Calcined hydrotalcite or its use under high temperatures (around 400 °C so that the hydrotalcite becomes oxide and can be rebuilt in contact with CO₂). LDH are poor CO₂ adsorbents in their natural or unburned form, which is due to a poor basic property and the presence of entities that hinder CO₂ adsorption. Hence, they are subjected to thermal treatment to obtain nearly amorphous metastable mixed solid solutions (CLDHs) [19]. Figure 1 shows two diagrams representing what happens during the calcination of hydrotalcite of Mg₃AlCO₃ as shown (A) by W.J. Long et al. [77] and (B) by Lauermannová et al. [73]. Both schemes attempt to represent the collapse of the structure due to the loss of interlayer anions and moisture.

LDH undergoes several stages until it reaches CLDHs. There is even research that attempts to explain this process in great detail and focuses on it alone [80,81]. It is very important to choose a suitable calcination temperature, avoiding it being too high (to avoid higher energy consumption), or being too low (not producing the collapse of the structure and hindering a higher CO₂ capture). To distinguish the different stages, it is useful to rely on thermogravimetric analysis (TGA) and differential thermal analysis (DTA) carried out by Suescum-Morales et al. [33], shown in Figure 2. It should be noted that different variations in temperature ranges may be encountered, approximately as shown below.

First of all, a loss of humidity is observed, up to a temperature of about 105 °C. The second stage occurs from 105 to 270 °C, where the water hydration of the hydrotalcite structure is lost (leading to a decrease in basal spacing) [82,83,84]. There are authors who indicate that between 200 and 300 °C, the OH⁻ groups attached to Al³⁺ are lost [19]. The third stage, from 270 to 540 °C, is where the dehydroxilation of the hydrotalcite takes place and the loss of the carbonate anion of the interlayer in the form of CO₂ occurs. The layer structure collapses (Figure 1), and the LDH converts to a mixed-oxide MgO-like phase [85]. In the last stage, from 540 to 1000 °C, very small weight losses are observed, attributed to the loss of residual OH⁻ groups.

From the above, it can be seen that the calcination temperature affects the capacity to capture CO₂ in CLDHs. Different structural characteristics are presented in the different stages of thermal decomposition. From this analysis, the ideal calcination temperature of the hydrotalcite under study can be determined, which is characteristic and unique depending on the type of hydrotalcite. Most research indicates that the calcination temperature of a Mg-Al LDH is around 400 °C [86,87,88]. However, the performance of a TGA/DTA for each specific case would allow observing the exact calcination temperature.

The appropriate calcination temperature can also be determined by XRD temperature variation analysis. The thermal decomposition sequence of Mg-Al-CO₃ hydrotalcite is well documented. Miyata, 1980 and Hibino et al., 1995 [82,89] studied the XRD variation at different temperatures, shown in Figure 3 (non-calcined, 150, 250, 350, 500, 850 and 1000 °C). The diffraction patterns of MgO can be identified between 400 °C and 850 °C, given the amorphous nature of Al₂O₃ at this temperature. For 900 °C the spinal phase (MgAl₂O₃) was formed. Given these experiences, the following chemical equations can be posed, with the different temperature ranges, to help us understand the process (Equations (3)–(5)):

Figure 4 shows the similar XRD obtained by several authors by calcination at 500 °C for different periods: firstly, the XRD obtained by W.J. Long et al. [77] (Figure 4a) shows how at a temperature of 500 °C for 3 h the layered structure collapses and also shows the production of mixed oxides. Figure 4b shows a similar result, but in this case using a calcination time of 2 h, and the same temperature (500 °C) [33]. This leads to a large saving of energy in the calcination of LDH, with a consequent lowering of the carbon footprint. Already, Z. Yang et al. [67] calcined at 500 °C for 3 h, obtaining a similar result. Even Q.Tao et al. [90] heated at 500 °C for 4 h, obtaining similar results (Figure 4c). Similar results in XRD were obtained by S.I. Garcés Polo et al. [91] for CLDH. None of the previous authors [33,67,77,91] indicated the amount of sample used in the calcination, which may have led to these observed differences. Although different calcination temperatures have been used with similar results, there are no economic studies of the cost of calcination; studies of this type, with times, temperatures and quantities of LDH to be calcined, would be very useful for these materials in industrial applications. It would also be very important to carry out a real carbon footprint calculation (UNE EN 15804:2012), which determines the real CO₂ sink capacity in each specific case (amount of hydrotalcite, type of furnace, etc.). Only two studies have been found that indicate the amount of hydrotalcite calcined (100 and 1 g respectively) [92,93]. Annotations of this type, i.e., what quantity is fed into the LDH kiln, are very important in order to maximise the efficiency of the calcination process.

Figure 5a shows SEM images of commercial LDH and CLDH Mg-Al from W.J. Long et al. [77]. After 3 h at 500 °C, the structure collapses. A decrease in size was also observed. P. Cai et al. [95] obtained similar results (Figure 5b) with the same temperature and time of calcination. After 4 h at 450 °C on Mg-Al LDH, C. Geng et al. [96] observed a decrease in particle size, and the hexagonal shape was hardly noticeable (Figure 5c). No other studies using different temperatures (different at 500 °C) and calcination times have been found that show SEM images of LDH and CLDH. Studies along these lines could fill this information gap.

Figure 6A shows the TEM images of LDH and CLDH, after 2 h at 500 °C, obtained by Suescum-Morales et al. [33]. In CLDH, small pores were formed, attributable to the dehydration process, dehydroxilation of the OH⁻ groups, and to the decomposition of the interlayer carbonate. C Hobbs et al. [97] studied the evolution of a Mg-Al LDH under different temperatures using a rate of 10 °C/min (Figure 6B). At a temperature of 20 °C (LDH), they have a well-defined platelet shape; the porous structure is clearly visible at 850 °C. S Luo et al. [98] also obtained the same porous structure in CLDH, as shown in Figure 6C.

Different heating rates have been used in LDH calcination; a heating rate of 10 °C/min was used in Mg/Al LDH by several authors [99,100,101,102] and a heating rate of 5 °C /min was used by [103]. It is known that if the heating rate is slower, the thermal effects are better observed (better porous structure). However, if the ramp is slower, it takes more time and wastes more energy; if the heating is too fast, the porous structure will be worse. These differences should be extensively studied, both from an energy point of view (higher consumption and, therefore, higher carbon footprint produced by the kiln when the larger ramp is used), and from the point of view of the efficiency of the CLDH itself. Another very important aspect is the kinetics of LDH, which has been extensively discussed in other research [104,105].

7. Combined Use of Hydrotalcites and Cement-Based Materials