Clay/fly ash bricks evaluated in terms of kaolin and vermiculite precursors of mullite and forsterite, and photocatalytic decomposition of the methanol-water mixture

Citation: Valášková, M.; Blah˚ušková, V.; Edelmannová, M.F.; Matˇejová, L.; Soukup, K.; Plevová, E. Clay/Fly Ash Bricks Evaluated in Terms of Kaolin and Vermiculite Precursors of Mullite and Forsterite, and Photocatalytic Decomposition of the Methanol–Water Mixture. Minerals 2023,13, 1114. https://doi.org/ 10.3390/min13091114 Academic Editor: Francisco Franco Received: 2 July 2023 Revised: 12 August 2023 Accepted: 21 August 2023 Published: 23 August 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). minerals Article Clay/Fly Ash Bricks Evaluated in Terms of Kaolin and Vermiculite Precursors of Mullite and Forsterite, and Photocatalytic Decomposition of the Methanol–Water Mixture Marta Valášková1,* , Veronika Blah˚ušková1, Miroslava Filip Edelmannová1, Lenka Matˇejová1, Karel Soukup 2and Eva Plevová3 1Institute of Environmental Technology, CEET, VSB-Technical University of Ostrava, 17. Listopadu 2172/15, 708 00 Ostrava, Czech Republic; [email protected] (V.B.); [email protected] (M.F.E.); [email protected] (L.M.) 2Institute of Chemical Process Fundamentals of the CAS, Rozvojova 135, 165 02 Prague, Czech Republic; [email protected] 3Institute of Geonics of the CAS, Studentska 1768, 708 00 Ostrava, Czech Republic; [email protected] *Correspondence: [email protected]; Tel.: +420-597-327-308 Abstract: This study focused on mullite-based and forsterite-based ceramic bricks fired at 1000 ◦ C from mixtures of fly ash (40 mass%) and kaolins or vermiculites (60 mass%). The structural, physical, and mechanical properties were characterized by X-ray powder diffraction, nitrogen physisorption, mercury porosimetry, thermogravimetry, and compressive strength. In the development of green-material-derived photocatalysts, we evaluated fly ash ceramic bricks based on kaolins and vermiculites, which deserve deeper research. Alkali potassium in the mixtures positively influenced the reduction of the firing temperature, shrinkage, small porosity, and high compressive strength of ceramic bricks. The crystallization of mullite in fly ash was observed on exotherm maxima from 813 to 1025 ◦ C. Muscovite/illite admixture in kaolins precursor of mullite-based ceramics reduced the crystallization temperature of mullite by up to 70 ◦ C. Vermiculite–hydrobiotite–phlogopite in mixed layers of a raw vermiculite precursor of forsterite-based ceramics controlled the formation of enstatite and forsterite in the temperature range from 736 ± 6 ◦ C to 827 ± 6 ◦ C. Mulliteand forsterite-based ceramic bricks were also investigated for photocatalytic hydrogen production. The photocatalytic generation of hydrogen in the presence of mullite-based ceramic bricks was positively correlated with the percentages of Fe 2 O 3 in the lattice of mullites and in the presence of forsterite-based ceramics with the presence of diopside. Mullite-based ceramic produced the highest yield of hydrogen (320 µ mol/g cat after 4 h of irradiation) in the presence of mullite with the highest 10.4% substitution of Fe 2 O 3 in the lattice. The forsterite-based ceramic produced the highest hydrogen yields (354 µ mol/g cat after 4 h of irradiation) over more active diopside than forsterite. Keywords: clays; fly ash; kaolin mullite precursor; vermiculite forsterite precursor; firing; compressive strength; photocatalytic capability 1. Introduction Clay minerals in clay deposits are accompanied by other rock-forming minerals that have a great influence on the final properties of fired clay bricks. Clay bricks have been used for the construction of buildings continuously because of their thermal, physical, and mechanical properties and simple manufacturing process. Fly ashes (FAs) are potentially valuable sources of oxides of silica (SiO 2 ), alumina (Al 2 O 3 ), and iron oxide (Fe 2 O 3 ) for the manufacture of fired clay masonry bricks [ 1 ]. Other oxides in FAs are CaO, MgO, K 2 O, and Na 2 O, as well as some toxic metals (including As, Pb, Cd, and Ni) in varying amounts. The main crystalline phases are mullite (3Al 2 O 3· SiO 2 ) and quartz. The amount of solid Minerals 2023,13, 1114. https://doi.org/10.3390/min13091114 https://www.mdpi.com/journal/minerals Minerals 2023,13, 1114 2 of 16 waste added to clays considering the quality of the products is normally less than 30% by weight [ 2 , 3 ]. Bricks from clay and fly ash in mass ratio up to 80% fired at 1050 ◦ C reached the compressive strength of 25 MPa [ 4 ]. A fly-ash-positive effect was found in the reduction of firing shrinkage and a decrease in bulk density [5]. The international classification of bricks is based on different specifications and international standard codes depending on the importance of structures and specific use [ 6 ]. Bricks are generally characterized by both interaction with water and mechanical resistance compressive strength. From the manufacturer’s point of view, after the clay is dried at above 105 ◦ C, the reduction in size in length or volume is closely related to particle size determined as drying shrinkage (DS). Changes in parameters at firing temperatures in the range of 900–1100 ◦ C are characterized as linear shrinkage (LS), bulk density (BD), and weight loss (WL) [ 7 ]. Kaolin clays used in the bricks should contain percentages of SiO 2 in the range between 50% and 60%, Fe 2 O 3 not exceeding 10%, and Al 2 O 3 in the range between 10% and 20% [ 7 ]. Al 2 O 3 contributes to the transformation into mullite during firing and to the increase in mechanical resistance [ 7 ]. Fly ash (40 mass%) and plastic clay (60 mass%) mixtures were fired between 900 and 1200 ◦ C to the ceramic building materials, including fired clay as building masonry bricks [ 8 ]. Kaolin in flay ash mixtures prevented the formation of open pores during mullite sintering [ 9 ]. Generally, kaolinite (Al 4 (OH) 8· SiO 4 O 10 ) in kaolins in temperatures above 450 ◦ C undergoes dehydroxylation to form metakaolinite (Al 2 SiO 2 O 7 ). Metakaolinite is converted into Al-Si spinel and amorphous silica around 950 ◦ C. According to the literature, mullites nucleate and crystallize around 1000 ◦ C, depending on the type of precursor, particle size, and heat treatment [ 10 ]. Metakaolin additive reduces pores and makes the ceramic waterproof, significantly reducing shrinkage, but increasing the mechanical strengths [11]. Raw vermiculites have recently been used as an inorganic pore-forming additive for the production of porous and lightweight clay bricks [ 12 , 13 ]. Vermiculite is a product of weathering or hydrothermal alteration of hydrobiotite or phlogopite. In the mixed layer structure of raw vermiculite, potassium occupies the interlayer of phlogopite, whereas hydrated cations balancing a net layer charge occupy the interlayer of hydrobiotite and the vermiculite mineral. The hydration state of vermiculite is defined by the 0, 1, or 2 water layers in the interlayer space, or the water layer hydration state (0-, 1-, or 2-WLHS) of the corresponding phases [ 14 ]. The hydration state of phlogopite and Mg-vermiculite corresponds to the 2-WLHS and 0-WLHS, respectively. The hydrobiotite is diffracted in the broad band (13.6–11.0 Å) due to the different stacking sequence of Mg-vermiculite and Kbiotite [ 15 ]. At temperatures around 550 ◦ C, vermiculite and hydrobiotite dehydrate to the 10 Å phase (0-WLHS). The high temperature phases show considerable variation, depending on the interlayer ions and lattice substitutions [ 16 ]. At 1000 ◦ C, Mg-vermiculite transforms to enstatite (Mg 2 Si 2 O 6 ) [ 17 ], while the vermiculite–hydrobiotite–phlogopite mixed layer structure transforms to forsterite (Mg2SiO4) and leucitelinebreak (KAlSi2O6) [18,19]. A comprehensive overview [ 20 ] summarizes recent advances in the utilization of natural materials (clay mineral and pumice) and waste materials (ash and foundry sand)-based metal oxide photocatalysts for the photodegradation of various pollutants. Silicate glass ceramics belong to the surface-active category, having the unique ability to form strong bonding with metal oxides. Larnite (Ca 2 SiO 4 ) and forsterite (Mg 2 SiO 4 ) degraded methylene blue dye under natural daylight (Sunlight) within the 90 min by 67.2 and 47.1%, respectively [ 21 ]. Similarly, the photocatalytic experiment with larnite and forsterite silicate ceramics with rhodamine B (RhB) dye for 120 min confirmed dye degradation of 87% and 79%, respectively [22]. The main aim of this work was the utilization of a kaolin–fly ash mixture into mullite-based ceramic bricks in comparison with the unconventionally used vermiculite–fly ash mixtures into forsterite-based ceramic bricks. Two objectives are focused on in terms of the evaluation of the ceramic bricks: (1) structural, physical, and mechanical properties, and (2) photocatalytic activity performed on the decomposition of the methanol– water mixture. Minerals 2023,13, 1114 3 of 16 2. Materials and Methods 2.1. Materials Fly ash (denoted FA) was obtained from the combustion of black coal captured on an electrostatic precipitator in the powder station (Czech Republic). Clays for the fly ash mixtures were kaolins (samples denoted Ka and Kb), supplied by LB Minerals, Ltd. (Horní Bˇríza, Czech Republic); metakaolin (sample denoted Mk), supplied by the KERAMOST, Plc. (Most, Czech Republic) under the trade name “thermally processed washed kaolin”; and vermiculites from the Palabora region of South Africa (sample denoted Va) and the Paraiba region of Brazil (sample denoted Vb), supplied by Grena Co. (Veselínad Lužnicí, Czech Republic). 2.2. Production of Brick Cubes Kaolins, vermiculites, and fly ash were pulverized by grinding in a PULVERISETTE 6 mill (Fritsch GmbH, Idar-Oberstein, Germany) and sieved to the size fraction under 63 µm. Dry mixtures of FA (40 mass%) and kaolins or vermiculites (60 mass%) prepared to the ceramic building bricks [ 8 ] were homogenized in a bottle at 40 rpm for 1 h (Heidolph MR Hei-Tec, Heidolph, Heidolph Instruments GmbH and Co., KG, Schwabach, Germany) and marked as FA/Ka, FA/Kb, FA/Mk, FA/Va, and FA/Vb. Finally, the homogenized mixtures were milled at 300 rpm for 15 min and prepared for the clay slurry of fine particles containing about 20 mass% distilled water. The slurry was kneaded by hand and formed in the molds of a size of 20 mm ×20 mm ×20 mm. The mols were covered with wet burlap and left to rest overnight. Samples in the molds were dried at 105 ◦ C for 5 h. Dry cubes were taken out of the mold and dried until the weight became constant. Cubes were sintered in a muffle furnace (NABERTHERM L9/11/SW, Nabertherm GmbH, Lilienthal, Germany) at the heating ramp 10 ◦ C/min to the desired temperature of 1000 ◦ C, maintained at this temperature for 2 h, and then slowly cooled to room temperature. 2.3. Characterization Techniques The chemical composition of fly ash, kaolins, and vermiculites was determined by X-ray fluorescence (XRF) using a SPECTRO XEPOS energy-dispersive X-ray fluorescence (ED-XRF) spectrometer (Spectro Analytical Instruments, Kleve, Germany). The samples (2 × 4 g) were mixed with the wax (0.9 g) and prepared for the pellets by manual hydraulic pressing at 10 tons. The total concentration of Fe was related to Fe 2 O 3 . The content of Fe(II) was subtracted from the total Fe when it was determined using titration with a 0.1 M solution of K2Cr2O7in a sample decomposed in HCl and HF in a CO2atmosphere. X-ray powder diffraction (XRD) analysis of mineral phases was performed using the Rigaku SmartLab diffractometer (Rigaku Corporation, Tokyo, Japan) under CoK α radiation at 40 kV and 40 mA. The XRD patterns were evaluated using PDXL2 software no. 2.4.2.0 (Rigaku Corporation, Tokyo, Japan) and compared with the database PDF-2, 2015 (ICDD, Newton Square, PA, USA). Lattice parameters of mullites were calculated using Rietveld refinement by PDXL2 software. Thermogravimetry and differential thermal curves (TG/DTA) were obtained using the thermal analyzer Setsys 24 Evolution (Setaram, Caluire, France), equipped with the thermocouple Pt-Pt90/Rh10. Clays and fly ash mixtures (about 25 mg) loose packed in a crucible were heated to the final temperature 1100 ◦ C at the heating rate 10 ◦ C/min (atmosphere Ar, reference material Al2O3). Determination of mesoporeand macropore-size distribution; specific intrusion volume; V intr ; apparent (mercury) density; ρHg ; and porosity, ε , of solids were realized with the aid of a high-pressure mercury porosimeter, AutoPore IV 9500 (Micromeritics, Norcross, GA, USA). Complementary information about the skeletal (true) density, ρHe , of solids was obtained by using an AccuPyc II 1340 helium pycnometer (Micromeritics, Norcross, GA, USA). Before both analyses, the solids were dried in an oven at 110 ◦ C for several days to remove physisorbed moisture. Minerals 2023,13, 1114 4 of 16 The specific surface area (S BET ) and the net pore volume (V net ) of solids were realized by using nitrogen physisorption at 77 K, performed using a 3Flex volumetric apparatus (Micromeritics, Norcross, GA, USA). Before physisorption analysis, the solids were degassed at a pressure of ≈ 0.6 bar for 12 h at least at 350 ◦ C to remove the physisorbed water. The specific surface area, S BET , was calculated according to classical BET theory for the p/p 0 range = 0.05–0.25, and net pore volume, V net , was calculated from the adsorption branch of the measured nitrogen adsorption–desorption isotherm at p/p0= 0.99. 2.4. Testing Procedures of Cube Bricks 2.4.1. Physical and Mechanical Tests Some physical average values of the five brick cubes of each sample were calculated by Equations (1)–(3) [23]: Mass Loss, WL (%) = (WLdry −WLfired/WLfired)×100 (1) Drying Shrinkage, DS (%) = (lplastic −ldry/ldry)×100 (2) Firing Shrinkage, FS (%) = (ldry −lfired/lfired)×100 (3) lplastic: width, height, and length of the clay brick samples in the plastic state. ldry: width, height, and length of the clay brick samples in the oven-dry state at 105 ◦C. lfired: width, height, and length of the clay brick samples after being fired at 1000 ◦C. Wdry: mass of the clay brick samples in the oven-dry state at 105 ◦C. Wfired: mass of the clay brick samples after being fired at 1000 ◦C. Compressive strength (CS) was determined on the press machine (BRIO, Hranice, Czech Republic) at a loading rate of 500 N/s. The highest and lowest MPa values were excluded, and the remaining measurements were averaged. 2.4.2. Photocatalytic Activity Test of Bricks Photocatalytic tests were performed in a home-made photoreactor (stainless steel, volume 305 mL, 8W Hg lamp peak intensity at 254 nm wavelength; Ultra-Violet Products Inc., Upland, CA, USA). Reaction of the methanol–water mixture (100 mL of 50% methanol) with a photocatalyst (0.1 g) was saturated by helium in order to purge the air from the solution. The gaseous products were analyzed by a gas chromatograph (Shimadzu Tracera GC-2010Plus, Shimadzu, Kyoto, Japan) equipped with a BID (barrier discharge ionization detector). All experiments were repeated at least 3 times, and similar results were calculated for the mean values. 3. Results and Discussion 3.1. Chemistry and Phases of Clays, FA, and FA/Clay Ceramic Bricks The concentrations of elements (from XRF analysis) were converted to the concentrations of stoichiometric oxides (mass%) (Table 1). FA met the requirements SiO 2 + Al 2 O 3 + Fe 2 O 3≥ 70% to a class F fly ash set forth in ASTM C618-19 [ 24 ]. Clays used for brick production should have SiO 2 in the range of 50%–60% and Al 2 O 3 in the range of 10%– 20% [ 6 ]. Manufacturers recommend not exceeding 10% of Fe 2 O 3 and 6% of CaO [ 7 ]. In this work, the kaolins Ka, Kb, and Mk had SiO 2 in the range from 49 to 58 mass%, while Al 2 O 3 exceeded this range by about 15%. Vermiculites Va and Vb had SiO 2 about 5% lower outside this range and Al2O3in the desired range. In this work, the kaolins Ka, Kb, and Mk had SiO 2 in the range from 49 to 58 mass%, while Al 2 O 3 exceeded this range by about 15%. Vermiculites Va and Vb had SiO 2 about 5% lower outside this range and Al2O3in the desired range. Minerals 2023,13, 1114 5 of 16 Table 1. Bulk chemical analysis of the major oxide elements (mass%) of the fly ash (FA), kaolins (Ka, Kb, and Mk), and vermiculites (Va and Vb), as well as the FA/clay ceramic bricks (normalized to 100%). Sample SiO2 (mass%) TiO2 (mass%) Al2O3 (mass%) Fe2O3 FeO (mass%) CaO (mass%) MgO (mass%) K2O (mass%) Na2O (mass%) L.O.I 1 (mass%) FA 53.57 1.21 24.20 7.40 3.36 1.28 2.87 0.81 5.2 Ka 51.35 0.75 31.60 0.71 0.26 0.28 1.40 <0.01 13.30 Kb 49.40 0.87 34.37 0.85 0.19 0.31 2.55 <0.01 10.62 Mk 57.58 0.59 37.08 0.94 0.04 0.28 0.97 <0.01 3.22 Va 43.39 1.11 8.88 5.06 1.43 23.48 4.81 0.15 8.38 Vb 44.02 0.75 11.02 6.65 1.30 23.23 1.82 0.35 9.73 FA 54.90 1.12 28.43 6.65 3.25 2.30 2.95 0.40 n.d. 2 FA/Ka 56.47 1.02 33.17 3.21 1.27 0.70 2.11 2.05 n.d. 2 FA/Kb 56.63 0.95 32.18 3.35 1.43 0.77 2.77 1.92 n.d. 2 FA/Mk 56.54 0.79 36.11 3.08 1.20 0.58 1.70 <0.01 n.d. 2 FA/Va 46.66 1.14 16.50 8.53 5.22 17.83 4.12 <0.01 n.d. 2 FA/Vb 47.82 1.09 17.05 7.92 5.58 16.94 3.60 <0.01 n.d. 2 1L.O.I. (loss on ignition) at 1000 ◦C; 2n.d. (not determined). Firing temperature highly influences brick properties and is one of the main factors in energy consumption. Although some reviewed works relate higher temperatures (above 1100 ◦C), these temperatures are infrequently used in the brick industry [25]. XRD phase analysis of kaolins and vermiculites have previously been published [ 26 , 27 ]. FA proved mullite (JCPDS card no. 00-079-1455), quartz (JCPDS card no. 03-065-0466), anorthite (JCPDS card no. 00-076-0948), and hematite (JCPDS card no. 00-089-0596). XRF analysis of FA showed CaO having 3.29 mass% and K 2 O having 3.54 mass% (Table 1), observed not only in crystalline phases, and therefore their presence in amorphous and weakly diffracting material was assumed. Kaolin samples Ka and Kb were composed of kaolinite ((JCPDF card no. 00-058-2005), muscovite (JCPDF card no. 01-076-0928), quartz (JCPDF card no. 01-086-2237), and orthoclase (JCPDS card no. 01-076-0824); more details are found in [ 26 ]. The metakaolin (Mk) sample was composed of kaolinite (JCPDF card no. 00-058-2028) and illite (JCPDF card no. 00-058-2016)/muscovite (JCPDF card no. 01-076-0928). Vermiculite (Va) contained hydrobiotite (JCPDF card no. 00-049-1057) predominating over vermiculite (JCPDF card no. 00-076-0847), a minority of phlogopite (JCPDF card no. 01–082-2699), and hydroxyapatite (JCPDS card no. 00-064-0738) impurity from the parent rocks. Vermiculite (Vb) is composed of vermiculite (JCPDF card no. 00-076-0847), prevailing slightly over hydrobiotite (JCPDF card no. 00-049-1057), a minority of phlogopite (JCPDF card no. 01–082-2699,) and the accessory minerals tremolite (JCPDF card no. 00-009-0437) and rutile (JCPDF card no. 00-034-0180)); more details can be found in [27]. Oxides in FA/clay mixtures when plotted in a ternary diagram of a SiO 2 –Al 2 O–(MgO + CaO + K 2 O) system fall into the areas of mullite and enstatite–forsterite [ 28 ] (Figure 1a) in fired ceramic bricks (Figure 1b). Dry powder FA/clay mixtures were fired at 1000 ◦ C for the mullite-based and forsteritebased ceramics (Figure 2). Crystalline phases in FA/Mk, FA/Ka, and FA/Kb mullitebased bricks (Figure 2a) were mullite (JCPDF card no. 01-079-1455), quartz (JCPDF card no. 01-086-2237), anorthite (JCPDS card no. 01-071-0748), nepheline (JCPDS card no. 01-083-2279), and hematite (JCPDF card no. 01-089-0596). Minerals 2023,13, 1114 6 of 16 Minerals 2023, 13, x 6 of 15 (a) (b) Figure 1. FA/clay mixtures of the ceramic bricks: (a) ternary phase diagram of SiO2–Al2O–(MgO + CaO + K2O) of the FA and FA/clay mixtures with the delineation of the chemistry of mullite and forsterite; (b) photograph of the fired ceramic brick cubes. Dry powder FA/clay mixtures were fired at 1000 °C for the mullite-based and forsterite-based ceramics (Figure 2). Crystalline phases in FA/Mk, FA/Ka, and FA/Kb mullitebased bricks (Figure 2a) were mullite (JCPDF card no. 01-079-1455), quartz (JCPDF card no. 01-086-2237), anorthite (JCPDS card no. 01-071-0748), nepheline (JCPDS card no. 01083-2279), and hematite (JCPDF card no. 01-089-0596). (a) (b) Figure 2. X-ray diffraction patterns (segments) of (a) mullite-based bricks and (b) forsterite-based bricks. An = anorthite; Di = diopside; Fo = forsterite; Hem = hematite; Lc = leucite; Mul = mullite; Nph = nepheline; Phl = phlogopite; Qz = quartz (symbols according to [29]). Forsterite-based FA/Va and FA/Vb bricks (Figure 2b) are composed of forsterite (JCPDS card no. 00-034-0189), diopside (JCPDS card no. 01-076-6317), leucite (JCPDS card no. 01-076-8735, quartz (JCPDS card no. 01-089-8936), hematite (JCPDS card no. 01-0898936), anorthite (JCPDS card no. 01-071-0748), and phlogopite (JCPDS card no. 01-0852271). Phlogopite is the remaining phase after the dehydration of vermiculite and hydrobiotite [28]. The presence of forsterite (Mg2SiO4) and leucite (KAlSi2O6) agree well with the phase analysis on single vermiculite flakes and commercial vermiculite above 800 °C [19]. 3.2. Porosity of FA and FA/Clay Ceramics All ceramic cubes, instead of FA/Ka and FA/Vb, showed uniform macropore-size distribution with macropore diameters above 230 nm (Figure 3). In general, the macropore size of individual solids increased as follows: FA/Mk (234 nm) < FA/Va (548 nm) < FA/Kb (755 nm) = Fa/Ka (754 and 4070 nm) < FA/Vb (943 and 4070 nm) < FA (1160 nm). FA/Ka Figure 1. FA/clay mixtures of the ceramic bricks: ( a ) ternary phase diagram of SiO 2 –Al 2 O– (MgO + CaO + K 2 O) of the FA and FA/clay mixtures with the delineation of the chemistry of mullite and forsterite; (b) photograph of the fired ceramic brick cubes. Minerals 2023, 13, x 6 of 15 (a) (b) Figure 1. FA/clay mixtures of the ceramic bricks: (a) ternary phase diagram of SiO2–Al2O–(MgO + CaO + K2O) of the FA and FA/clay mixtures with the delineation of the chemistry of mullite and forsterite; (b) photograph of the fired ceramic brick cubes. Dry powder FA/clay mixtures were fired at 1000 °C for the mullite-based and forsterite-based ceramics (Figure 2). Crystalline phases in FA/Mk, FA/Ka, and FA/Kb mullitebased bricks (Figure 2a) were mullite (JCPDF card no. 01-079-1455), quartz (JCPDF card no. 01-086-2237), anorthite (JCPDS card no. 01-071-0748), nepheline (JCPDS card no. 01083-2279), and hematite (JCPDF card no. 01-089-0596). (a) (b) Figure 2. X-ray diffraction patterns (segments) of (a) mullite-based bricks and (b) forsterite-based bricks. An = anorthite; Di = diopside; Fo = forsterite; Hem = hematite; Lc = leucite; Mul = mullite; Nph = nepheline; Phl = phlogopite; Qz = quartz (symbols according to [29]). Forsterite-based FA/Va and FA/Vb bricks (Figure 2b) are composed of forsterite (JCPDS card no. 00-034-0189), diopside (JCPDS card no. 01-076-6317), leucite (JCPDS card no. 01-076-8735, quartz (JCPDS card no. 01-089-8936), hematite (JCPDS card no. 01-0898936), anorthite (JCPDS card no. 01-071-0748), and phlogopite (JCPDS card no. 01-0852271). Phlogopite is the remaining phase after the dehydration of vermiculite and hydrobiotite [28]. The presence of forsterite (Mg2SiO4) and leucite (KAlSi2O6) agree well with the phase analysis on single vermiculite flakes and commercial vermiculite above 800 °C [19]. 3.2. Porosity of FA and FA/Clay Ceramics All ceramic cubes, instead of FA/Ka and FA/Vb, showed uniform macropore-size distribution with macropore diameters above 230 nm (Figure 3). In general, the macropore size of individual solids increased as follows: FA/Mk (234 nm) < FA/Va (548 nm) < FA/Kb (755 nm) = Fa/Ka (754 and 4070 nm) < FA/Vb (943 and 4070 nm) < FA (1160 nm). FA/Ka Figure 2. X-ray diffraction patterns (segments) of ( a ) mullite-based bricks and ( b ) forsterite-based bricks. An = anorthite; Di = diopside; Fo = forsterite; Hem = hematite; Lc = leucite; Mul = mullite; Nph = nepheline; Phl = phlogopite; Qz = quartz (symbols according to [29]). Forsterite-based FA/Va and FA/Vb bricks (Figure 2b) are composed of forsterite (JCPDS card no. 00-034-0189), diopside (JCPDS card no. 01-076-6317), leucite (JCPDS card no. 01-076-8735, quartz (JCPDS card no. 01-089-8936), hematite (JCPDS card no. 01-089-8936), anorthite (JCPDS card no. 01-071-0748), and phlogopite (JCPDS card no. 01-085-2271). Phlogopite is the remaining phase after the dehydration of vermiculite and hydrobiotite [ 28 ]. The presence of forsterite (Mg 2 SiO 4 ) and leucite (KAlSi 2 O 6 ) agree well with the phase analysis on single vermiculite flakes and commercial vermiculite above 800 ◦C [19]. 3.2. Porosity of FA and FA/Clay Ceramics All ceramic cubes, instead of FA/Ka and FA/Vb, showed uniform macropore-size distribution with macropore diameters above 230 nm (Figure 3). In general, the macropore size of individual solids increased as follows: FA/Mk (234 nm) < FA/Va (548 nm) < FA/Kb (755 nm) = Fa/Ka (754 and 4070 nm) < FA/Vb (943 and 4070 nm) < FA Minerals 2023,13, 1114 7 of 16 (1160 nm). FA/Ka and FA/Vb were characterized by bidisperse macropore-size distribution. Moreover, FA/Vb showed broad macropore-size distribution of smaller macropores with a maximum at 943 nm. The porosity was in the range between 28% and 46%. Minerals 2023, 13, x 7 of 15 and FA/Vb were characterized by bidisperse macropore-size distribution. Moreover, FA/Vb showed broad macropore-size distribution of smaller macropores with a maximum at 943 nm. The porosity was in the range between 28% and 46%. Figure 3. Mesoporeand macropore-size distributions of the investigated solids determined from high-pressure mercury porosimetry. The results from nitrogen physisorption measurements (Figure 4) indicated that FA, FA/Ka, and FA/Kb (Figure 4a) in comparison with FA/Mk, FA/Va, and FA/Vb (Figure 4b) showed very low surface areas of 0.79, 1.26, and 1.07 m 2 /g, respectively (Table 2), originating from macropores (Figure 4c). Additional smaller macropores from 80 to 200 nm were determined only in FA/Mk, FA/Va, and FA/Vb (Figure 4d). Therefore, FA/Mk, FA/Va, and FA/Vb ceramic samples had higher surface areas of 5.16, 2.09, and 2.30 m 2 /g, respectively, as well as higher net pore volumes of 17.77, 6.34, and 10 cm 3 (STP)/g, respectively (Table 2). From the measured nitrogen isotherm (Figure 4b) and evaluated macropore-size distribution (Figure 4d), it is evident that FA/Vb possessed some macropores of 100 nm diameter not differentiated by high-pressure mercury porosimetry. Table 2. Textural properties determined by high-pressure mercury porosimetry, helium pycnometry, and nitrogen physisorption at 77 K. Sample Hg Porosimetry and He Pycnometry Physisorption V intr (cm 3 /g) ρ Hg (g/cm 3 ) ρ He (g/cm 3 ) ε (%) S BET (m 2 /g) V net (cm 3 (STP)/g) FA 0.23 1.49 2.67 44 0.79 1.20 FA/Ka 0.28 1.43 2.27 37 1.26 2.63 FA/Kb 0.20 1.67 2.33 28 1.07 2.36 FA/Mk 0.31 1.40 2.40 42 5.16 17.77 FA/Va 0.24 1.64 2.59 37 2.09 6.73 FA/Vb 0.35 1.41 2.60 46 2.30 10.05 V intr : specific intrusion volume determined from high-pressure mercury porosimetry. ρ Hg : apparent (mercury) density determined from high-pressure mercury porosimetry. ρ He : skeletal (true) density determined from helium pycnometry. ε: porosity determined by following calculation: ε (%) = (1 − (ρ Hg /ρ He )) × 100. S BET : specific surface area determined from nitrogen physisorption at 77 K. V net : net pore volume determined from nitrogen physisorption at 77 K. Figure 3. Mesoporeand macropore-size distributions of the investigated solids determined from high-pressure mercury porosimetry. The results from nitrogen physisorption measurements (Figure 4) indicated that FA, FA/Ka, and FA/Kb (Figure 4a) in comparison with FA/Mk, FA/Va, and FA/Vb (Figure 4b) showed very low surface areas of 0.79, 1.26, and 1.07 m 2 /g, respectively (Table 2), originating from macropores (Figure 4c). Additional smaller macropores from 80 to 200 nm were determined only in FA/Mk, FA/Va, and FA/Vb (Figure 4d). Therefore, FA/Mk, FA/Va, and FA/Vb ceramic samples had higher surface areas of 5.16, 2.09, and 2.30 m2/g, respectively, as well as higher net pore volumes of 17.77, 6.34, and 10 cm 3 (STP)/g, respectively (Table 2). From the measured nitrogen isotherm (Figure 4b) and evaluated macropore-size distribution (Figure 4d), it is evident that FA/Vb possessed some macropores of 100 nm diameter not differentiated by high-pressure mercury porosimetry. 3.3. Thermal Transformation of Clays and FA/Clay Mixtures The results of thermal changes in FA, clay samples, and FA/clay mixtures was obtained from thermogravimetry and derivate thermogravimetry (Table 3, Figure 5). Mass loss percentages obtained from TG curves and temperatures at maximum mass loss rate (T m ) obtained from derivative TG curves (Figure 5a) characterized the thermal decomposition steps of the studied samples (Table 3). Total mass loss percentages ( ∆ m) were comparable to the L.O.I. value shown in Table 1, within a error of up to 2%. Minerals 2023,13, 1114 8 of 16 Minerals 2023, 13, x 8 of 15 Figure 4. (a,b) Nitrogen adsorption–desorption isotherms at 77 K; (c,d) evaluated mesoporeand macropore-size distributions of the investigated solids from physisorption. 3.3. Thermal Transformation of Clays and FA/Clay Mixtures The results of thermal changes in FA, clay samples, and FA/clay mixtures was obtained from thermogravimetry and derivate thermogravimetry (Table 3, Figure 5). Mass loss percentages obtained from TG curves and temperatures at maximum mass loss rate (Tm) obtained from derivative TG curves (Figure 5a) characterized the thermal decomposition steps of the studied samples (Table 3). Total mass loss percentages (∆ m) were comparable to the L.O.I. value shown in Table 1, within a error of up to 2%. Table 3. Thermal characteristics of FA, clays, and FA/clay mixtures. Sample ∆ m (%) Tm 1 (°C) Tm 2 (°C) Tm 3 (°C) Tm 4 (°C) Tm 5 (°C) Tm 6 (°C) Tm 7 (°C) T1exo (°C) T2exo (°C) FA −5.5 119 – – 589 679 – – 813 1025 Ka −11.7 110 – 395 528 – – – 980 Kb −11.2 108 – 326 519 – – – 975 Mk −0.7 120 – 349 520 – – – 981 Va −7.8 115 230 – 505 682 789 998 924 Vb −10.9 112 217 – 501 658 827 989 857 FA/Ka −12.1 104 – 361 532 – 866 – 966 FA/Kb −10.8 102 – 344 510 – 840 – 954 FA/Mk −4.3 112 – – 538 699 845 – 978 FA/Va −9.3 110 219 – 512 689 822 991 743 833 FA/Vb −8.8 107 216 – 491 637 815 963 730 821 ∆ m: mass loss percentages obtained from TG curves and temperature intervals at maximum mass loss rate. Tm: mass loss rate from DTG curves. T1 and T2: temperatures of exotherm peak maxima from DTA curves. Figure 4. ( a , b ) Nitrogen adsorption–desorption isotherms at 77 K; ( c , d ) evaluated mesoporeand macropore-size distributions of the investigated solids from physisorption. Table 2. Textural properties determined by high-pressure mercury porosimetry, helium pycnometry, and nitrogen physisorption at 77 K. Sample Hg Porosimetry and He Pycnometry Physisorption Vintr (cm3/g) ρHg (g/cm3) ρHe (g/cm3) ε (%) SBET (m2/g) Vnet (cm3(STP)/g) FA 0.23 1.49 2.67 44 0.79 1.20 FA/Ka 0.28 1.43 2.27 37 1.26 2.63 FA/Kb 0.20 1.67 2.33 28 1.07 2.36 FA/Mk 0.31 1.40 2.40 42 5.16 17.77 FA/Va 0.24 1.64 2.59 37 2.09 6.73 FA/Vb 0.35 1.41 2.60 46 2.30 10.05 V intr : specific intrusion volume determined from high-pressure mercury porosimetry. ρHg : apparent (mercury) density determined from high-pressure mercury porosimetry. ρHe : skeletal (true) density determined from helium pycnometry. ε : porosity determined by following calculation: ε (%) = (1 − ( ρHg / ρHe )) × 100. S BET : specific surface area determined from nitrogen physisorption at 77 K. V net : net pore volume determined from nitrogen physisorption at 77 K. Minerals 2023,13, 1114 9 of 16 Table 3. Thermal characteristics of FA, clays, and FA/clay mixtures. Sample ∆m (%) Tm 1 (◦C) Tm 2 (◦C) Tm 3 (◦C) Tm 4 (◦C) Tm 5 (◦C) Tm 6 (◦C) Tm 7 (◦C) T1exo (◦C) T2exo (◦C) FA −5.5 119 – – 589 679 – – 813 1025 Ka −11.7 110 – 395 528 – – – 980 Kb −11.2 108 – 326 519 – – – 975 Mk −0.7 120 – 349 520 – – – 981 Va −7.8 115 230 – 505 682 789 998 924 Vb −10.9 112 217 – 501 658 827 989 857 FA/Ka −12.1 104 – 361 532 – 866 – 966 FA/Kb −10.8 102 – 344 510 – 840 – 954 FA/Mk −4.3 112 – – 538 699 845 – 978 FA/Va −9.3 110 219 – 512 689 822 991 743 833 FA/Vb −8.8 107 216 – 491 637 815 963 730 821 ∆ m: mass loss percentages obtained from TG curves and temperature intervals at maximum mass loss rate. Tm: mass loss rate from DTG curves. T1and T2: temperatures of exotherm peak maxima from DTA curves. Minerals 2023, 13, x 9 of 15 (a) (b) Figure 5. DTG curves of (a) kaolins and vermiculites; (b) FA and FA–clay mixtures. Kaolinite transforms at temperatures below 1000 °C during the removal of water and hydroxyl groups directly into mullite and, therefore, kaolinite is classified as a mullite precursor of type I [30]. The DTG or DTA peak, which reflects the dehydroxylation of kaolinite, displays some variability of the formation of disordered metakaolinite and mullite. Kaolins Ka and Kb, as well as metakaolin (Mk), were dehydrated at a low temperature interval of up to 150 °C. Dehydroxylation of kaolinite and transformation to metakaolinite occurred at the temperature interval of 350–600 °C, and exothermic transformation of metakaolinite to mullite at about 980 °C (Table 3). Mass loss of raw vermiculites during firing is a result of dehydration and dehydroxylation of vermiculite (Vm) and hydrobiotite (Hb). The three thermal mass losses are generally observed [13] (Table 3). The first loss is caused by the removal of physically adsorbed water as well as the first hydration sphere of interlayer cations up to 150 °C, the second mass loss of remaining bound water is associated with interlayer cations in the range of 180 to 250 °C, and the third mass loss and heat effects observed in the temperature range of 650 to 950 °C is due to the dehydroxylation of vermiculite. The dehydroxylation of raw Va takes place in the Hb > Vm mixed layer structure and was recorded on DTG curves of two different intensive endothermal maxima at 682 °C and at 789 °C. Similarly, raw Vb dehydroxylation of the Vm > Hb mixed layer structure produced two similar less intensive endo peaks at 658 and 827 °C (Figure 5a, Table 3). The exotherm maxima at 924 and 857 °C in Va and Vb, respectively, confirmed about a 67 °C higher temperature of forsterite crystallization from Hb > Vm than from Vm > Hb. Thermodynamic calculations on similar vermiculites suggests that forsterite can exist with leucite and hematite in potassium aluminosilicate melt at 970 °C [18]. Enstatite was not found in the XRD patterns, but leucite (KAlSi 2 O 6 ) was formed instead [19]. It is obvious that magnesium silicates in FA/Va and FA/Vb mixtures undergo transformation to different crystalline phase states at lower temperatures than pure FA, Va, and Vb (Table 3, Figure 5b). Exothermal maxima at 743 °C in FA/Va and 730 °C in FA/Vb were assigned to the crystallization of forsterite [31]. The XRD phase analysis of forsteritebased bricks FA/Va and FA/Vb confirmed the presence of crystalline forsterite (Mg 2 SiO 4 ), diopside (CaMgSi 2 O 6 ), leucite (KAlSi 2 O 6 ), and phlogopite (KMg 3 AlSi 3 O 10 ) (Figure 2b). Crystallization of phases can be observed on the exotherm maxima at 924 and 857 °C in Va and Vb, respectively, as well as the maxima that dropped to 833 and 821 °C in FA/Va and FA/Vb, respectively. Enstatite (MgSiO 3 ) and forsterite crystallizing from 743 °C to 833 °C in FA/Va and from 730 °C to 821 °C in FA/Vb was already documented in literature [31–33]. Figure 5. DTG curves of (a) kaolins and vermiculites; (b) FA and FA–clay mixtures. Kaolinite transforms at temperatures below 1000 ◦ C during the removal of water and hydroxyl groups directly into mullite and, therefore, kaolinite is classified as a mullite precursor of type I [ 30 ]. The DTG or DTA peak, which reflects the dehydroxylation of kaolinite, displays some variability of the formation of disordered metakaolinite and mullite. Kaolins Ka and Kb, as well as metakaolin (Mk), were dehydrated at a low temperature interval of up to 150 ◦ C. Dehydroxylation of kaolinite and transformation to metakaolinite occurred at the temperature interval of 350–600 ◦ C, and exothermic transformation of metakaolinite to mullite at about 980 ◦C (Table 3). Mass loss of raw vermiculites during firing is a result of dehydration and dehydroxylation of vermiculite (Vm) and hydrobiotite (Hb). The three thermal mass losses are generally observed [ 13 ] (Table 3). The first loss is caused by the removal of physically adsorbed water as well as the first hydration sphere of interlayer cations up to 150 ◦ C, the second mass loss of remaining bound water is associated with interlayer cations in the range of 180 to 250 ◦ C, and the third mass loss and heat effects observed in the temperature range of 650 to 950 ◦ C is due to the dehydroxylation of vermiculite. The dehydroxylation of raw Va takes place in the Hb > Vm mixed layer structure and was recorded on DTG curves of two different intensive endothermal maxima at 682 ◦ C and at 789 ◦ C. Similarly, raw Vb dehydroxylation of the Vm > Hb mixed layer structure produced two similar less intensive endo peaks at 658 and 827 ◦ C (Figure 5a, Table 3). The exotherm maxima at 924 and 857 ◦ C in Va and Vb, respectively, confirmed about a 67 ◦ C higher temperature of forsterite crystallization from Hb > Vm than from Vm > Hb. Thermodynamic calculations on similar vermiculites Minerals 2023,13, 1114 16 of 16 40. Valášková, M.; Blah˚ušková, V.; Vlˇcek, J. Effects of kaolin additives in fly ash on sintering and properties of mullite ceramics. Minerals 2021,11, 887. [CrossRef] 41. Okada, K.; Otsuka, N.; Somiya, S. Review of mullite synthesis routes in Japan. Am. Ceram. Soc. Bull. 1991,70, 1633–1640. Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.