*2.2. Reaction Temperature*

The reaction temperature was varied between 20 ◦C and 90 ◦C (T1 = 20 ◦C, T2 = 40 ◦C, T3 = 60 ◦C, T4 = 80 ◦C and T5 = 90 ◦C). The corresponding XRD (a) and ATR-FTIR (b) results are shown in Figure 5.

**Figure 5.** (**a**) XRD results of the temperature series T1, T2, T3, T4 and T5 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, oHC = secondary HC-like phase, Kat = katoite, P = portlandite and C = calcite. (**b**) FTIR-ATR scans of the temperature series T1, T2, T3, T4 and T5 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. T1 = 20 ◦C, T2 = 40 ◦C, T3 = 60 ◦C, T4 = 80 ◦C and T5 = 90 ◦C.

The [Ca4Al2(OH)12][(CO3) · 5 H2O] anorthic phase once again proved to be the best fit for the phase produced. However, in this series some additional phases were present, depending on the synthesis temperature. As shown in the inset of Figure 5a, an increase in temperature led to first the formation and then the disappearance of an LDH-like phase with a layered structure, imitating HC. This phase was most prominent in the synthesis at 20 ◦C and was not present at 60 ◦C. The phase was best fit by 3 CaO · Al2O3 · 0.5Ca(OH)2 · 0.5CaCO3 · 11.5H2O, a rhombohedral structure. Being an outdated entry, however, the closest fit that could be used for Rietveld refinement is the structure identified by [34] for calcium hemicarboaluminate, a partially carbonated (low carbonate content) phase that is formed at the start of carbonation reactions in cement Ca4Al2(OH)12(OH)(CO3)0.5 · 4 H2O. The first phase, a calcium aluminate carbonate hydrate was studied by [35]. At the time it was noted that this compound could be a meta-stable phase that forms in cement but had not been found in Portland cement at the time. In T2, this phase was still present but its reflection shifted slightly to the right—closer resembling calcium hemicarboaluminate—and more of the always-observed HC phase [Ca4Al2(OH)12][(CO3) · 5 H2O] was present. With an increase in temperature, HC formation increased up to 80 ◦C. Katoite and portlandite were present in all samples, while calcite was only present in T4 and T5. Calcite content increased at 90 ◦C. Table 1 shows the progression of the phase contents with temperature as determined by Rietveld refinement.

**Table 1.** Rietveld refinement of the temperature series of LDHs T1 = 20 ◦C, T2 = 40 ◦C, T3 = 60 ◦C, T4 = 80 ◦C and T5 = 90 ◦C. HC indicated the phase [Ca4Al2(OH)12][(CO3) · 5 H2O], oHC indicates the phase Ca4Al2(OH)12(OH)(CO3)0.5 · 4 H2O, Kat is katoite, P is portlandite and C is calcite. All values are given in percentages of the crystalline phases.


Some differences in spectra could also be observed with FTIR-ATR, most notably in T1 and T2—which showed a shoulder left of the vibrations at 3669 cm<sup>−</sup><sup>1</sup> and changed intensity of other vibrations as indicated in the insets of Figure 5b and in the region between 3517 cm<sup>−</sup><sup>1</sup> and 2828 cm<sup>−</sup>1. With an increase in temperature, the 3010 cm<sup>−</sup><sup>1</sup> and 2828 cm<sup>−</sup><sup>1</sup> vibrations decreased in intensity. The 3620 cm<sup>−</sup><sup>1</sup> Al–OH str. vibration was significantly reduced in T1. Further, T1 showed less definition of the 1414 cm<sup>−</sup>1/1361 cm<sup>−</sup><sup>1</sup> doublet and a greater resemblance of the calcite vibration in this region shown in Figure 2b. T1 also showed less definition in the vibrations between 1000 cm<sup>−</sup><sup>1</sup> and 550 cm<sup>−</sup>1.

SEM micrographs showed clear differences in the materials obtained and a change in distribution of phases (Figure 6).

**Figure 6.** SEM micrographs of the HC LDHs T1 = 20 ◦C, T2 = 40 ◦C, T3 = 60 ◦C, T4 = 80 ◦C and T5 = 90 ◦C at 1 keV and 2k magnification. The scale bar is indicated under the label.

All samples showed plate-like structures. In T1, these structures were highly agglomerated and packed into stacks of ill-defined platelets. Round, ball-like structures were mixed in-between these platelet agglomerates. It is possible that these were ill-defined katoite structures. T2 showed better definition of the platelet structures, albeit with them still being small and unevenly crystallised, and a large amount of katoite, which was omnipresent between stacks of platelets. No cubic calcite structures were visible in these micrographs. T3, T4 and T5 showed well-defined platelets, with the best-defined platelets being present in T4. Katoite and cubic calcite structures were visible in all three micrographs, corresponding to the Rietveld analysis. Particulate matter was visible in all samples and could again be either unreacted Ca(OH)2 or amorphous Al(OH)3. The secondary calcium aluminate carbonate hydrate phase or calcium hemicarboaluminate phase present in T1 and T2 could not be discerned from other phases.

The pH documented during synthesis varied greatly with temperature. Figure 7 shows the sampled pHs (adjusted to 25 ◦C) as a function of time for comparative purposes.

**Figure 7.** pHs of the temperature series of LDHs T1 = 20 ◦C, T2 = 40 ◦C, T3 = 60 ◦C, T4 = 80 ◦C and T5 = 90 ◦C. The pH was adjusted to 25 ◦C to facilitate comparison.

The recorded pH rose with an increase in temperature. The pHs at 20 ◦C and 40 ◦C were almost identical. Increasing the temperature to 60 ◦C led to the start of the reaction at a higher pH, lowering with the progression of time. At 80 ◦C the pH stayed constant (only decreasing slightly with time), while at 90 ◦C, the pH was very high at the start of reaction and dipped before rising again after 2 h of reaction time had passed.
