*2.7. Water-to-Solids Ratio*

The final parameter investigated was the effect of the water-to-solids ratio on the formation of HC. Three water-to-solids ratios were tested, being WS1 = 70:30, WS2 = 80:20 and WS3 = 90:10. During the reaction, these three ratios behaved very differently. Where, usually, reaction mixtures with an 80:20 ratio thickened considerably after 1 h to 1.5 h, WS1 had already thickened considerably after 30 min. After 2 h, this mixture became difficult to agitate even with the pH probe during measurement and was mixing with grea<sup>t</sup> difficulty until the 3 h of reaction time had passed. WS3, on the contrary, never thickened to a considerable degree, fully incorporating all mixture contents continuously.

The XRD and FTIR-ATR results depicted in Figure 22 show some differences between the three materials formed.

**Figure 22.** (**a**) XRD results of the water-to-solids ratio series WS1, WS2 and WS3 between 5◦ 2*θ* and 90◦ 2*θ*. Each scan is y-shifted by 2500 counts. The inset depicts the primary LDH peaks and change in position. HC = hydrocalumite, Kat = katoite, P = portlandite and C = calcite. (**b**) FTIR-ATR scans of the water-to-solids ratio series WS1, WS2 and WS3 between 4000 cm<sup>−</sup><sup>1</sup> and 550 cm<sup>−</sup>1. The three insets show the regions: (**1**) 3700 cm<sup>−</sup><sup>1</sup> – 3600 cm<sup>−</sup>1, (**2**) 1500 cm<sup>−</sup><sup>1</sup> – 1250 cm<sup>−</sup>1, (**3**) 1000 cm<sup>−</sup><sup>1</sup> – 550 cm<sup>−</sup><sup>1</sup> in detail. Dashed and solid grey lines indicate the maxima of vibrations. WS1 = 70:30, WS2 = 80:20 and WS3 = 90:10.

The primary LDH peak of HC was shifted somewhat to the right for WS3 with respect to WS1 and WS2 (inset in Figure 22a). [Ca4Al2(OH)12][(CO3) · 5 H2O] remained the best fitting crystal structure analogue in all three materials, however. FTIR-ATR showed little difference between the three materials, although there was a clear reduction in the 3641 cm<sup>−</sup><sup>1</sup> peak associated with unreacted Ca(OH)2. Intensity of vibration also decreased in the 1414 cm<sup>−</sup>1/1361 cm<sup>−</sup><sup>1</sup> doublet with an increase in water-to-solids ratio; as did the vibrations between 1000 cm<sup>−</sup><sup>1</sup> and 550 cm<sup>−</sup>1. Rietveld refinement, however, showed very large differences between these three materials. WS1 led to a HC purity greater than 70%, while WS3 achieved only approximately 55%. There were also large differences in the amount of katoite formed and the amount of portlandite left unreacted, as well as the amount of calcite remaining, as shown in Table 6.

**Table 6.** Rietveld refinement of the water-to-solids ratio series of LDHs WS1 = 70:30, WS2 = 80:20 and WS3 = 90:10. HC indicates the phase [Ca4Al2(OH)12][(CO3) · 5 H2O], Kat is katoite, P is portlandite and C is calcite. All values are given in percentages of the crystalline phases.


Even with this large difference in the amount of each phase present, crystallisation of the LDH phases appeared similar on the SEM micrographs, shown in Figure 23. However, less particulate matter was present in WS1 and more in WS3, in good correlation with the decreased amount of portlandite present. An increased amount of katoite was also present in WS2 and WS3. The large amount of calcite determined through the Rietveld analysis for WS1 was not observed in its micrograph.

**Figure 23.** SEM micrographs of the water-to-solids ratio series WS1 = 70:30, WS2 = 80:20 and WS3 = 90:10 taken at 1 keV and 2k magnification. The scale bar is indicated below the label.

The pHs recorded for WS1, WS2 and WS3 showed that significant differences existed between the starting pH of the reaction mixture and the behaviour during synthesis with time (Figure 24). WS1, as expected due to the larger amount of reactants present, had a very high initial pH but then exhibited a considerable reduction in pH over time. WS3, on the other hand started off with a much lower pH, which remained almost constant throughout the reaction. WS1 and WS3 had opposing viscosities, the one being very thick, the other very thin. Yet they still ended on a similarly low pH after 3 h.

**Figure 24.** pHs of the water-to-solids ratio series of LDHs WS1 = 70:30, WS2 = 80:20 and WS3 = 90:10. The pH was adjusted to 25 ◦C to facilitate comparison.
