**1. Introduction**

Hydrocalumite (HC) is a CaAl-LDH with the general formula

$$[\text{Ca}\_{1-x}\text{Al}\_x(\text{OH})\_2][\text{X}\_{\text{x}/\text{q}}\text{ ${}^{\text{q}}$ }\cdot \text{nH}\_2\text{O}] \tag{1}$$

where *x* is the ratio of trivalent to total cations in the layered double hydroxide (LDH) lattice, *X* is the interlayer anion, *q* its charge and *n* the amount of water present in the interlayer [1]. As a result of its corrugated-iron-like structure, the ratio of Ca:Al is limited to 2:1 (*x* = 0.33¯ ) [2]. In nature, HC occurs with chloride and hydroxide anions in the interlayer, but many other anions can be intercalated.

HC is used in numerous scientific fields, including catalysis [3–5], sensors [6,7], medical applications [8,9], environmental remediation [10–12], agriculture [13], polymers [6,14,15] and occurs in

cementitious phases during curing [16]. As with other LDHs for study in applications, this material is typically produced using co-precipitation—the most widely used synthesis technique for LDHs [1,17]. While co-precipitation has many advantages, such as simplicity, speed and high tailorability of the materials produced, it is also one of the most polluting synthesis methods available to produce LDHs, causing large amounts of salt-rich waste water.

Co-precipitation synthesis utilises a mixture of metal salts (typically metal chlorides, nitrates or sulphates) and a base (frequently NaOH or KOH) which are added dropwise to a beaker, typically containing the anion to be intercalated. While many derivatives of this method exist, the basic concept remains. During synthesis, LDH precipitates out of solution. After synthesis, this precipitate must be filtered from the resulting slurry to further process the material. During filtering, large amounts of water are used to wash the filtrate liquor—that would otherwise dry with the LDH filter cake and contaminate the final product—out of the material. Depending on the chemicals used, the resulting filtrate can be rich in sodium or potassium and chlorides, nitrates or sulphates and, of course, excess intercalant ions. In addition to large-scale environmental pollution associated with these untreated waste-streams (if released to the environment), the chemicals required for the synthesis of LDHs are expensive. Considering worsening environmental pollution and the drive to reduce the impact of the chemical industry on the environment, it has become evermore important to find alternative syntheses for these materials.

There exist a multitude of synthesis routes to produce LDHs. Urea hydrolysis and sol-gel syntheses are often used. These methods use urea (for urea hydrolysis) or metal alkoxides, alcohol and acids as chelating agents (for sol-gel synthesis). While these are not environmentally friendly alternatives to co-precipitation, attempts have been made to make sol-gel synthesis more environmentally-friendly [18]. In addition to these, there exist less frequently used methods that can be used in an environmentally friendly manner, such as hydrothermal or mechanochemical synthesis. In both methods, metal hydroxides or metal oxides are common starting materials that are mixed with water and processed at elevated temperatures and pressures (hydrothermal method) or milled together (mechanochemical synthesis). Use of the metal hydroxides and oxides as starting materials is hereby key to producing less polluting waste streams. In previous work, we have shown that the hydrothermal process can produce very pure phases of MgAl-LDH, even with a recycle-based system that reduces waste-streams [19]. It has been shown that mechanochemical synthesis (wet-milling) of Ca(OH)2 and Al(OH)3 could lead to an HC-phase content similar to that achieved in this work [20].

HC has been synthesised using a hydrothermal method in previous works, with a grea<sup>t</sup> interest in the effects of the presence of CaO/Ca(OH)2 and Al(O)OH/Al(OH)3/Al2O3 in presence of water and CO2 on the curing of cement [16,21–23]. In fact, some of the earliest reports concerning the study of the material and its characterisation used a hydrothermal synthesis [22,24,25]. HC is especially well suited to hydrothermal synthesis because of the metal oxides and hydroxides that can be used at mild conditions to produce the desired phase. In these early reports of hydrothermal HC synthesis and related studies, HC (also frequently referred to in cement and concrete literature as tetracalcium monocarboaluminate, a carbonate intercalated CaAl-LDH) was synthesised using several approaches. Ref. [25] synthesised HC by reacting Ca(OH)2, Al(OH)3 and CaCO3 in water for one month at 2 kbar and at 100 ◦C. Ref. [26] prepared HC using different sources of aluminium (gibbsite and boehmite) and CaCO3 (with different surface areas and particle sizes), and Ca(OH)2 at varying temperatures and reacting the mixture for 24 h to 48 h. They also did a small study on the effect of temperatures between 70 ◦C and 90 ◦C, and found that the largest fraction of HC is formed at 80 ◦C. It was found that the reaction temperature significantly affects the product formed, being HC or katoite (Ca3Al2(OH)12). In our own study of the effect of temperatures between 30 ◦C and 90 ◦C on the hydrothermal synthesis of HC using Ca(OH)2 and highly crystalline Al(OH)3, similar results were obtained [27]. Ref. [28] investigated the crystal structure and phase transitions in HC from −115 ◦C to 45 ◦C. They used Ca(OH)2, Al(OH)3, CaCl2 · 6 H2O and CaCO3 to create single crystals at 120 ◦C and 2 kbar in two months. They prepared powder HC samples by reacting Ca3Al2O6, CaCl2 · 6 H2O and CaCO3 in water at room temperature and under inert atmosphere for four weeks. Ref. [29] synthesised HC using CaO, Al2O3 and K2CO3 within an hour at 100 ◦C with microwave irradiation assistance. There also exist several thermodynamic studies [21–24] and studies concerning pressure-induced reactions [16] in the system CaO-Al2O3-H2O. These studies have shown that the species formed can be similar to HC [24]. Ref. [3] used a carbonate-free approach for the synthesis of HC by reacting Ca(OH)2 and Al(O)OH in water under an inert atmosphere at 80 ◦C for 3 h. However, it was shown that the calcium hydroxide used for synthesis was contaminated with calcite. No FTIR analysis was conducted to determine the interlayer anions, but preparation of the HC-like compounds with nitric acid led to the formation of a pure HC-like phase similar to nitrate intercalated HC.

From the above literature it is evident that several attempts on the hydrothermal synthesis of HC have been made; however, most of these attempts were either time-intensive or time- and energy-intensive, because of the long reaction times, temperatures and pressures required. Further, most work has focused on the synthesis of a carbonate-intercalated LDH. To create a CaAl-CO3-LDH, a carbonate source is required during synthesis. In others works, this was achieved using CaCO3 or K2CO3. However, as carbonate uptake is favoured, it is difficult to reverse. As many HC applications have anion-exchange reactions as a subsequent step or end goal, the presence of carbonate is frequently undesirable. Removing the interlayer carbonate, usually involves calcination—a process that may alter the structure and morphology of the LDH, which can be undesirable. Thus, this contribution focuses on the preparation of CaAl-OH-LDH by using Ca(OH)2, Al(OH)3 and water under low-temperature and atmospheric-pressure conditions. The purpose of this paper is to investigate whether it is possible to achieve a high purity HC using hydrothermal synthesis with this minimal number of chemicals and at which synthesis conditions this is possible, if at all. Several parameters can influence the hydrothermal synthesis of HC, these being temperature, time, molar calcium-to-aluminium ratio, chemical morphology/crystallinity of the reactants, mixing and the water-to-solids ratio used. The influence of these on the conversion of reactants to HC will be discussed in the following text.
