*2.6. Mixing*

The mixing during synthesis was varied by altering the rotational speed of the magnetic stirrer used. Three rotational speeds were used: M1 = 500 rpm, M2 = 750 rpm and M3 = 1000 rpm. During the synthesis, these rotational speeds led to mixing at full incorporation at all times (1000 rpm), sufficient mixing to keep the system visibly agitated but cause a thin condensate film forming (750 rpm) and mixing that kept the reaction mixture agitated but led to the formation of a thicker condensate film on top of the mixture (500 rpm). Little difference was observed in the outcome of the reaction in XRD and FTIR-ATR results, as shown in Figure 19a,b.

**Figure 19.** (**a**) XRD results of the mixing series M1, M2 and M3 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 mixing series M1, M2 and M3 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. M1 = 500 rpm, WS2 = 750 rpm and WS3 = 1000 rpm.

Rietveld refinement further confirmed the similarity between these three reaction outcomes. As shown in Table 5, only small variations in HC, katoite, portlandite and calcite content were observed. Unsurprisingly, SEM (Figure 20) also showed very little difference in the morphologies of M1, M2 and M3.

**Table 5.** Rietveld refinement of the mixing series of LDHs M1 = 500 rpm, M2 = 750 rpm and M3 = 1000 rpm. 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.


**Figure 20.** SEM micrographs of the mixing series M1 = 500 rpm, M2 = 750 rpm and M3 = 1000 rpm taken at 1 keV and 2k magnification. The scale bar is indicated below the label.

However, surprisingly, even though there was almost no difference seen between M1, M2 and M3 in terms of the other results, the pH of M1 and M3 decreased notably with time in comparison to that of M2 (Figure 21).

**Figure 21.** pHs of the mixing series of LDHs M1 = 500 rpm, M2 = 750 rpm and M3 = 1000 rpm. The pH was adjusted to 25 ◦C to facilitate comparison.
