**2. Results**

The method chosen leads to a self-regulated-pH synthesis through a dissolution-precipitation mechanism. The only externally variable parameters during the reaction are those that will be discussed in this paper (temperature, time, molar calcium-to-aluminium ratio, mixing, chemical morphology/crystallinity and water-to-solids ratio) and the use of other reactants, such as oxides, etc. which will not be discussed here. In addition to these, the chosen system was self-contained. During the synthesis, the system remained in an inert atmosphere to mitigate atmospheric carbonate contamination. The synthesis pH was sampled at regular intervals and was recorded continuously during some syntheses.

#### *2.1. The Standard LDH and Its Carbonate Form*

One LDH was common between all sets of experiments, with it synthesised at 80 ◦C for 3 h, a stirring speed of 750 rpm, a water-to-solids ratio of 80:20, at a molar calcium-to-aluminium ratio of 4:2 and using amorphous aluminium hydroxide (SA).

The standard sample was synthesised in triplicate to determine the repeatability of the synthesis method used and the trustworthiness of the Rietveld refinement applied to the wet-phase XRD data. The XRD and FTIR-ATR results for the standard LDHs are shown in Figure 1a,b, respectively.

**Figure 1.** (**a**) XRD results of the three standard hydrocalumite (HC) layered double hydroxides (LDHs) S1, S2 and S3 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 three standard HC LDHs S1, S2 and S3 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>1–3600 cm<sup>−</sup>1, (**2**) 1500 cm<sup>−</sup>1–1250 cm<sup>−</sup>1, (**3**) 1000 cm<sup>−</sup>1–550 cm<sup>−</sup><sup>1</sup> in detail. Dashed and solid grey lines indicate the maxima of vibrations.

As a result of cation ordering and interlayer offsets, HC is typically presented in monoclinic form but has also been found in nature in the 6T polytype [2]. Other polytypes also exist, especially in the synthetically produced HCs, although a comprehensive review of all different polytypes of HC is lacking. The standard HCs were best described by HC of the formula [Ca4Al2(OH)12][(CO3) · 5 H2O] in the anorthic (triclinic) crystal system and P1 space group with crystal parameters a = 5.7747 Å, b = 8.4689 Å and c = 9.9230 Å (reference code: 98-005-9327). The XRD patterns of the three standard HC LDHs were very similar with the exception of S2 which had a secondary phase that could not be identified. Contaminants of katoite (Ca3Al2(OH)12), portlandite (Ca(OH)2) and calcite (CaCO3) were found in all three LDHs to varying degrees.

FTIR-ATR analysis showed that the standard HC LDHs S1, S2 and S3 all portrayed almost identical vibrational responses at 3675 cm<sup>−</sup>1, 3669 cm<sup>−</sup>1, 3641 cm<sup>−</sup>1, 3620 cm<sup>−</sup>1, 3540 cm<sup>−</sup>1, 3517 cm<sup>−</sup>1, 3358 cm<sup>−</sup>1, 3243 cm<sup>−</sup>1, 3010 cm<sup>−</sup>1, 2828 cm<sup>−</sup>1, 1640 cm<sup>−</sup>1, 1414 cm<sup>−</sup>1, 1361 cm<sup>−</sup>1, 943 cm<sup>−</sup>1, 883 cm<sup>−</sup>1, 870 cm<sup>−</sup>1, 803 cm<sup>−</sup>1, 750 cm<sup>−</sup>1, 717 cm<sup>−</sup><sup>1</sup> and 663 cm<sup>−</sup>1. Typically, for LDHs, vibrations in the region between 3700 cm<sup>−</sup><sup>1</sup> and 2500 cm<sup>−</sup><sup>1</sup> are ascribed to OH str. vibrations of hydroxides bonded to the metal ions, of interlayer water or of water bonded to carbonate in the interlayer region. Ref. [30] assigned the vibrations around 3500 cm<sup>−</sup><sup>1</sup> and 3305 cm<sup>−</sup><sup>1</sup> to the Al–OH str. and Ca–OH str. vibration in the brucite-like lattice, respectively and the vibrations around 3100 cm<sup>−</sup><sup>1</sup> and between 2915 cm<sup>−</sup><sup>1</sup> and 2935 cm<sup>−</sup><sup>1</sup> to the OH str. vibrations of interlayer water and water bonded to interlayer carbonate, respectively. A similar assignment has been made for MgAl-LDHs, spanning different synthesis methods [31]. Thus, the vibrations at 3517 cm<sup>−</sup>1, 3358 cm<sup>−</sup>1, 3243 cm<sup>−</sup><sup>1</sup> and 3010 cm<sup>−</sup><sup>1</sup> were assigned to the OH str. vibrations of Al–OH and Ca–OH bonds, and interlayer H2O and CO3<sup>2</sup> – bonded to interlayer H2O, respectively. The vibration at 3641 cm<sup>−</sup><sup>1</sup> could be assigned to the remaining portlandite in the samples (see Figure 2b). Some of the other vibrations between 3700 cm<sup>−</sup><sup>1</sup> and 3500 cm<sup>−</sup><sup>1</sup> have been observed in spectra of gibbsite (Al(OH)3) [32] and have also been ascribed to different OH species [26]. The vibration at 2828 cm<sup>−</sup><sup>1</sup> could be a combination band as mentioned by [31]. On the lower end of the spectrum, the vibration at 1640 cm<sup>−</sup><sup>1</sup> could be present due to both, *<sup>ν</sup>asym* vibrations of interlayer anions (such as hydroxides or carbonates) and *ν*2 (H2O) [31,32]. The vibration at 1414 cm<sup>−</sup><sup>1</sup> corresponds strongly to calcite (see Figure 2b) and is frequently assigned with 1365 cm<sup>−</sup><sup>1</sup> as the doublet *ν*3 (CO3<sup>2</sup> – ) vibration [33]. However, [31] have found this vibration to also correspond to the *<sup>ν</sup>sym* vibration of the interlayer anion (for both, CO3<sup>2</sup> – and OH– ). The vibrations at 943 cm<sup>−</sup>1, 870 cm<sup>−</sup>1, 717 cm<sup>−</sup><sup>1</sup> and 663 cm<sup>−</sup><sup>1</sup> could be assigned to *<sup>ν</sup>sym* (OH), *ν*2 (CO3<sup>2</sup> – ) (interlayer and contaminant CaCO3), contaminant CaCO3 and *ν*4 (CO3<sup>2</sup> – ), respectively. Vibrations at 803 cm<sup>−</sup><sup>1</sup> and 750 cm<sup>−</sup><sup>1</sup> have been observed in gibbsite spectra but remained unassigned [32]. The vibration at 803 cm<sup>−</sup><sup>1</sup> has also previously been assigned to katoite [26].

The best-fit crystal structure of HC—as identified by XRD—contained carbonate in the interlayer, contrary to the carbonate-free synthesis utilised. Although this, of course, is not evidence enough of carbonate intercalation, FTIR also showed the existence of carbonate-related vibrations. An HC LDH containing carbonate (sample ID: CO3) was, therefore, synthesised, the characteristics of which are shown in Figure 2.

**Figure 2.** (**a**) XRD results of the HC-CO3-LDH plotted against the standard HC LDH S2 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 HC-CO3-LDH plotted against the standard HC LDH S2 and the starting materials used (Al(OH)3, Ca(OH)2 and CaCO3) 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.

The standard and CO3 specimens had the anorthic [Ca4Al2(OH)12][(CO3) · 5 H2O] phase as best fit (Figure 2a). However, the primary reflection of CO3 proved to be shifted to the left of that of S3. The FTIR-ATR results (Figure 2b) showed that some differences exist between these two phases. Synthesis of HC-CO3-LDH was conducted with adjustment for the stoichiometric amount of CO3<sup>2</sup> – required through replacement of Ca(OH)2) with CaCO3. No Ca(OH)2 was detected at the end of this synthesis but a large amount of CaCO3 remained. This is clearly evident on the FTIR scans (Figure 2b) through an increase in vibration strength at 1414 cm<sup>−</sup>1, 870 cm<sup>−</sup>1, 717 cm<sup>−</sup><sup>1</sup> and 663 cm<sup>−</sup>1, and the disappearance of the vibration at 3641 cm<sup>−</sup>1.

Rietveld refinement of S1 and S3 showed that a material consisting of approximately 60% HC could be obtained, the rest being katoite (approximately 28%), portlandite (approximately 10%) and calcite (approximately 2%). No Rietveld refinement could be performed on S2 due to the presence of the unidentifiable phase. CO3 consisted of approx 70% LDH, 10% katoite and 20% calcite. The analysis of S3 will be used for comparative purposes in the remainder of the text.

Figure 3 depicts the micrographs obtained for S1, S2, S3, CO3 and the Ca(OH)2, Al(OH)3 and CaCO3 used in the synthesis.

LDH platelet formation was evident in all of the synthesised materials in Figure 3 and constituted the majority of the identifiable phase with thin, large hexagonal platelets, typically with an elongated facet. Katoite was visually identifiable as faceted garnet-like balls and calcite as cubic crystals. Small particulate matter was present in all samples, sticking on the LDH platelets and other phases and sometimes forming small agglomerates. This could be either Ca(OH)2 or Al(OH)3 left over. While no left-over Al(OH)3 was identified through XRD (unsurprisingly, since an amorphous Al(OH)3 was used in the synthesis) it is possible that the small structures are either portlandite or amorphous Al(OH)3, which proved to be similar in size and appearance. SEM showed that CO3 consisted of smaller platelets than the standard LDHs. It was also more difficult to find remnants of the left-over CaCO3 used, even though this LDH consisted of 72.47% LDH, 9.24% katoite and 18.29% calcite, according to Rietveld refinement. This could be attributed to the CaCO3 consisting of ill-defined oblong shapes that would be difficult to identify on SEM micrographs. It does, however, introduce a difference between the calcite fed with Ca(OH)2 (cubic crystals) and the calcite supplied to CO3.

**Figure 3.** SEM micrographs of the HC LDHs S1, S2, S3 and CO3 at 1 keV and 2k magnification, and Ca(OH)2, Al(OH)3-SA (amorphous) and CaCO3 at 1 kEV and 10k magnification. The scale bar is indicated under the label. LDH: turquoise hexagon, katoite: turquoise circle, calcite: turquoise asterisk.

pH measurements during the reaction showed that the self-regulated pH remained close to 11 for the duration of the synthesis at 80 ◦C for S1, S2 and S3. pH fluctuations were, however, common—especially after 1 h to 1.5 h of reaction time when the reaction mixture considerably thickened up—and, overall, the pH decreased slightly with time during the 3 h of synthesis. The pH during synthesis of CO3 was significantly lower than that of S1, S2 and S3. The pH probe (used for sampled pH recording) required substantial agitation to read the pH correctly after 2 h. As a result of these fluctuations and sampling difficulties, even though the results showed very good comparability, the pHs shown in this work are to be taken as an estimate. Figure 4 shows the pH readings obtained for S1, S2 and S3 over 3 h.

**Figure 4.** pHs of the standard HC LDHs S1, S2, S3 and CO3. The pH was adjusted to 25 ◦C to facilitate comparison.

For S1 a sampling pH method was used where the pH probe was cleaned before every measurement. The pH of S2 and S3 was measured continuously. The pH of S2 exhibited greater fluctuations than that of S3. This could be attributed to the encased pH probe used for S2 which was

subject to material build-up. Due to the sampling method employed for other experiments, the pH measurements of S1 will be used for comparative purposes in the remainder of the text.

In the following sections, only deviations from the standard S3 discussed in this section—caused by varying one of the studied parameters—will be highlighted. S3 will thus, in the following text, be synonymous with T4, t2, MR2, A1, M2 and WS2.
