, sufficient mixing, the use of amorphous Al(OH)3 with a high surface area, reaction at an adequate temperature and, most surprisingly, the use of a calcium-to-aluminium ratio that stoichiometrically favours katoite formation. X-ray diffraction (XRD) and Rietveld refinement were used to determine the composition and crystal structures of the materials formed. Scanning electron microscopy (SEM) was used to determine morphological differences and Fourier-transform infrared analysis with attenuated total reflectance (FTIR-ATR) was used to identify possible carbonate contamination, inter alia. While the synthesis was conducted in an inert environment, some carbonate contamination could not be avoided. A thorough discussion on the topic of carbonate contamination in the hydrothermal synthesis of hydrocalumite was given, and the route to improved conversion as well as the possible reaction pathway were discussed.

**Keywords:** HC; hydrothermal synthesis; layered double hydroxide; AFm phase; calcium hemicarboaluminate; cement phases; cement hydration; C3AH6; C4ACH¯ 11; katoite
