Performances Assessment of Tricalcium Aluminate As an Innovative Material for Thermal Energy Storage Applications

applied sciences

Article Performances Assessment of Tricalcium Aluminate as an Innovative Material for Thermal Energy Storage Applications

Fabrizio Alvaro 1,* , Elpida Piperopoulos 1,2 , Luigi Calabrese 1,* , Emanuele La Mazza 1, Maurizio Lanza 3 and Candida Milone 1,2

1 Department of Engineering, University of Messina, 98166 Messina, Italy; [email protected] (E.P.); [email protected] (E.L.M.); [email protected] (C.M.) 2 National Interuniversity Consortium of Materials Science and Technology (INSTM), 50121 Firenze, Italy 3 Istituto per i Processi Chimico Fisici, Consiglio Nazionale delle Ricerche (CNR), 98158 Messina, Italy; [email protected] * Correspondence: [email protected] (F.A.); [email protected] (L.C.)

Featured Application: Innovative material for thermochemical energy storage applications.

Abstract: In this paper, tricalcium aluminate hexahydrate (Ca3Al2O6·6H2O), thanks to its appropriate features, was assessed as an innovative, low-cost and nontoxic material for thermochemical energy storage applications. The high dehydration heat and the occurring temperature (200–300 ◦C) suggest that this material could be more effective than conventional thermochemical storage materials

operating at medium temperature. For these reasons, in the present paper, Ca3Al2O6·6H2O hydration/dehydration performances, at varying synthesis procedures, were assessed. Experimentally, a co-precipitation and a solid–solid synthesis were studied in order to develop a preparation method that better provides a performing material for this specific application field. Thermal analysis (TGA, Citation: Alvaro, F.; Piperopoulos, E.; Calabrese, L.; La Mazza, E.; Lanza, DSC) and structural characterization (XRD) were performed to evaluate the thermochemical behavior M.; Milone, C. Performances at medium temperature of the prepared materials. Furthermore, reversibility of the dehydration Assessment of Tricalcium Aluminate process and chemical stability of the obtained materials were investigated through cycling dehy- as an Innovative Material for Thermal dration/hydration tests. The promising results, in terms of de/hydration performance and storage Energy Storage Applications. Appl. density (≈200 MJ/m3), confirm the potential effectiveness of this material for thermochemical energy Sci. 2021, 11, 1958. https://doi.org/ storage applications and encourage further developments on this topic. 10.3390/app11041958 Keywords: thermochemical storage materials; tricalcium aluminate hexahydrate; hydrothermal Academic Editor: Luisa F. Cabeza stability; thermal cycle testing

Received: 7 February 2021 Accepted: 18 February 2021 Published: 23 February 2021 1. Introduction

Publisher’s Note: MDPI stays neutral The great changes that are affecting our planet have undeniably led to a significant with regard to jurisdictional claims in and radical transformation of the energy system, which needs to be remodeled on the published maps and institutional affil- basis of better efficiency, especially in regards to the industrial sector. From this point of iations. view, improving energy efficiency means principally acting on the recovery of low-grade (100–200 ◦C) and medium-grade (200–300 ◦C) waste heat, which represent about 90% of the thermal energy lost in industrial processes [1]. To this purpose, in the last twenty years, the awareness of an urgent upgrade of thermal energy storage systems has led to the development of chemical heat pumps (CHPs), devices whose operation is based on Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. a thermochemical storage material (TCM) capable of accumulating and releasing energy This article is an open access article through a reversible reaction, schematized in Equation (1). distributed under the terms and A + heat B + C (1) conditions of the Creative Commons Attribution (CC BY) license (https:// where the direct process (endothermic) represents the thermal charge, in which the waste creativecommons.org/licenses/by/ heat allows the reaction to take place and the energy is therefore recovered. On the contrary, 4.0/).

Appl. Sci. 2021, 11, 1958. https://doi.org/10.3390/app11041958 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 1958 2 of 17

the reverse (exothermic) process represents the thermal discharge, in which the products (B and C), when mixed together, regenerate the initial compound (A), releasing the previously stored energy. Thermochemical storage promises to offer numerous advantages over latent heat and sensible heat technologies, despite these technologies currently being in an advanced stage of development [2,3]. Firstly, the conversion of thermal energy into chemical energy ensures a long-term durability; in addition, the typical high-storage densities allow for significantly reduced volumes, making the storage systems easier to handle [4,5]. Although it is relatively easier with sorption materials (which involve intermolecular bond- ing) to set up a system that operates cyclically with high performances, thermochemical storage materials (TCMs), which operate through proper chemical reactions, present many issues related to several factors, like chemical and structural stability of each reagent and product [6–8]. Within this research field, materials based on solid–gas reactions are the most investigated, ascribed to the possibility of collecting, in an isolated tank, the formed gas when the thermal charge ends, subsequently keeping the products separated until the thermal discharge is needed [9–12]. In recent years, many charge/discharge working pairs operating by reduction/oxidation [13–16] (oxides, salts), decarbonation/carbonation [17] (carbonates) and dehydration/hydration (hydroxides) [18] have been assessed. However, among these materials, it could be noted that the range between 200–250 ◦C has the great- est lack of developments; in fact, there is no material operating in this range that can be competitive, in relation to the storage density, with pairs operating at higher temperature and which are currently in an advanced stage of application (magnesium hydroxide, calcium hydroxide [19–24]). As mentioned above, chemical and structural stability play a fundamental role, and it is not always easy to find systems capable of providing a good compromise between a high storage capacity and a long-term durability. Hence, the development of new TCMs operating in the 200–250 ◦C temperature range is still an open challenge. The purpose is to identify new materials that possibly meet the Thermal Energy Storage (TES) technological expectations in terms of performances, stability and economic industrial applicability [25]. Indeed, the heat storage at medium temperature represents a crucial technology that still requires an improvement of knowledge. In this context, the aim of this work is to investigate the calcium aluminate compounds as potentially effective and practical alternatives to conventional TES materials. Calcium aluminates are cheap and nontoxic compounds; they are basic components of cements called calcium aluminate cements (CAC) [26] and often related to construction materials [27,28]. They generally provide suitable mechanical properties to the concrete and play a fundamental role in the hydration process of the cements, releasing a high amount of heat during the product hydration. In cement technology, the release of heat is an issue that needs to be avoided, although, for other applications, such as energy storage, this aspect could be a key factor, if properly exploited. From this point of view, the main compound responsible for heat release is the most thermodynamically stable tricalcium aluminate Ca3Al2O6, when it forms the corresponding hexahydrate specie with a standard reaction enthalpy (∆H0) equal to −258.6 kJ/mol [29,30]. Tricalcium aluminate hexahydrate, Ca3Al2O6·6H2O, known as katoite mineral, be- longs to the hydrogarnet group [31]. Studies on thermal decomposition [32] reported a two-step process in which the first dehydration takes place at 240–300 ◦C and the 1.5- hydrate product is formed. The reaction is reversible; indeed, Ca3Al2O6·1.5H2O is used as drying agent due to the high tendency to adsorb water vapor from the surrounding atmosphere (Equation (2)) [33]. At higher temperature, Ca3Al2O6·1.5H2O transforms into mayenite (Ca12Al14O33)[34].

Ca3Al2O6·6H2O(s)Ca3Al2O6·1.5H2O(s) + 4.5H2O(g) (2)

In this work, katoite was prepared via hydration starting from synthetic Ca3Al2O6. In order to achieve, selectively, the Ca3Al2O6 formation, several preparation methods have been investigated, such as co-precipitation and solid–solid reaction. Morphological and structural characterization were necessary to underline the differences between the samples. Appl. Sci. 2021, 11, 1958 3 of 17

Appl. Sci. 2021, 11, 1958 been investigated, such as co-precipitation and solid–solid reaction. Morphological3 ofand 17 structural characterization were necessary to underline the differences between the samples. Therefore, the characterization was carried out on the anhydrous and hydrated batches. Therefore, the characterization was carried out on the anhydrous and hydrated batches. Moreover, the materials were analyzed by means of differential scanning calorimetry and Moreover, the materials were analyzed by means of differential scanning calorimetry and thermogravimetry. The coupling of these techniques, in fact, allowed us to determine thermogravimetry. The coupling of these techniques, in fact, allowed us to determine operating temperature and endothermic heat associated with the dehydration process. These operating temperature and endothermic heat associated with the dehydration process. studies were useful to highlight how the preparation process affects the features of the These studies were useful to highlight how the preparation process affects the features thermochemical storage material. Finally, the stability during low-term operating cycles was of the thermochemical storage material. Finally, the stability during low-term operating studied through consecutive dehydration/hydration reactions. cycles was studied through consecutive dehydration/hydration reactions.

2.2. MaterialsMaterials andand MethodsMethods 2.1.2.1. SamplesSamples PreparationPreparation TricalciumTricalcium aluminate aluminate was was prepared prepared by by solid–solid solid–solid synthesis synthesis and and co-precipitation. co-precipitation. Solid–solidSolid–solid synthesissynthesis waswas carriedcarried outout usingusing aa mixture mixture of of calcium calcium oxide oxide (CaO, (CaO, anhydrous anhydrous powder,powder, ≥ ≥99.0%,99.0%, Sigma–Aldrich, Sigma–Aldrich, Darmstadt,Darmstadt, Germany)Germany) and and aluminum aluminum oxide oxide (Al (Al2O2O3,3, ≥≥99.0%,99.0%, Carlo Erba, Cornaredo (MI) (MI),, Italy) Italy) at at molar molar ratio ratio CaO:Al CaO:Al22OO3 3== 3:1, 3:1, then then calcined calcined in inair air at at1100 1100 °C◦ Cfor for 2 h. 2 h. TheThe co-precipitationco-precipitation waswas carriedcarried outout byby addingadding sodiumsodium hydroxidehydroxide solutionsolution (NaOH,(NaOH, pellets,pellets, Sigma–Aldrich, Sigma–Aldrich, Germany) Germany) to to a a solution solution containing containing calcium calcium nitrate nitrate (Ca(NO (Ca(NO3)23)·24H·4H2O,2O, ≥≥99.0%,99.0%, Sigma–Aldrich,Sigma–Aldrich, Germany)Germany) and and aluminium aluminium nitrate nitrate (Al(NO (Al(NO3)33·)9H3·9H2O,2O,≥ ≥99.0%,99.0%, CarloCarlo Erba,Erba, Italy). Italy). Two Two different different Ca/Al Ca/Al molar molar ratios ratios werewere used, used, respectively, respectively, 1:11:1 and and 3:2. 3:2. The The procedure,procedure, represented represented in in FigureFigure1 ,1, can can be be described described as as follows: follows: The The nitrate nitrate salts salts were were weightedweighted andand dissolveddissolved inin distilleddistilled water,water, upup to to a a ﬁnal final volume volume (100 (100 mL), mL), and and the the solution solution waswas kept kept under under magnetic magnetic stirring. stirring. Then, Then, the the sodium sodium hydroxide hydroxide solution solution (50 (50 mL) mL) was was added added (2(2 mL/min)mL/min) through through a aperistaltic peristaltic pump. pump. At Atthe the end end of the of reaction, the reaction, the pH the values pH values were 12÷13. were 12The÷13. formed The formedwhite solid white was solid aged was for aged 24 h, for filt 24ered, h, ﬁltered,washed washedwith distilled with distilledwater (150 water mL), ◦ ◦ (150dried mL), at 105 dried °C for at 10512 h andC for then 12 hcalcined and then at calcined1100 °C for at 11002 h. TableC for 1 2shows h. Table the1 sample shows code the sampleand the code preparation and the preparationmethod. method.

FigureFigure 1. 1.Co-precipitation Co-precipitation method method ﬂowchart. flowchart.

Table 1. Sample code and preparation method. Table 1. Sample code and preparation method. Sample Code nCa (mmol) nAl (mmol) Method Ca/Al Molar Ratio Sample Code nCa (mmol) nAl (mmol) Method Ca/Al Molar Ratio CA-S 15 10 Solid–solid 3:2 CA-S 15 10 Solid–solid 3:2 CA-11 10 10 Co-precipitation 1:1 CA-11 10 10 Co-precipitation 1:1 CA-32CA-32 1515 1010 Co-precipitationCo-precipitation 3:23:2

2.2. Samples Hydration Hydration of calcium aluminates generally involves a large and complex variety of secondary processes, and it is strongly dependent on temperature. Although the most stable hydrate form is Ca3Al2O6·6H2O, its nucleation can be very slow at low temperatures, and it is usually preceded by the formation of metastable hydrates, which could persist for Appl. Sci. 2021, 11, 1958 4 of 17

2.2. Samples Hydration Appl. Sci. 2021, 11, 1958 Hydration of calcium aluminates generally involves a large and complex variety4 of 17 of secondary processes, and it is strongly dependent on temperature. Although the most stable hydrate form is Ca3Al2O6·6H2O, its nucleation can be very slow at low temperatures, and it is ausually long time preceded before by a thermodynamicthe formation of metastable conversion hydrates, process occurs.which could Previous persist studies for a long demon- time ◦ stratedbefore a that, thermodynamic during the early conversion stage of process hydration, occurs. at temperatures Previous studies between demonstrated 15 and 70 thatC,, CaAl O ·10H O and Ca Al O ·8H O are mainly formed, while Ca Al O ·6H O becomes during2 4the early2 stage of2 hydration,2 5 2 at temperatures between 15 and3 702 °C,6 CaAl2 2O4·10H2O the most prominent when the process occurs at temperatures higher than 70 ◦C[35]. A and Ca2Al2O5·8H2O are mainly formed, while Ca3Al2O6·6H2O becomes the most prominent correlationwhen the process between occurs the hydration at temperature temperatures higher and than the 70 primary °C [35]. calciumA correlation aluminate between phase the ishydration summarized temperature in Table 2and. the primary calcium aluminate phase is summarized in Table 2. Table 2. Most-formed calcium aluminates in dependence on the hydration temperature. Table 2. Most-formed calcium aluminates in dependence on the hydration temperature. Hydration Temperature (◦C) Main Formed Phase(s) Ref. Hydration Temperature (°C) Main Formed Phase(s) Ref. <15 CaAl O ·10H O[36] <15 2 4 CaAl2 2O4·10H2O [36] 15–27 CaAl2O4·10H2O/Ca2Al2O5·8H2O [36] 15–27 CaAl2O4·10H2O/Ca2Al2O5·8H2O [36] 27–70 Ca2Al2O5·8H2O/Ca3Al2O6·6H2O [36] 2 2 5 2 3 2 6 2 >7027–70 CaCa3AlAl2OO6·6H·8H2O[O/Ca Al O ·6H O 15] [36] >70 Ca3Al2O6·6H2O [15] FollowingFollowing these these considerations, considerations, the the samples samples obtained obtained by by the the synthesis synthesis procedure procedure were were hydratedhydrated viavia thethe procedureprocedure described described below: below:In Inthe the internal internal and and external external sides sides of of a a double double shellshell crucible, crucible, the the sample sample (≈ (≈100100 mg) mg) and and a a bed bed of of water water were were placed, placed, respectively. respectively. The The ◦ systemsystem waswas hermeticallyhermetically closed and and heated heated for for 30 30 min min at at 180 180 °C.C. Under Under these these conditions, conditions, the thegenerated generated high high-pressure-pressure water water vapor vapor favor favoreded the the material material hydration. hydration. As As the lastlast step, step, ◦ dryingdrying isis carriedcarried outout atat 105 105 °CC toto eliminate eliminate thethe nonchemically nonchemically bondedbonded water.water. The The samples samples werewere weighed weighed before before and and after after this this treatment, treatment, thus thus verifying verifying the the occurred occurred mass mass change. change. In In orderorder toto validatevalidate thethe procedureprocedure repeatability,repeatability, for for each each batch,batch,three three replicasreplicas werewere carriedcarried out.out. TheThe hydratedhydrated samples were were codified codiﬁed based based on on Table Table 11, ad, addingding a letter a letter “H “H”” to the to the codes codes:: e.g., e.g.,CA- CA-32H32H indicates indicates a hydrated a hydrated sample sample obtained obtained by byco-precipitation co-precipitation process process by byusing using a 3:2 a 3:2Ca/Al Ca/Al ratio. ratio. Figure Figure 2 reports2 reports a schematization a schematization of the of described the described procedure. procedure.

Figure 2. Schematized hydration procedure sequences. Figure 2. Schematized hydration procedure sequences.

2.3.2.3. CharacterizationCharacterization 2.3.1.2.3.1. StructuralStructural andand MorphologicalMorphological CharacterizationCharacterization InIn order order to to explain explain the the chemical chemical transformation transformation of of the the materials materials during during the the preparation preparation andand afterafter thethe cyclingcycling tests,tests, thesethese have have been been structurally structurally and and morphologically morphologically characterized characterized byby means means of of X-ray X-ray diffraction diffraction (XRD) (XRD) and and scanning scanning electron electron microscopy microscopy (SEM). (SEM). XRD mea- XRD surementsmeasurements were were conducted conducted by usingby using a D8 a D8 Advance Advance Bruker Bruker instrument instrument equipped equipped with with a a monochromaticmonochromatic Cu Cu K Kαα radiation radiation source source (40 (40 kV, kV,40 mA). 40 mA). Bragg–Brentano Bragg–Brentano theta–2theta theta–2theta con- ◦ figurationconfiguration and and a scanning a scanning speed speed of 0.1 of 0.1°/s/s were were used used to to examine examine the the samples samples in in the the 2 2θθ range range ◦ 10–8010–80°.. TheThe phasesphases identificationidentification waswas donedone usingusing the the PDF-4+ PDF-4+ database. database. SEMSEM micrographs micrographs werewere collected collected byby aa QuantaQuanta 450450 FEIFEI withwith aa large-fieldlarge-field detectordetector (LFD),(LFD), onon Cr-coatedCr-coated samples,samples, −6 withwith an an accelerating accelerating voltage voltage of of 5 5 kV kV in in high high vacuum vacuum (10 (10−6mbar). mbar). EnergyEnergy dispersivedispersive X-rayX-ray analysisanalysis (EDX)(EDX) waswas alsoalso usedused to collectcollect punctual elemental analysis analysis.. 2.3.2. Differential Scanning Calorimetry (DSC) DSC analysis (Q100, TA Instruments) was performed to evaluate the amount of involved dehydration heat. Specifically, 6–7 mg hydrated sample was put in an aluminum Appl. Sci. 2021, 11, 1958 5 of 17

pan, covered with an aluminum cap and pressed in a sample crimper. On the cap, a micro hole was made to ensure that the formed water vapor exited during the analysis. This precaution avoids a pressure increase inside the pan due to water vapor formation. Mea- surements were performed in inert gas ﬂow (N2, 50 mL/min). Preliminarily, all samples were maintained at 105 ◦C for 5 min to remove the moisture content, and subsequently, a constant heating step (10 ◦C/min) up to 400 ◦C was carried out.

2.3.3. Thermogravimetric Analysis (TGA) Thermogravimetric analysis (TGA), performed by a STA 449 F3 Jupiter (Netzsch, Selb, Germany), was used to study the dehydration/hydration cycles and stability. The Thermo-Gravimetric (TG) unit is coupled with an evaporation system for the water vapor ◦ supply. After a preliminary sample (≈20 mg) drying at 125 C in inert gas flow (N2 flow rate: 100 mL/min) for 60 min, each dehydration/hydration cycle was performed by the following procedure: (i) Heating. The temperature was increased (10 ◦C/min) up ◦ to the chosen dehydration temperature (Td = 250 C); (ii) Isothermal. The sample was held at these conditions for 120 min, allowing for the complete dehydration reaction; (iii) Cooling. The temperature was decreased (−10 ◦C/min) to the hydration temperature ◦ (Th = 125 C); (iv) Stabilization step. Performed after cooling the temperature down to the preset value, and therefore, this interval is essential to stabilize the system temperature before the following step. (v) Hydration reaction. A gas mixture flow of water vapor (2.22 g/h) and N2 (35 mL/min) was supplied (pH2O/p = 0.58), and the isothermal hydration reaction proceeded for 120 min; (vi) Release of adsorbed water. Finally, the water vapor ◦ supply was stopped, and the sample was kept at 125 C for 30 min under a constant N2 flow (100 mL/min) to remove the physically adsorbed water from the sample. Table3 summarizes a single TG cycle.

Table 3. Schematization of a single thermogravimetric cycle.

Step n◦ Temperature (◦C) Atmosphere Method Time (Min)

1 125→250 N2 Heating - 2 250 N2 Isothermal 120 3 250→125 N2 Cooling - 4 125 N2 Stabilization - 5 125 N2/H2O Hydration 120 6 125 N2 Release of adsorbed water 30

3. Results and Discussion 3.1. Ca3Al2O6 Synthesis in Anhydrous and Hydrated Forms

As shown in Table1, two preparation methods were attempted to achieve Ca 3Al2O6: (i) solid–solid reaction at 1100 ◦C and (ii) co-precipitation followed by thermal treatment at 1100 ◦C. In the case of solid–solid reaction, a Ca2+/Al3+ molar ratio 3:2 was used. In the case of co-precipitation, the Ca2+:Al3+ molar ratios 3:2 and 1:1 were used. Figure3 shows the representative diffraction spectra of CA-11 as a function of the calcination temperature (in the range 500–1100 ◦C). It is worth noting that the thermal treatment in ◦ static air at 1100 C allows for Ca3Al2O6 formation. Indeed, by evaluating the XRD spectra evolution at increasing calcination time, a clear relationship between thermal treatment and sample crystallinity can be observed. A calcination temperature of 500 ◦C leads to a poor crystallinity. The aluminum is mainly present in the form of calcium dialuminate, CaAl2O4 (JCPDS # 01-0888), and calcite, CaCO3 (JCPDS # 83-1762). Appl. Sci. 2021, 11, 1958 6 of 17 Appl. Sci. 2021, 11, 1958 6 of 17

FigureFigure 3.3. CA-11CA-11 diffraction patterns after after 2 2 h h calcination calcination under under static static air air at at 500 500 °C,◦C, 700 700 °C,◦C, 900 900 °C◦ C ◦ andand 11001100 °C.C. (JCPDS reference cards: CaAl 2OO44: :01 01-0888;-0888; Ca Ca1212AlAl14O1433O:33 09:- 09-0413;0413; Ca Ca3Al32AlO62: O386-:1429; 38-1429; CaO: 70-4068; CaCO3: 83-1762/19-3758; AlO(OH): 78-4581). CaO: 70-4068; CaCO3: 83-1762/19-3758; AlO(OH): 78-4581).

AfterAfter calcinationcalcination atat 700700 ◦°CC,, thethe samplesample stillstill presentedpresented amorphousamorphous phases;phases; however,however, somesome peakspeaks relatedrelated to to crystalline crystalline phases phases were were identifiable. identifiable. Calcium Calcium carbonate carbonate underwent underwent a partiala partial decomposition decomposition into into calcium calcium oxide oxide (JCPDS (JCPDS # # 70-4068), 70-4068), while while the the remainingremaining portionportion waswas convertedconverted intointo vaterite,vaterite, CaCOCaCO33 (JCPDS(JCPDS ## 19-3758).19-3758). AluminumAluminum waswas observableobservable onlyonly asas boehmite,boehmite, AlO(OH) AlO(OH) (JCPDS (JCPDS # 78-4581). # 78-4581). The The diffraction diffraction pattern pattern continued continue itsd evolution its evolution after calcinationafter calcination at 900 at◦C, 900 where °C, the where crystallinity the crystallinity increased increase and thed mainand phasethe main was phase mayenite, was Camayenite,12Al14O 33Ca(JCPDS12Al14O33 #09-0413). (JCPDS #09 Additionally,-0413). Additionally, peaks of calcium peaks oxideof calcium were alsooxide observable, were also demonstratingobservable, demonstrating a complete decarbonation a complete decarbonation of the previously of the detected previously calcium detected carbonate calcium phases.carbonate As phases. the latest As diffraction the latest patterndiffraction showed, pattern tricalcium showed, aluminate,tricalcium Caaluminate,3Al2O6 (JCPDS Ca3Al2O #6 ◦ 38-1429),(JCPDS # appeared38-1429), appear predominantlyed predominantly only after only calcination after calcination at 1100 C; at however,1100 °C; however a low amount, a low ofamount mayenite of mayenite was still wa present.s still present. In order In to order better to correlate better correlate the phase the transformationsphase transformations with thewith production the production process, process, the diffractionthe diffraction patterns patterns of of the the calcined calcined products products CA-32, CA-32, CA-11CA-11 ◦ andand CA-S CA-S at at 1100 1100 °CC are are compared compared in in Figure Figure4 a. 4a. For For CA-32, CA-32, the the main main crystal crystal phase phase was was tricalciumtricalcium aluminate, aluminate, Ca3AlAl22OO6 6(JCPDS(JCPDS # # 38 38-1429).-1429). At At the the same time, a small small presence presence of of calciumcalcium oxide, CaO, CaO, wa wass confirmed confirmed (JCPDS (JCPDS # #70 70-4068).-4068). For For CA CA-11,-11, as asdescribed described before, before, the themain main phase phase was was Ca3Al Ca2O3Al6, 2andO6, a and moderate a moderate amou amountnt of mayenite of mayenite was also was appreciable. also appreciable. CA-S, CA-S,after calcination, after calcination, did not show did not tricalcium show tricalcium aluminate aluminate peaks, while peaks, it wa whiles possible it was to possible mainly to mainly identify the presence of unreacted CaO and aluminum oxide, Al O (JCPDS # identify the presence of unreacted CaO and aluminum oxide, Al2O3 (JCPDS2 3# 34-0440). 34-0440). Furthermore, peaks related to mayenite, Ca Al O , could be identified. The Furthermore, peaks related to mayenite, Ca12Al14O33, 12could14 be33 identified. The solid–solid solid–solidsynthesis, therefore, synthesis, d therefore,id not lead did to the not formation lead to the of formation tricalcium of aluminate, tricalcium as aluminate, observed for as observed for the co-precipitation products, at 1100 ◦C for 2 h. Probably, temperatures the co-precipitation products, at 1100◦ °C for 2 h. Probably, temperatures close to melting closepoint to(above melting 1350 point °C) are (above necessary 1350 C)to obtain are necessary this product to obtain through this this product method through [37]. this method [37]. Appl. Sci. 2021, 11, 1958 7 of 17 Appl. Sci. 2021, 11, 1958 7 of 17

Figure 4. DiffractionFigure 4. patternsDiffraction after patterns calcination after (acalcination) and after ( hydrationa) and after (b hydration) for CA-32/CA-32H (b) for CA-32/CA (red), CA-11/CA-11H-32H (red), (blue) and CA-S/CA-SHCA-11/CA (orange).-11H (blue) (JCPDS and referenceCA-S/CA cards:-SH (orange). Ca3Al2 (JCPDSO6: 38-1429; reference CaO: cards: 70-4068; Ca3Al Ca2O126:Al 3814-1429;O33: 09-0413;CaO: Al2O3: 34-0440; Ca370Al-24068;O6·6H Ca2O:12Al 24-0217;14O33: 09 Al(OH)-0413; Al3:2 33-0018;O3: 34-0440; Ca(OH) Ca3Al2:2O 44-1481;6·6H2O: CaCO 24-0217;3: 83-1762). Al(OH)3: 33-0018; Ca(OH)2: 44-1481; CaCO3: 83-1762). For TES application, a relevant aspect that requires attention to assess the potential For TES application,suitability of a therelevant material aspect in this that context requires is to attention evaluate to the assess hydrated the potential phases that are formed suitability of thedue material a hydration in this process. context Inis to particular, evaluate the Figure hydrated4b shows phases the diffractionthat are formed patterns after the due a hydrationhydration process. step In particular, (as described Figure in Section4b shows 2.2 the). Hydrated diffraction samples patterns are after named the CA-32H, CA- hydration step11H (as anddescribed CA-SH. in ForSection CA-32H, 2.2.). Hydrated katoite, Ca samples3Al2O6· 6Hare2 namedO (JCPDS CA #-32H, 24-0217) CA- was detected. A small amount of calcium hydroxide, Ca(OH) , was also observed (JCPDS # 44-1481), 11H and CA-SH. For CA-32H, katoite, Ca3Al2O6·6H2O (JCPDS 2# 24-0217) was detected. A formed by the hydration of calcium oxide previously observed in CA-32. The diffractogram small amount of calcium hydroxide, Ca(OH)2, was also observed (JCPDS # 44-1481), formed by the hydrationshowed of calcium a low oxide peak previously intensity. observed In the CA-11H in CA-32. diffraction The diffractogram pattern, only show theed characteristic · a low peak intensitpeaksy. of In katoite, the CA Ca-11H3Al2 Odiffraction6 6H2O, were pattern, present, only thus the indicating characteristic that, peaks besides of the hydration of the Ca Al O phase, mayenite was also converted into the most thermodynamically katoite, Ca3Al2O6·6H2O,3 we2re 6 present, thus indicating that, besides the hydration of the stable katoite. Mayenite-to-katoite conversion was also observed for the CA-SH sample. Ca3Al2O6 phase, mayenite was also converted into the most thermodynamically stable Gibbsite, Al(OH) (JCPDS # 33-0018) and calcium hydroxide, Ca(OH) , were also present katoite. Mayenite-to-katoite conversion3 was also observed for the CA-SH sample. Gibbsite,2 as hydrated products. Moreover, the characteristic peaks of unreacted Al2O3 were still Al(OH)3 (JCPDS # 33-0018) and calcium hydroxide, Ca(OH)2, were also present as hydrated evident, and calcium oxide was partially converted into calcite, CaCO3 (JCPDS # 83-1762). products. Moreover, the characteristic peaks of unreacted Al2O3 were still evident, and The obtained results demonstrated that Ca3Al2O6 was almost quantitatively achieved calcium oxide was partially converted into calcite, CaCO3 (JCPDS # 83-1762). The obtained by the co-precipitation method using a Ca2+/Al3+ molar ratio 1:1, which was in defect results demonstrated that Ca3Al2O6 was almost quantitatively achieved by the co-precipitation of the amount of calcium with respect the stoichiometry of Ca Al O , and it was fully method using a Ca2+/Al3+ molar ratio 1:1, which was in defect of the amount of calcium3 2 6with converted into the hexahydrate form. With regard to the co-precipitation products, it is respect the stoichiometry of Ca3Al2O6, and it was fully converted into the hexahydrate form. important to point out the discrepancy between what could be expected from the initial With regard to the co-precipitation products, it is important to point out the discrepancy Ca/Al ratio and what is shown by the diffraction patterns of materials CA-32 and CA-11, between what could be expected from the initial Ca/Al ratio and what is shown by the in terms of phase composition. A rational explanation is given by the acid–base behavior diffraction patterns of materials CA-32 and CA-11, in terms of phase composition. A rational of aluminium cations: amphoterism of Al3+ makes its solubility increasingly relevant after 3+ explanation is pHgiven ~9, by in the accordance acid–base with behavior the tetrahydroxy of aluminium aluminate cations: ionamphoterism formation of [38 Al], as described in makes its solubilityEquation increasing (3). ly relevant after pH ~9, in accordance with the tetrahydroxy aluminate ion formation [38], as described in Equation (3). − − Al(OH)3(s) + OH (aq)Al(OH)4 (aq) (3) − − It is thereforeAl(OH) understandable3(s) + OH (aq)⇄ thatAl(OH) part4 of(aq) the initially employed aluminum(3) is not 2+ It is thereforefound understandable in the ﬁnal product. that part Consequently, of the initially in CA-32, employed some ofaluminum Ca precipitates is not as Ca(OH)2 found in the final(dehydrated product. Consequently, to CaO, subsequently, in CA-32 during, some of calcination), Ca2+ precipitates while as in Ca(OH) CA-11,2 the initial alu- (dehydrated tominum CaO, excess subsequently, led to the formation during calcination), of tricalcium while aluminate. in CA The-11 morphology, the initialof the samples aluminum excessafter led calcination, to the formation evaluated of by tricalcium SEM micrographs, aluminate. is The shown morphology in Figure5 .of A the similar morphol- samples after calcination,ogy is observable evaluated for samplesby SEM CA-32micrographs, and CA-11 is shown after in calcination Figure 5. A (Figure similar5a,b), in which morphology isrounded observable particles for samples (red arrows) CA-32 and and ﬂat CA hexagonal-11 after calcination particles of (Figure different 5a sizes,b), in (black arrows), which roundedascribable particles to(red tricalcium arrows) aluminate,and flat hexagonal are present particles [39]. CA-S of different shows sizes particles (black relatively bigger arrows), ascribablethan the to co-precipitates tricalcium aluminate (black circle), are after present calcination [39]. CA (Figure-S shows5c). particles relatively bigger than the co-precipitates (black circle) after calcination (Figure 5c). Appl. Sci. 2021, 11, 1958 8 of 17 Appl. Sci. 2021, 11, 1958 8 of 17

Figure 5. ScanningFigure 5. electron Scanning microscopy electron microscopy (SEM) images (SEM at)magniﬁcation images at magnification = 100 k of calcined= 100 k of products calcined CA-32 (a), CA-11 (b) and CA-S (cproducts). CA-32 (a), CA-11 (b) and CA-S (c).

Furthermore, FigureFurthermore, 6 reports Figure the 6 products reports theviewed products with viewedSEM micrographs with SEM micrographsafter after hydration. For hydration.a better clarity, For apunctual better clarity, EDX elemental punctual EDXanalysis elemental on the analysishydrated on products the hydrated products was useful to estimatewas useful the to chemical estimate thecomposition chemical composition on a specified on a point specified by means point by of means the of the Ca/Al Ca/Al molar ratiomolar (see ratio the (see attached the attached table in table Figure in Figure 6). Hexagonal6). Hexagonal katoite katoite plates plates can can be be noted for all noted for all thethe samples samples (black (black arrows) arrows);; these these have have a more a more regular regular shape shape in in CA CA-11-11 batch batch (a), in which (a), in which katoite XRD XRD peaks peaks were were also also more more intense intense (Figure (Figure4b). The 4b). punctual The punctual elemental analysis on elemental analysisthese on particles these particles (spots n (spots◦ 2,3,4 n° and 2,3,4 5) confirmedand 5) confirmed a Ca/Al a ratioCa/Al close ratio to close 1.5, corresponding to 1.5, correspondingto the Cato3 theAl2 OCa6·36HAl22OO6·6H one.2O CA-32Hone. CA- (b)32H clearly (b) clearly shows shows a significant a significant amount of calcium ◦ amount of calciumhydroxide hydroxide nanoparticles nanoparticles (blue (blue arrow), arrow), where where the elemental the elemental analysis analysis (spot n 1) gives a (spot n° 1) givessignificantly a significantly increased increased Ca/Al Ca/Al molar molar ratio. ratio. For For the the solid–solid solid–solid product product (c), the presence (c), the presenceof bigger of bigger particles particles can can still still be noted be noted (black (black circle); circle) however; however it is also it is possible also to observe possible to observeother other morphologies, morphologies, the most the evident most evident of which of is which the typical is the cubic typical shape cubic of α -alumina [40] ◦ shape of α-alumina(green [40] arrow). (green The arrow). spot nThe6 onspot a cubicn° 6 on particle a cubi confirmsc particle a localconfirms large a excesslocal of aluminum, large excess of withaluminum, a Ca/Al with molar a Ca/Al ratio molar < 0.1. ratio < 0.1.

Figure 6. SEM images at magniﬁcation = 100 k and punctual energy dispersive X-ray analysis (EDX)

estimated Ca/Al molar ratios of hydrated products CA-32H (a), CA-11H (b) and CA-SH (c). Figure 6. SEM images at magnification = 100 k and punctual energy dispersive X-ray analysis (EDX) estimated3.2. Ca/Al Dehydration molar ratios and of hydrated Heat of Reaction products CA-32H (a), CA-11H (b) and CA-SH (c). In order to thermally characterize the hydrated materials, thermogravimetric analysis 3.2. Dehydrationand and differentialHeat of Reaction scanning calorimetry measurements were performed. TGA scan of the In order ﬁrst to thermally dehydration characterize process enabled the hydrated the evaluation materials, of the thermogravimetricoperating temperature range and analysis and differentialthe quantiﬁcation scanning of calorimetry the related measurements weight loss. At were the performed. same time, TGA DSC scan measured the of the first dehydrationamount ofprocess involved enabl endothermiced the evaluation heat ofof thethe observedoperating processes.temperature The range comparison of the Appl. Sci. 2021, 11, 1958 9 of 17

and the quantification of the related weight loss. At the same time, DSC scan measured the amount of involved endothermic heat of the observed processes. The comparison of the thermogravimetric dehydration results of the different tricalcium aluminate hexahydrate Appl. Sci. 2021, 11, 1958 9 of 17 samples, shown in Figure 7a, demonstrates that the dehydrated fraction at 375 °C is significantly influenced by synthesis process. Indeed, CA-11H sample shows the highest weight reductionthermogravimetric equal to 21.12%, dehydrationquite close to results the theoretical of the different mass loss tricalcium due to the aluminate katoite hexahydrate partial dehydrationsamples, (Equation shown (4 in)): Figure7a, demonstrates that the dehydrated fraction at 375 ◦C is

signiﬁcantlyCa3Al inﬂuenced2O6·6H2O(s) by→Ca synthesis3Al2O6·1.5H process.2O(s) + Indeed, 4.5H2O CA-11H(g) sample shows(4) the highest weight reduction equal to 21.12%, quite close to the theoretical mass loss due to the katoite The masspartial loss decreases dehydration about (Equation 19.04% for (4)): CA-32H batch and 10.05% for CA-SH one due to a lower amount of hydrated calcium aluminate compound in the latter (see Figure

7a). Consequently, the thermogravimetricCa3Al2O6· 6Hcurves2O(s) show→Ca 3aAl clearly2O6·1.5H more2O (s)effective+ 4.5H 2behaviorO(g) (4) for the co-precipitated specimens compared to the solid–solid one.

Figure 7. CA-32HFigure (red), 7. CA CA-11H-32H (red), (blue) CA and-11H CA-SH (blue)(orange) and CA- thermogravimetricSH (orange) thermogravimetric (TG) (a) and differential (TG) (a) and scanning calorimetry (DSC) (bdifferential) curves, recorded scanning between calorimetry 150 ◦C (DSC and) 375 (b) ◦curves,C, scan recorded rate 10 ◦C/min. between 150 °C and 375 °C, scan rate 10 °C/min. The mass loss decreases about 19.04% for CA-32H batch and 10.05% for CA-SH Furthermore,one duethe heat to a of lower dehydration amount has of hydrated been determined calcium through aluminate integration compound of the in the latter (see peak obtainedFigure by calorimetric7a ). Consequently, analysis the (Figure thermogravimetric 7b). The results curves confirm showed awhat clearly was more effective highlighted bybehavior TGA measurements. for the co-precipitated CA-11H revealed specimens a dehydration compared to heat the solid–solid of 807.0 kJ/kg, one. higher than CA-32HFurthermore, and CA-SH, therespectively heat of dehydration, 655.8 kJ/kg hasand been530.2 determinedkJ/kg. Based through on these integration of considerations,the a peak preliminary obtained explanation by calorimetric of the analysis dehydration (Figure process7b). The can results be assessed. confirmed what was According to XRDhighlighted phase identification, by TGA measurements. the observed CA-11H chemical revealed process a dehydration is mainly related heat of 807.0 kJ/kg, to the thermalhigher decomposition than CA-32H of katoite, and CA-SH, which respectively, leads to 4.5 655.8water kJ/kg molecules and 530.2 loss (See kJ/kg. Based on Equation (4)). theseThis process considerations, represents a preliminary the only one explanation observed for of theco-precipitated dehydration products. process can be assessed. In contrast, it Accordingis still possible to XRD to perceive phase identification, that for CA-SH the product observed there chemical is the presence process is of mainly related an additionalto process, the thermal as a decomposition secondary DSC of katoite, peak shows which clearly,leads to ascribed 4.5 water to molecules the loss (See decompositionEquation of aluminum (4)). This hydroxide process to represents boehmite the(Equation only one (5) observed): for co-precipitated products. In contrast, it is still possible to perceive that for CA-SH product there is the presence of an additional process,Al(OH) as a secondary3(s)→AlO(OH) DSC(s) peak+ H2O shows(g) clearly, ascribed to the(5 decomposition) This resultof aluminumconfirms that hydroxide this batch to boehmite is characterized (Equation by (5)): a relevant microstructural heterogeneity that hinders the hydration/dehydration efficiency. In contrast, the CA-11H Al(OH) →AlO(OH) + H O (5) and CA-32H samples show a microstructural homogeneity3(s) whi(s)ch acts2 effectively(g) on the vapor phase mass exchangeThis result processes. confirms The that phenomenon, this batch is m characterizedanaged almost by exclusively a relevant by microstructural Equation (3), heterogeneityallows for high that thermal hinders capacities the hydration/dehydration in a very narrow efficiency. temperature In contrast, range, the CA-11H enhancing theand selectivity CA-32H of samplesthe material show for a microstructurala suitable and efficient homogeneity use of whichthermal acts waste. effectively on the Table 4 summarizesvapor the phase results mass obtained exchange from processes. the analysis. The phenomenon, managed almost exclusively by Equation (3), allows for high thermal capacities in a very narrow temperature range, enhancing the selectivity of the material for a suitable and efficient use of thermal waste. Table4 summarizes the results obtained from the analysis. Appl. Sci. 2021, 11,, 1958 10 of 17

Table 4. Thermogravimetric analysis (TGA) and DSC characterization results. Table 4. Thermogravimetric analysis (TGA) and DSC characterization results. ◦ Sample ID Tonset ( C) Weight Loss (%) Heat (kJ/kg) Katoite (%) Sample ID Tonset (°C) Weight Loss (%) Heat (kJ/kg) Katoite (%) CA-32H 247.5 −19.04 655.8 88.9 CA-32H 247.5 −19.04 655.8 88.9 CA-11H 245.8 −21.12 807.0 98.6 CACA-SH-11H 245.8 248.7 −21.12−10.15 807.0 530.2 <46.998.6 CA-SH 248.7 −10.15 530.2 <46.9

The last column shows the amount amount of of katoite katoite on on the the respective respective material. material. It was determined as the conversion percentage compared to the theoretical reference value for pure katoitekatoite (−221.421.42 w wt.%),t.%), calculated calculated as as follows follows (Equation (Equation ( (6)):6)):

4.54.5·MW∙ 푀푊퐻2푂 ∆푚 = − H2O ∙ 100 (6) ∆mth 푡=ℎ − 푀푊 ·100 (6) MWCa퐶푎3Al3퐴푙22O푂66·6∙6H퐻22O푂 Regarding the material obtained by solid–solid synthesis, the amount of katoite Regarding the material obtained by solid–solid synthesis, the amount of katoite cannot cannot be determined quantitatively, due to the existence of two simultaneous processes. be determined quantitatively, due to the existence of two simultaneous processes. Thus, Thus, in lack of further experimental evidence, it is only possible to claim that the sample in lack of further experimental evidence, it is only possible to claim that the sample has hasa katoite a katoite content content lower lower than than 46.9% 46.9% (calculated (calculated assuming assuming that that the the whole whole weightweight lossloss is ascribed to the dehydration of thethe katoite phase).

3.33.3.. Dehydration/ Dehydration/HydrationHydration Stability After structural and thermal characterization of as synthesized materials, cyclic TG experiments were were carried carried out out in in order order to to assess assess the the dehydration/hydration dehydration/hydration stability stability of all of batches.all batches. Figure Figure 8 shows8 shows the whole the whole first thermogravimetric ﬁrst thermogravimetric cycle for cycle CA- for11H/32H CA-11H/32H and CA- SH,and and CA-SH, the highlighted and the highlighted sections 1 sections–6 correspond 1–6 correspond to the analys to theis step analysiss previously steps previously reported inreported Section in 2.3.3 Section and 2.3.3 listed and in listed Table in 3. Table The3 . plot The is plot useful is useful to visually to visually perceive perceive how how the material’sthe material’s mass mass changes changes along along each each step, step, relating relating these these transitions transitions to the to dehydration the dehydration and hydrationand hydration reactions. reactions.

Figure 8. First dehydration/hydration cycle cycle for for CA CA-32H-32H (red), (red), CA CA-11H-11H (blue) (blue) and and CA CA-SH-SH (orange). (orange).

◦ InitiallyInitially,, aa heatingheating phasephase andand subsequentsubsequent sustainment sustainment of of 250 250 C°C were were applied applied (steps (step 1s 1and and 2). 2). As As shown, shown, this this temperature temperature allowed allowed the the materialsmaterials toto bebe dehydrateddehydrated in a relatively ◦ short time. In In fact, fact, already already at about 20 200–2200–220 °CC the material beg beganan to undergo a partial dehydrationdehydration,, which which wa wass completed completed when when the the temperature temperature plateau plateau is reached. is reached. The Thetime time that lapsethat lapsedd between between the theachievement achievement of the of the onset onset temperature temperature and and the the end end of of the process process (identified(identiﬁed as as weight weight loss loss stabilization) stabilization) wa wass around around 40 40 min. min. The The dehydration dehydration results results at equilibriumat equilibrium were were compatible compatible to TGA to ones. TGA This ones. means This that means the chosen that theoperating chosen temperature operating was adequate to obtain a complete reaction conversion of the materials into their respective Appl. Sci. 2021, 11, 1958 11 of 17

temperature was adequate to obtain a complete reaction conversion of the materials into their respective dehydrated forms. Step 3 and 4 are necessary after the dehydration reaction to decrease and stabilize the system at the target hydration temperature (125 ◦C) before the atmosphere shift. Hydration kinetics on co-precipitated batches was faster; the mass suddenly increased when the water vapor was supplied (5), reaching a plateau after 35–40 min. CA-32H exhibited a slightly higher hydration rate and, at the same time, a more marked slope variation. In contrast, CA-11H exhibited a larger yield at the end of the hydration process. However, both co-precipitated batches exhibited a comparable hydration trend. This behavior could be related to their similar composition (katoite, mostly) and morphology (different sized hexagonal plates). CA-SH had a different hydration kinetic; the mass gain associated to the hydration reaction was more gradual, if compared to the other two batches. This behavior could be related to its structural heterogeneity that kinetically limited the hydration processes. The release of physisorbed water (6) occurred when the water vapor supply was over and led to a small weight loss for all the materials; hence, it was deducted that just a limited portion of adsorbed water was not chemically bonded. However, to estimate the effective yield of the hydration reaction, this surplus of mass was not calculated, since the surface interaction with physisorbed water clearly had a weaker energy than a chemisorption. Hence, to avoid an overestimation of the released heat, the real hydration conversion in mass was calculated as the difference between the mass at the end of Section 6 and the beginning of Section 5. Additionally, in Section 6, the cycle drift is evidenced (double pointed arrow). It shows a less hydration yield than the previous dehydration, which led to an efﬁciency loss throughout the cycle. This aspect therefore involved an evaluation of the behavior in subsequent cycles. Table5 lists the conversion values for each dehydration and hydration step, related to a ﬁve-cycle experiment. The reported data are expressed by means of Rd/h (%), which is the normalized conversion obtained by Equation (7):

∆md/h Rd/h(%) = ·100 (7) ∆m1◦d where d and h refer to the dehydration and hydration steps, respectively. The term ∆m refers to the mass variation at the end of the considered process, while ∆m1◦d is the mass variation relative to the ﬁrst dehydration, which is supposed to correspond to a 100% yield.

Table 5. Thermogravimetric performances per cycle.

◦ Sample ID Cycle Dehydration Tonset ( C) Rd (%) Rh (%) CA-32H 1 241.5 100 66.5 2 231.1 67.6 50.9 3 229.8 45.7 40.4 4 230.2 40.4 40.2 5 230.0 40.1 39.8 CA-11H 1 242.7 100 67.5 2 231.4 68.1 48.3 3 229.4 47.6 37.1 4 229.3 42.5 33.6 5 229.6 33.3 32.5 CA-SH 1 241.0 100 52.7 2 233.2 51.0 31.7 3 233.3 32.1 15.4 4 231.7 12.2 11.0 5 230.5 11.4 8.5

Normalized conversions represent the most proper method to compare how the materials work along the cycling, taking into account their different katoite content. The previously described cycle drift could be numerically evaluated by observing the hydration Appl. Sci. 2021, 11, 1958 12 of 17 Appl. Sci. 2021, 11, 1958 12 of 17

hydration yields:yields: co-precipitated co-precipitated presented presented a sim a similarilar Rh1° R h1(66.5%◦ (66.5% and and 67.49% 67.49% for for CA CA-32H-32H and CA-11H, and CA-11H, respectively)respectively),, while while the the solid solid–solid–solid one one was was moderately moderately lower lower (52.68%) (52.68%) at at the first cycle. the first cycle.This This effect, effect, however, however, became beca lessme enhancedless enhanced cycle bycycle cycle. by Figure cycle. 9 Figure graphically 9 shows the graphically showscycle the drift cycle reduction, drift reduction, where the w singlehere normalizedthe single normalized dehydrations dehydrations and hydrations are plotted and hydrationsfor are each plotted material. for each CA-32H material. (a) CA reached-32H (a) comparable reached comparable conversions conversions from the end of the third from the end ofcycle, the third corresponding cycle, corresponding to a Rh around to a 40%.Rh around Similar 40%. conversions Similar conversions were also appreciable for were also appreciableCA-11H for at theCA end-11H of at the the last end two of cycles, the last around two cycles, a Rh of around 33%. The a solid–solidRh of 33%. sample, on the The solid–solidcontrary, sample, didon the not contrary, reach a stable did not conversion reach a stable throughout conversion the experiment, throughout andthe its conversion experiment, andat its the conversion end of the at fifth the cycleend of was the muchfifth cycle lower wa thans much the lower other than two the materials other (8.51%). The two materials progressive (8.51%). The decrease progressive suggests decrease that the suggests material that may the have material become may totally have deactivated in the become totally subsequentdeactivated cycles. in the Thesubsequent nonpurity cycles. of the The CA-SH nonpurity sample of had,the CA therefore,-SH sample a detrimental effect had, therefore,on a detrimental the thermal effect stability on the of the thermal hydration stability process. of the Still hydration in Figure process.9, it should Still be noted that in Figure 9, it shotheuld hydration be noted kinetic that the of thehydration second cyclekinetic undergoes of the second a clear cycle change undergoes with respect a to the first clear change withcycle respect but then to the remains first cycle similar but forthen the remain followings similar ones. for the following ones.

Figure 9. TheFigure 1st, 2nd,9. The 3rd, 1st, 4th 2nd, and 3rd, 5th 4th normalized and 5th normalized dehydration/hydration dehydration/hydration curves for curves CA-32H for (red), CA-32H CA-11H (blue) and CA-SH (orange).(red), CA-11H (blue) and CA-SH (orange).

An explanationAn could explanation be related could to a bestructural related change to a structural that occur changered during that occurred the first during the ﬁrst cycle, which involvecycle,d which rearrangements involved rearrangements that persisted thatuntil persisted a condition until of a stability condition wa ofs stability was achieved. The numberachieved. of cycles The number require ofd to cycles achieve required a structural to achieve stabilization a structural depend stabilizationed on depended the kind of materialon the and kind the of conditions material and applied the conditions during cycling. applied The during structural cycling. change The that structural change the materials graduallythat the materials underwent gradually was coupled underwent with a wasreduction coupled of the with dehydration a reduction onset of the dehydration temperature, Tonsetonset, whic temperature,h was observable Tonset, which after the was first observable cycle (Table after 5). the This ﬁrst temperature cycle (Table 5). This tem- Appl. Sci. 2021, 11Appl., 1958 Sci. 2021, 11, 1958 13 of 17 13 of 17

perature was found to be settled at an almost constant value from the third cycle for the was found toco-precipitated be settled at an batches almost (around constant 230 value◦C), fromsimultaneously the third cycle with anfor occurring the co- conversion precipitated batchesstabilization, (around as described 230 °C), before. simultaneously For CA-SH, with contrarily, an occurring the onset conversion temperature was still stabilization, asdecreasing described at before. the fifth For cycle. CA- InSH, order contrarily, to figure the out onset the temperature heat efficiency wa ofs still the materials, the decreasing at the fifth cycle. In order to figure out the heat efficiency of the materials, the amount of released heat per mass unit (Qrel,m) and volume unit (Qrel,V), referred to by the amount of releasedsingle heat cycle per (n), mass was calculatedunit (Qrel,m) by and Equations volume (8)unit and (Q (9):rel,V), referred to by the single cycle (n), was calculated by Equations (8) and (9): R%;h(n) Q 푅%;ℎ=(푛Q) · (8) 푄 = 푄 rel,m∙ (n) DSC (8) 푟푒푙,푚(푛) 퐷푆퐶 100 100 Q = Q · 푄푟푒푙,푉(푛) = 푄푟푒푙rel,푚,(V푛()n∙)휌 rel,m(n) ρ (9) (9)

where QDSC is wherethe previouslyQDSC is thedetermined previously dehydration determined heat dehydration by calorimetric heat by analysis, calorimetric R%;h(n) analysis,is R%;h(n) the normalizedis hydration the normalized conversion hydration referred conversion to by the referred n cycle to and by theρ is n the cycle experimentally and ρ is the experimentally 3 determined bulkdetermined density for bulk the density hydrated for thematerials, hydrated corresponding materials, corresponding to 745 kg/m3 for to 745CA kg/m- for CA- 3 3 32H, 854 kg/m32H,3 for CA 854-11H kg/m andfor 699 CA-11H kg/m3 for and CA 699-SH. kg/m Figurefor 10 CA-SH. shows the Figure released 10 shows heat per the released heat mass unit (a) andper volume mass unit unit (a) (b) and as volumea function unit of (b)the as cycle a function number of (n the). cycle number (n).

(a) (b)

Figure 10. ReleasedFigure 10. heatReleased per mass heat unit per (a mass) and unitvolume (a) andunit volume (b) per unitcycle. (b ) per cycle.

What can be observedWhat can in be Figure observed 10a is in that Figure CA -1011Ha is under that CA-11Hwent a notable underwent performance a notable performance drop during thedrop five during cycles, the which five cycles, brought which the material brought to the exchange material a to quantity exchange of a heat quantity of heat numerically comparablenumerically with comparable the CA-32H with one. the Furthermore, CA-32H one. the Furthermore, trend of CA the-11H trend material of CA-11H material suggested thatsuggested a further small that loss a further in efficiency small could loss be in potentially efficiency observed could be in potentially the following observed in the cycles. CA-32H,following on the other cycles. hand, CA-32H, underwent on the a more other moderate hand, underwent performance a more drop moderate that was performance greatly softeneddrop in the that third was cycle. greatly Insoftened fact, the incurve the trend third cycle.between In fact,the third the curve and fifth trend cycles between the third reached an almostand constant fifth cycles value. reached Hence, an we almost expected constant a stabilizati value.on Hence, for the we following expected cycles a stabilization for as well. CA-SHthe show followinged a progressive cycles as well.decreasing CA-SH of showed exchanged a progressive heat, reaching decreasing the value of exchangedof heat, 45.12 kJ/kg. Similarreaching considerations the value regarding of 45.12 kJ/kg. the co-precipitated Similar considerations materials can regarding be argued the from co-precipitated Figure 10b; howevermaterials, there can is be a arguedremarkable from di Figurefference 10 dueb; however, to the higher there density is a remarkable of CA-11H, difference due to which ensuresthe a highergreater density efficiency, of CA-11H, despite whichthe lower ensures reaction a greater yield. efficiency, The materials despite were the lower reaction structurally andyield. morphologically The materials were characterized structurally again and by morphologically XRD and SEM characterized after cyclic again by XRD experiment. Inand Figure SEM 11, after the diffractio cyclic experiment.n patterns of In the Figure materials 11, theare diffractionshown. CA- patterns32H, which of the materials exhibited the highestare shown. conversion CA-32H, and which the highest exhibited tendency the highest to stabilize conversion (Table 5), and show theed highest a tendency similar diffractionto stabilize pattern (Tableto that5 of), showedthe initial a material, similar diffraction where katoite, pattern Ca3Al to2 thatO6·6H of2O, the and initial material, calcium hydroxidewhere, Ca(OH) katoite,2, Ca we3Alre 2recognizable.O6·6H2O, and No calcium additional hydroxide, phases Ca(OH)were identified.2, were recognizable. In No CA-11H, originallyadditional composed phases almo werest exclusively identified. Inby CA-11H,katoite (98.6 originally%) and composedstill identifiable almost exclusively after five cycles,by katoite showed (98.6%) the further and still presence identifiable of after characteristic five cycles, peaks showed of mayenite, the further presence of Ca12Al14O33, andcharacteristic calcium hydroxide. peaks of mayenite,This can be Ca justified12Al14O, 33considering, and calcium that hydroxide. katoite suffer Thised can be justified, a partial decompositionconsidering, which that katoite was reasonably suffered apartial associated decomposition, to the formation which wasof mayenite reasonably associated aggregates and the consequent release of calcium oxide, subsequently converted in Appl. Sci. 2021, 11, 1958 14 of 17

calcium hydroxide by the water vapor atmosphere. A possible reaction scheme could be (Equations (10) and (11)): Under heating:

7Ca3Al2O6·6H2O(s)→Ca12Al14O33(s) + 9CaO(s) + 42H2O(g) (10) Appl. Sci. 2021, 11, 1958 14 of 17 Under water vapor supply:

to the formation of mayenite aggregatesCaO(s) + H and2O(g)→ theCa(OH) consequent2(s) release of calcium oxide,(11) subsequentlyMoreover, converted broadening in calcium and less hydroxide intensity byof the the peaks, water vaporin comparison atmosphere. to the A possiblestarting materials,reaction scheme indicate could a less be crystallinity. (Equations (10) and (11)):

Figure 11. DiffractionDiffraction pattern pattern after after the the five ﬁve-cycle-cycle experiment experiment for for CA CA-32H-32H (red), (red), CA CA-11H-11H (blue) (blue) and and CA-SH (orange). (JCPDS reference cards: Ca3Al2O6·6H2O: 24-0217; Ca12Al14O33: 09-0413; Ca(OH)2: CA-SH (orange). (JCPDS reference cards: Ca3Al2O6·6H2O: 24-0217; Ca12Al14O33: 09-0413; Ca(OH)2: 44-1481; Al2O3: 04-2852; CaCO3: 12-0489). 44-1481; Al2O3: 04-2852; CaCO3: 12-0489).

CAUnder-SH, heating: characterized by the lowest conversion value, still exhibited typical katoite peaks, although less intense. Similarly to CA-11H, mayenite peaks were visible as a consequence of 7Cakatoite3Al2 Odecomposition.6·6H2O(s)→Ca The12Al 14mainO33(s) crystalline+ 9CaO(s) phase+ 42H wa2O(g)s alumina, Al(10)2O3, which prevented the process reversibility. In fact, the high stability of the aluminum oxide inhibitUndered itswater rehydration vaporsupply: step. Calcium oxide, which was present in the initial material, had not been identified anymore; it is very likely that the contact with atmospheric carbon CaO + H O →Ca(OH) (11) dioxide led to a total conversion (s) of the2 la(g)tter into calcium2(s) carbonate (Equation (12)), CaCOMoreover,3, already broadeningdetected on andthe pristine less intensity material. ofthe peaks, in comparison to the starting

materials, indicate a less crystallinity.CaO(s) + CO2 (g)→CaCO3(s) (12) CA-SH, characterized by the lowest conversion value, still exhibited typical katoite peaks,The although aluminum less hydroxide intense. Similarly phase, previously to CA-11H, detected mayenite in CA peaks-SH were sample, visible was as no a longer observable because of the decomposition during the cycles; in fact, as mentioned consequence of katoite decomposition. The main crystalline phase was alumina, Al2O3, above,which preventedboehmite (AlO(OH)) the process wa reversibility.s formed, and In fact,subsequently the high stability, it is possible of the to aluminum reason that oxide this phaseinhibited was its slowly rehydration converted step. into Calcium alumina. oxide, This whichsuggestion was presentis also supported in the initial by material,the slow hydrationhad not been kinetic identiﬁed, which anymore;aluminum it oxides is very and likely oxides that hydroxides the contact withpresent atmospheric [40]. SEM images carbon showdioxiden in led Figure to a total 12 (magnification conversion of the = 100 latter k) into lead calcium to further carbonate considerations (Equation regarding (12)), CaCO the3, investigatedalready detected materials. on the Comparingpristine material. samples’ morphologies before (Figure 6) and after (Figure 11) cycling, some differences were observable. In particular, CA-32H was characterized by rounded nanoparticlesCaO(s) + CO 2 around (g)→CaCO 1003(s)–200 nm (calcium hydroxide),(12) homogeneously distributed, covering hexagonal plates (katoite). In contrast, CA-11H was characterizedThe aluminum by nanoparticles, hydroxide probably phase, previously mayenite and detected calcium in CA-SH hydroxide sample, (red arrows), was no depositedlonger observable on coalesced because hexagonal of the decomposition plates of katoite during (black the arrows).cycles; in Ag fact,ain, as the mentioned smaller above, boehmite (AlO(OH)) was formed, and subsequently, it is possible to reason that this phase was slowly converted into alumina. This suggestion is also supported by the slow hydration kinetic, which aluminum oxides and oxides hydroxides present [40]. SEM images shown in Figure 12 (magniﬁcation = 100 k) lead to further considerations regarding the investigated materials. Comparing samples’ morphologies before (Figure6 ) and after (Figure 11) cycling, some differences were observable. In particular, CA-32H was characterized by rounded nanoparticles around 100–200 nm (calcium hydroxide), Appl. Sci. 2021, 11, 1958 15 of 17

Appl. Sci. 2021, 11, 1958 homogeneously distributed, covering hexagonal plates (katoite). In contrast,15 of 17 CA-11H was

characterized by nanoparticles, probably mayenite and calcium hydroxide (red arrows), deposited on coalesced hexagonal plates of katoite (black arrows). Again, the smaller particle size afterparticle five cycles size aftercould ﬁve be cyclesrelated could to the be less related crystallinity to the less evidenced crystallinity in Figure evidenced 10. in Figure 10. CA-SH still showCA-SHed less still uniformity. showed less Large uniformity. particles Largewere predominantly particles were predominantlyobserved (white observed (white arrows) and differentarrows) structures, and different as confirmed structures, by asXRD conﬁrmed analysis by (Figure XRD analysis11), were (Figure present. 11 ), were present.

Figure 12. SEMFigure images, 12. SEM magniﬁcation images, magnification = 100 k, for = CA-32H 100 k, for (a), CA CA-11H-32H (a (b),) CA and-11H CA-SH (b) and (c) after CA- ﬁveSH ( dehydration-hydrationc) after cycles. five dehydration-hydration cycles.

The detected morphologicalThe detected morphologicaland structural materials’ and structural modification materials’s can modiﬁcations address the can address the different behaviordifferent of the behavior two co-precipitated of the two co-precipitated materials. The materials. absence of The decomposition absence of decomposition processes of katoiteprocesses phase of katoite in CA phase-32H in could CA-32H be could related be to related the presenceto the presence of calcium of calcium hydroxide hydroxide impurities:impurities: this phase this phase could could play a play fundamental a fundamental role in roleterms in of terms surface of surface basicity basicity by pro- by promoting moting a better a thermodynamicbetter thermodynamic stability stability of katoite, of katoite, avoiding avoiding the the formation formation of of mayenite, as mayenite, as descdescribedribed in in Equation Equation ( (11)11) forfor CA-11H CA-11H and and CA-SH, CA-SH thus, thus guaranteeing guaranteeing a larger a conversion. larger conversion.At theAt samethe same time, time, it is reasonableit is reasonable to assume to assume that thethat surface the surface basicity basicity imposed by calcium imposed by calciumhydroxide hydroxide is also is responsible also responsible for the for stabilization the stabilization observed observed for CA-11H for CA (in- which calcium 11H (in which hydroxide calcium hydroxide is formed is during formed the during cycling), the and cycling) the difference, and the differencein percentage in conversion is percentage conversiondue to the is formation due to the of formation mayenite of in mayenite this second in case, this secondwhich involves case, which a loss of the active involves a loss portionof the active of the portion material. of the material. 4. Conclusions 4. Conclusions The highlights of this work can be summarized as follows: The highlights of this work can be summarized as follows: 1. Co-precipitation and solid–solid methods have been studied and compared as possible 1. Co-precipitation and solid–solid methods have been studied and compared as strategies to obtain low-cost tricalcium aluminate hexahydrate materials. The solid– possible strategiessolid to reaction obtain pointedlow-cost out tricalcium that it is aluminot possiblenate hexahydrate to obtain a material materials. with appropriate The solid–solid thermochemicalreaction pointed features out that without it is not passing possible through to obtain a preliminary a material with synthetic route that appropriate thermochemicalleads to a better features chemical without interaction passing between through Ca2+ and a Al preliminary3+. 2+ 3+ synthetic route2. thatThe structuralleads to a better and thermochemical chemical interaction characterization between Ca showedand Al higher. katoite content 2. The structural and(98.6%) thermochemical if a 1:1 Ca/Al characterization ratio was initially showed used in higher co-precipitation, katoite content which also involves (98.6%) if a 1:1 Ca/Ala greater ratio heat wa ofs initially dehydration used in (807.0 co-precipitation, kJ/kg), compared which to also the involves other products. a greater heat3. ofThe dehydration study on dehydration/hydration (807.0 kJ/kg), compared cycled to the materials other pro evincedducts. that the initial samples 3. The study on dehydration/hydrationunderwent a structural cycled change materials during evinced the cycles. that the As initial a consequence, samples a decrease in underwent a structuralconversion change yields during and, therefore, the cycles. in stored–releasedAs a consequence, heat, a isdecrease observed, in with respect to conversion yieldsthe and, starting therefore, materials. in stored–released heat, is observed, with respect to the starting4. materials.Calcium hydroxide appears to be important on the stabilization of the material, 4. Calcium hydroxidepartially appears preventing to be decompositionimportant on the phenomena. stabilization of the material, partially preventing5. Once decomposition a cycle stability phenomena. is reached, the amount of stored/released heat per mass unit 5. Once a cycle stabilityis similar is reached, for both the co-precipitated amount of stored/released products (about heat 260 per kJ/kg mass ofunit hydrated is material), similar for bothwhile co-precipitated in terms of storage products density, (about CA-11H 260 kJ/kg exhibits of hydrated the highest material), capacity (224.2 MJ/m3). 3 while in terms ofIn storage future development,density, CA-11H further exhibits studies the highest will aim capacity to enhance (224.2 the MJ/m obtained). promising In future development,results. Cyclic further stability studies will be will investigated aim to enhance and improved the obtained in order promising to make this low-cost results. Cyclic staandbility nontoxic will be material investigated a worthy and improved competitor in toorder current to make thermochemical this low-cost energy storage and nontoxic materialsystems. a worthy competitor to current thermochemical energy storage systems.

Author Contributions: conceptualization: C.M., methodology: L.C. and E.P., validation: F.A., E.P. and L.C., formal analysis: F.A., E.L.M. and M.L.; investigation: F.A. and E.L.M.; data curation: F.A., Appl. Sci. 2021, 11, 1958 16 of 17

Author Contributions: Conceptualization: C.M., methodology: L.C. and E.P., validation: F.A., E.P. and L.C., formal analysis: F.A., E.L.M. and M.L.; investigation: F.A. and E.L.M.; data curation: F.A., L.C. and E.P.; writing original draft: F.A.; writing review: F.A., L.C. and E.P.; supervision: C.M. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available within the article. Acknowledgments: This study was conducted as part of the project, “Thermochemical materials for heat storage: development and characterization,” sponsored by INSTM (Consorzio Interuniversitario Nazionale per la Scienza e Tecnologia dei Materiali), within the IEA SHC Task 58 “Material and Component Development for Thermal Energy Storage.” Authors thank Emanuela Mastronardo for helpful discussion. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References 1. Haddad, C.; Périlhon, C.; Danlos, A.; François, M.X.; Descombes, G. Some efficient solutions to recover low and medium waste heat: Competitiveness of the thermoacoustic technology. Energy Procedia 2014, 50, 1056–1069. [CrossRef] 2. Liu, J.; Chang, Z.; Wang, L.; Xu, J.; Kuang, R.; Wu, Z. Exploration of Basalt Glasses as High-Temperature Sensible Heat Storage Materials. ACS Omega 2020, 5, 19236–19246. [CrossRef] 3. McGillicuddy, R.D.; Thapa, S.; Wenny, M.B.; Gonzalez, M.I.; Mason, J.A. Metal-organic phase-change materials for thermal energy storage. J. Am. Chem. Soc. 2020, 142, 19170–19180. [CrossRef] 4. Cabeza, L.F.; Martorell, I.; Miró, L.; Fernández, A.I.; Barreneche, C. Introduction to Thermal Energy Storage (TES) Systems; Woodhead Publishing Limited: Cambridge, UK, 2015; ISBN 9781782420965. 5. Henninger, S.K.; Jeremias, F.; Kummer, H.; Schossig, P.; Henning, H.M. Novel sorption materials for solar heating and cooling. Energy Procedia 2012, 30, 279–288. [CrossRef] 6. Cabeza, L.F.; Solé, A.; Barreneche, C. Review on sorption materials and technologies for heat pumps and thermal energy storage. Renew. Energy 2017, 110, 3–39. [CrossRef] 7. Alva, G.; Lin, Y.; Fang, G. An overview of thermal energy storage systems. Energy 2018, 144, 341–378. [CrossRef] 8. Linder, M. Using Thermochemical Reactions in Thermal Energy Storage Systems; Woodhead Publishing Limited: Cambridge, UK, 2014; ISBN 9781782420965. 9. Sunku Prasad, J.; Muthukumar, P.; Desai, F.; Basu, D.N.; Rahman, M.M. A critical review of high-temperature reversible thermochemical energy storage systems. Appl. Energy 2019, 254, 113733. [CrossRef] 10. Chan, C.W.; Ling-Chin, J.; Roskilly, A.P. A review of chemical heat pumps, thermodynamic cycles and thermal energy storage technologies for low grade heat utilisation. Appl. Therm. Eng. 2013, 53, 160–176. [CrossRef] 11. Neveu, P.; Castaing, J. Solid-gas chemical heat pumps: Field of application and performance of the internal heat of reaction recovery process. Heat Recover. Syst. CHP 1993, 13, 233–251. [CrossRef] 12. Cot-Gores, J.; Castell, A.; Cabeza, L.F. Thermochemical energy storage and conversion: A-state-of-the-art review of the experimental research under practical conditions. Renew. Sustain. Energy Rev. 2012, 16, 5207–5224. [CrossRef] 13. Albrecht, K.J.; Jackson, G.S.; Braun, R.J. Thermodynamically consistent modeling of redox-stable perovskite oxides for thermochemical energy conversion and storage. Appl. Energy 2016, 165, 285–296. [CrossRef] 14. Huo, J.; Ge, R.; Liu, Y.; Guo, J.; Lu, L.; Chen, W.; Liu, C.; Gao, H.; Liu, H. Recent advances of two-dimensional molybdenum disulfide based materials: Synthesis, modification and applications in energy conversion and storage. Sustain. Mater. Technol. 2020, 24.[CrossRef] 15. Zhao, R.; Cui, D.; Dai, J.; Xiang, J.; Wu, F. Morphology controllable NiCo2O4 nanostructure for excellent energy storage device and overall water splitting. Sustain. Mater. Technol. 2020, 24, e00151. [CrossRef] 16. André, L.; Abanades, S.; Cassayre, L. Experimental Investigation of Co-Cu, Mn-Co, and Mn-Cu Redox Materials Applied to Solar Thermochemical Energy Storage. ACS Appl. Energy Mater. 2018, 1, 3385–3395. [CrossRef] 17. Gravogl, G.; Knoll, C.; Artner, W.; Welch, J.M.; Eitenberger, E.; Friedbacher, G.; Harasek, M.; Hradil, K.; Werner, A.; Weinberger, P.; et al. Pressure effects on the carbonation of MeO (Me = Co, Mn, Pb, Zn) for thermochemical energy storage. Appl. Energy 2019, 252, 113451. [CrossRef] 18. André, L.; Abanades, S.; Flamant, G. Screening of thermochemical systems based on solid-gas reversible reactions for high temperature solar thermal energy storage. Renew. Sustain. Energy Rev. 2016, 64, 703–715. [CrossRef] 19. Mastronardo, E.; Bonaccorsi, L.; Kato, Y.; Piperopoulos, E.; Milone, C. Efficiency improvement of heat storage materials for MgO/H2O/Mg(OH)2 chemical heat pumps. Appl. Energy 2016, 162, 31–39. [CrossRef] Appl. Sci. 2021, 11, 1958 17 of 17

20. Kato, Y.; Takahashi, R.; Sekiguchi, T.; Ryu, J. Study on medium-temperature chemical heat storage using mixed hydroxides. Int. J. Refrig. 2009, 32, 661–666. [CrossRef] 21. Sakellariou, K.G.; Karagiannakis, G.; Criado, Y.A.; Konstandopoulos, A.G. Calcium oxide based materials for thermochemical heat storage in concentrated solar power plants. Sol. Energy 2015, 122, 215–230. [CrossRef] 22. Sánchez Jiménez, P.E.; Perejón, A.; Benítez Guerrero, M.; Valverde, J.M.; Ortiz, C.; Pérez Maqueda, L.A. High-performance and low-cost macroporous calcium oxide based materials for thermochemical energy storage in concentrated solar power plants. Appl. Energy 2019, 235, 543–552. [CrossRef] 23. Álvarez Criado, Y.; Alonso, M.; Abanades, J.C. Composite Material for Thermochemical Energy Storage Using CaO/Ca(OH)2. Ind. Eng. Chem. Res. 2015, 54, 9314–9327. [CrossRef] 24. Shkatulov, A.; Krieger, T.; Zaikovskii, V.; Chesalov, Y.; Aristov, Y. Doping magnesium hydroxide with sodium nitrate: A new approach to tune the dehydration reactivity of heat-storage materials. ACS Appl. Mater. Interfaces 2014, 6, 19966–19977. [CrossRef] 25. Wang, X.; Chen, H.; Xu, Y.; Wang, L.; Hu, S. Advances and prospects in thermal energy storage: A critical review. Kexue Tongbao Chin. Sci. Bull. 2017, 62, 1602–1610. [CrossRef] 26. Scrivener, K.L.; Capmas, A. Calcium Aluminate Cements. Lea’s Chem. Cem. Concr. 2003, 713–782. [CrossRef] 27. Kapeluszna, E.; Kotwica, Ł.; Pichór, W.; Nocuń-Wczelik,W. Cement-based composites with waste expanded perlite—Structure, mechanical properties and durability in chloride and sulphate environments. Sustain. Mater. Technol. 2020, 24.[CrossRef] 28. Liiv, J.; Teppand, T.; Rikmann, E.; Tenno, T. Novel ecosustainable peat and oil shale ash-based 3D-printable composite material. Sustain. Mater. Technol. 2018, 17, e00067. [CrossRef] 29. Geiger, C.A.; Dachs, E.; Benisek, A. Thermodynamic behavior and properties of katoite ( hydrogrossular ): A calorimetric study. Am. Mineral. 2012, 97, 1252–1255. [CrossRef] 30. Stein, H.N. Thermodynamic considerations on the hydration mechanisms of Ca3SiO5 and Ca3Al2O6. Cem. Concr. Res. 1972, 2, 167–177. [CrossRef] 31. Lacivita, V.; Mahmoud, A.; Arco, P.D.; Mustapha, S. Hydrogrossular, Ca3Al2(SiO4)3–x(H4O4)x: An ab initio investigation of its structural and energetic properties. Am. Mineral. 2015, 100, 2637–2649. [CrossRef] 32. Cavenett The Dehydration of Tricalcium aluminate exahydrate. J. Chem. Inf. Model. 1941, 19, 1689–1699. [CrossRef] 33. Ball, M.C.; Ladner, N.G.; Taylor, J.A. The dehydroxylation of tricalcium aluminate 6-hydrate. Thermochim. Acta 1976, 17, 207–215. [CrossRef] 34. Rivas-Mercury, J.M.; Pena, P.; de Aza, A.H.; Turrillas, X. Dehydration of Ca3Al2(SiO4)y(OH)4(3−y) (0 < y < 0.176) studied by neutron thermodiffractometry J.M. J. Eur. Ceram. Soc. 2008, 28, 1737–1748. [CrossRef] 35. Antonoviˇc,V.; Keriene, J.; Boris, R.; Alekneviˇcius,M. The effect of temperature on the formation of the hydrated calcium aluminate cement structure. Procedia Eng. 2013, 57, 99–106. [CrossRef] 36. Ukrainczyk, N.; Matusinović,T. Thermal properties of hydrating calcium aluminate cement pastes. Cem. Concr. Res. 2010, 40, 128–136. [CrossRef] 37. Mohamed, B.M.; Sharp, J.H. Kinetics and mechanism of formation of tricalcium aluminate, Ca3Al2O6. Thermochim. Acta 2002, 388, 105–114. [CrossRef] 38. Hem, J.D.; Roberson, C.E. Form and Stability of Aluminum Hydroxide Complexes in Dilute Solution. USGS Water Supply Pap. 1967, 1827-A, 55. [CrossRef] 39. Whittington, B.I.; Fallows, T.M.; Willing, M.J. Tricalcium aluminate hexahydrate (TCA) filter aid in the Bayer industry: Factors affecting TCA preparation and morphology. Int. J. Miner. Process. 1997, 49, 1–29. [CrossRef] 40. Wefers, K.; Misra, C. Oxides and Hydroxides of Aluminum. Alcoa Tech. Pap. 1987, 19, 100.