**5. Conclusions**

In conclusion, HC formation was favoured in all experiments, making up between approximately 50% and 85% of the final crystalline phases obtained. The formation of HC stood in competition with the formation of katoite, with constituted a large fraction of the remaining phases. It is expected that only small amounts of carbonate were present in the CaAl-OH-LDH and that any carbonate contamination came from the Ca(OH)2 (as calcite) and from the Al(OH)3 as surface adsorbed carbonate species rather than from the air as previously suggested. At high temperatures, CaCO3 formation seemed to be favoured instead of carbonate intercalation. The low solubility of carbonate species at elevated temperatures could be contributing factor to the low amount of carbonate intercalated.

The largest effect on HC purity was seen using a low water-to-solids ratio, increasing the reaction time, having sufficient mixing, using an amorphous Al(OH)3 with a high surface area, using an adequate reaction temperature and most surprisingly, by using a calcium-to-aluminium ratio stoichiometrically favouring katoite formation instead of HC formation. The morphology, surface area and crystallinity of the starting materials played a significant role. pH effects caused by the amount of reactants supplied could also have played a role in the increased purity observed—possibly by facilitating better dissolution of the Al(OH)3 phase—especially for low water-to-solids ratios and the stoichiometrically unfavoured molar calcium-to-aluminium ratios.

Finally, it was possible to obtain a hint regarding the reaction mechanism at elevated temperatures. At lower temperatures, it is possible that the formation of HC follows through the formation of calcium aluminate carbonate hydrate phases in conjunction with katoite, while at high temperatures, katoite formation seems to precede the formation of HC.

**Author Contributions:** Conceptualisation, B.R.G. and F.J.W.J.L.; methodology, B.R.G.; validation, B.R.G.; formal analysis, B.R.G.; investigation, B.R.G.; resources, F.J.W.J.L.; data curation, B.R.G.; writing—original draft preparation, B.G.; writing—review and editing, B.R.G. and F.J.W.J.L.; visualisation, B.R.G.; supervision, F.J.W.J.L.; project administration, F.J.W.J.L.; funding acquisition, F.J.W.J.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Techsparks (Pty) Ltd and the Technology and Human Resources for Industry Programme (THRIP) administered by the Department of Trade and Industry, South Africa, (grant number THRIP/133/31/03/2016). The APC was funded by the University of Pretoria, South Africa.

**Acknowledgments:** Particular thanks are extended to Wiebke Gröte (X-ray diffraction analyst) at the Department of Geology, University of Pretoria, South Africa, for making these time sensitive wet XRD measurements possible and performing the subsequent Rietveld refinement. We also thank David Viljoen for his assistance in the laboratory to allow for a timely publication of the results and editing the manuscript for language.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study, in the collection, analyses, or interpretation of data or in the writing of the manuscript. They consented to publish the results.
