Kaolinite-Magnesite Or Kaolinite–Talc-Based Ceramics. Part II: Microstructure and the Final Properties Related Sintered Tapes

Article Kaolinite-Magnesite or Kaolinite–Talc-Based Ceramics. Part II: Microstructure and the Final Properties Related Sintered Tapes

Aghiles Hammas 1 , Gisèle Lecomte-Nana 2,* , Imane Daou 2, Nicolas Tessier-Doyen 2, Claire Peyratout 2 and Fatima Zibouche 1 1 Laboratoire de Traitement et Mise en Forme des Polymères Fibreux, Faculté des Sciences, Université M’Hamed Bougara Boumerdes, Boumerdes 35000, Algeria; [email protected] (A.H.); [email protected] (F.Z.) 2 Institut de Recherche sur les Céramiques (IRCER, UMRCNRS 7315), ENSIL-ENSCI, Université de Limoges, CEC, 12 rue Atlantis, CEDEX, 87068 Limoges, France; [email protected] (I.D.); [email protected] (N.T.-D.); [email protected] (C.P.) * Correspondence: [email protected]

Received: 28 October 2020; Accepted: 27 November 2020; Published: 1 December 2020

Abstract: In recent decades, talc and kaolinite have been widely used as raw materials for the ceramic industry. In this study, the final characteristics of kaolinitic clay mixed with 6 mass% of magnesite obtained in our previous work were compared with those obtained with mixtures of kaolin (kaolin BIP) and talc (as the source of magnesium oxide). However, different amounts of talc in the kaolin powder were studied, namely 10, 30, and 50 mass% of added talc (with respect to kaolin + talc). The tape casting process was used during this work in order to manufacture the green tapes in an aqueous system with 0.2 mass% of dispersant. Subsequently, the green tapes were heated to 1000 and 1100 ◦C with a dwelling time of 12 min. The green and sintering tapes were characterized using the following techniques: DTA/TG, X-ray diffraction, porosity, and flexural strength analyses. The results obtained from our previous work indicate that the specimen with 6 mass% of MgCO3 sintered at 1200 ◦C for 3 h exhibited the best performances, with high flexural strength and weak porosity value—117 MPa and 27%—respectively. As results from this study, the optimal mechanical and thermal properties of sintering tapes were obtained for the specimen with 10 mass% of added talc sintered at 1100 ◦C. Indeed, this specimen exhibited 50 MPa and 43% of stress to rupture and apparent porosity, respectively.

Keywords: kaolinite; talc; magnesite; tape casting; biaxial ﬂexural strength

1. Introduction Many traditional and advanced ceramic materials are made from clay (or clay mixed with other materials) [1–4]. Indeed, the mixture of two or more clays of different composition and texture to form a paste is performed to improve the workability and firing behavior in comparison to the behavior of each single clay. Additionally, the blending of sediments with different proportions of clay and non-clay particles and/or different clay minerals is commonly used to enhance the final properties of some ceramics [5,6]. However, kaolinite and talc are both phyllosilicate minerals which present considerable interest to produce several types of ceramic materials (traditional and advances ceramics) for having certain valuable properties [7–11]. The addition of talc has positive effects on ceramic—for example, it increases the mechanical and optical properties of the ceramic material and glazes [12–14]. Kaolinite has a 1:1 sheet structure with chemical formulae Al2Si2O5(OH)4 (39.8 mass% Al2O3, 46.38 mass% SiO2, and 13.9 mass% H2O). It is a layered clay mineral consisting of the pilling up of two

Minerals 2020, 10, 1080; doi:10.3390/min10121080 www.mdpi.com/journal/minerals Minerals 2020, 10, x FOR PEER REVIEW 2 of 12 Minerals 2020, 10, x FOR PEER REVIEW 2 of 12

MineralsKaolinite2020, 10, 1080has a 1:1 sheet structure with chemical formulae Al2Si2O5(OH)4 (39.8 mass% Al2 of2O 123, Kaolinite has a 1:1 sheet structure with chemical formulae Al2Si2O5(OH)4 (39.8 mass% Al2O3, 46.38 mass% SiO2, and 13.9 mass% H2O). It is a layered clay mineral consisting of the pilling up of 46.38 mass% SiO2, and 13.9 mass% H2O). It is a layered clay mineral consisting of the pilling up of two sheets—one tetrahedral sheet of silica (SiO4) joined to one octahedral sheet of alumina (AlO6) two sheets—one tetrahedral sheet of silica (SiO4) joined to one octahedral sheet of alumina (AlO6) sheets—one tetrahedral sheet of silica (SiO4) joined to one octahedral sheet of alumina (AlO6)[15–18]. [15–18].[15–18]. Indeed,Indeed, thethe morphologymorphology ofof kaolinitekaolinite mineralmineral isis plate-like.plate-like. ItsIts crystalcrystal systemsystem isis triclinictriclinic withwith aa Indeed,C1-based the space morphology group. According of kaolinite to mineral literature is plate-like.[19], the unit Its crystalcell structure system of is kaolinite triclinic with is illustrated a C1-based in C1-basedspace group. space According group. According to literature to literature [19], the unit [19], cell the structure unit cell ofstructure kaolinite of iskaolinite illustrated is illustrated in Figure1 in. FigureFigure 1.1.

FigureFigure 1. 1. StructureStructure ofof kaolinitekaolinite primitiveprimitive cellcell (white,(white, hydrogen;hydrogen; redred oxygen;oxygen; pink,pink, aluminum;aluminum; yellow,yellow, yellow, silicon)silicon) [19]. [[19].19].

On thethe other other hand, hand, talc is atalc magnesium is a magnesium hydrosilicate hydrosilicate with the theoretical with formulathe theoretical 3MgO 4SiO formulaH O, On the other hand, talc is a magnesium hydrosilicate with the theoretical· formula2· 2 corresponding3MgO·4SiO2·H2 toO, 63.35corresponding mass% SiO 2to, 31.9063.35 mass% mass% MgO, SiO2 and, 31.90 4.75 mass% mass% MgO, of chemically and 4.75 bound mass% water of 3MgO·4SiO2·H2O, corresponding to 63.35 mass% SiO2, 31.90 mass% MgO, and 4.75 mass% of (hydroxide) [20]. In contrast, other minerals can be associated with talc, such as, chlorites, mica, chemicallychemically boundbound waterwater (hydroxide)(hydroxide) [20].[20]. InIn contracontrast,st, otherother mineralsminerals cancan bebe associatedassociated withwith talc,talc, actinolite, feldspars, rutile, pyrrhotite, carbonates, pyrite, magnetite, and hematite [12]. The crystalline suchsuch as,as, chlorites,chlorites, mica,mica, actinolite,actinolite, feldspars,feldspars, rutile,rutile, pyrrhotite,pyrrhotite, carbonates,carbonates, pyrite,pyrite, magnetite,magnetite, andand structure of talc has a 2:1 or TOT layer type with one octahedral (O) magnesium layer sandwiched hematitehematite [12].[12]. TheThe crystallinecrystalline structurestructure ofof talctalc hahass aa 2:12:1 oror TOTTOT layerlayer typetype withwith oneone octahedraloctahedral (O)(O) between two tetrahedral silica layers (Figure2)[21]. magnesiummagnesium layerlayer sandwichedsandwiched betweenbetween twotwo tetetrahedraltrahedral silicasilica layerslayers (Figure(Figure 2)2) [21].[21].

Figure 2. Schematic representation of the talc structures. FigureFigure 2.2. SchematicSchematic representationrepresentation ofof thethe talctalc structures.structures. In our previous work [1,17], the effect of magnesite amount on the different properties of kaolinitic InIn ourour previousprevious workwork [1,17],[1,17], thethe effecteffect ofof mamagnesitegnesite amountamount onon thethe didifferentfferent propertiesproperties ofof clay ceramics was studied. However, in this study, we aimed to compare the final characteristics of the kaolinitickaolinitic clayclay ceramicsceramics waswas studied.studied. However,However, inin thisthis study,study, wewe aimedaimed toto comparecompare thethe finalfinal kaolinitic clay from Algeria (KT2) mixed with 6 mass% of magnesite, with those resulting from the use characteristicscharacteristics ofof thethe kaolinitickaolinitic clayclay fromfrom AlgeriaAlgeria (KT2)(KT2) mixedmixed withwith 66 mass%mass% ofof magnesite,magnesite, withwith thosethose of a commercial kaolin (kaolin BIP) mixed with talc (as the source of magnesium oxide). Several mass resultingresulting fromfrom thethe useuse ofof aa commercialcommercial kaolinkaolin (kaolin(kaolin BIP)BIP) mixedmixed withwith talctalc (as(as thethe sourcesource ofof contents of talc in the kaolin powder were investigated—10, 20, 30, 40, and 50 mass% of added talc. magnesiummagnesium oxide).oxide). SeveralSeveral massmass contentscontents ofof talctalc inin thethe kaolinkaolin powderpowder werewere investigated—10,investigated—10, 20,20, 30,30, In a former paper, we suggested a firing temperature of up to 1200 C, but the use of talc (melting of 40,40, andand 5050 mass%mass% ofof addedadded talc.talc. InIn aa formerformer paper,paper, wewe suggestedsuggested aa◦ firingfiring temperaturetemperature ofof upup toto 12001200 samples) did not allow performing heat treatments above 1100 C. The structural changes (XRD, SEM), °C,°C, butbut thethe useuse ofof talctalc (melting(melting ofof samples)samples) diddid notnot allowallow performingperforming◦ heatheat treatmentstreatments aboveabove 11001100 °C.°C. the mechanical, and thermal properties of the manufactured samples using tape casting were analyzed before and after sintering at 1000 and 1100 ◦C.

Minerals 2020, 10, 1080 3 of 12

2. Materials and Methods

2.1. Raw Materials In the present study, two kaolins were used: kaolinitic clay KT2 from Algeria (Tamazert, El Milia, Jijel, Algeria) and a commercial kaolin noted kaolin BIP the kaolin (Kaolin de Beauvoir, Imerys Limoges, France). The magnesite and the talc were provided by x and y, respectively. The chemical compositions of these raw materials are shown in Table1. In this study, the kaolin KT2 was mixed with magnesite (KxMy), and kaolin BIP was mixed with talc (KxTy); in addition, T100 is 100% of talc.

Table 1. Chemical composition (XRF analyses) and other characteristic physical properties of the

starting powders. LOI, loss on ignition at 1050 ◦C.

Raw SSA (BET) Chemical Composition (mass%) ( 0.1 Density Material ± 3 m2 g 1) (g cm− ) SiO2 Al2O3 MgO Fe2O3 TiO2 Li2O CaO Na2OK2O LOI · − · Kaolinitic 25.0 2.6 47.8 32.9 0.6 3.5 0.5 - 0.1 0.1 2.9 11.5 clay KT2 Kaolin 11.7 2.6 48.1 39.9 0.17 0.26 <0.25 0.27 <0.2 <0.2 1.9 10 BIP Talc 11.0 2.7 63 0.2 30.5 1.4 <0.1 - 0.25 0.03 0.01 4.7 Magnesite 11.7 2.9 3.2 0.1 42.7 0.4 - - 3.2 - - 50.4 K94M6 45.0 30.9 3.1 3.3 0.4 - 0.3 0.1 2.7 13.8 K90T10 49.6 35.9 3.2 0.4 <0.23 0.24 0.2 0.18 1.7 9.5 K80T20 51.1 31.9 6.2 0.5 <0.2 0.22 0.2 0.16 1.5 8.9 K70T30 52.6 28 9.3 0.6 <0.17 0.2 0.2 0.14 1.3 8.4 K60T40 54.1 24 12.3 0.7 <0.15 0.16 0.2 0.12 1.14 7.9 K50T50 55.6 20.1 15.3 0.8 0.18 0.13 0.2 0.1 0.95 7.4

Diﬀerent organics additives were added to the mixture slurries, such as dispersant, binder, and plasticizer, which are dolaﬂux, polyvinyl alcohol (PVA 2200, VWR Prolabo, Geldenaaksebaan, Belgium), and polyethylene glycol (PEG 300, VWR, Prolabo, Germany), respectively.

2.2. Dispersions Preparation Before dispersing of mixture slurries (kaolin/talc), 0.1 g mL 1 dispersant solutions were prepared · − via dissolving of Dolaﬂux in a 3 M NaOH solution. The aqueous suspensions with 62.5 mass% of solid loading were dispersed with 0.1, 0.2, and 0.3 mass% of dispersant (respect to the clay content). The kaolinitic clay KT2 with and without 6 mass% of magnesite was consider in this paper. Nevertheless, diﬀerent talc amounts were added to the kaolin suspension, namely, 10, 20, 30, 40, and 50 mass%. At the same time, all suspensions were mixed using a roll mixer for 24 h at room temperature in order to promote interactions between the clay particles and additives while removing residual air bubbles. The dispersion and formulation optimization have been already described in our former paper (part I.) [17]. After the dispersion step for the kaolin BIP and talc mixtures, the organic additives such as binder and plasticizer were added to these suspensions, polyvinyl alcohol (PVA 2200) and polyethylene glycol (PEG 300), respectively, which was ground in a planetary mill for 16 h at 100 rpm. The relative mass ratio between binder and plasticizer was equal to 1 in this study. However, a solution of 0.16 g mL 1 · − binder was prepared by incorporating PVA slowly in deionized water, which was previously heated at 80 ◦C. During the addition of PVA, the solution was continuously stirred in order to avoid the formation of a gel. Prior to the tape casting, the slurries were de-aired on rollers for 22 h and sieved at 100 µm to ensure the elimination of the non-dissolved binder or impurities. Minerals 2020, 10, 1080 4 of 12

2.3. Preparation of Green and Sintered Bodies Diﬀerent slurries were cast under 10 mm/s of speed with a gap of 1 mm (thickness) between the Minerals 2020, 10, x FOR PEER REVIEW 4 of 12 blade and the casting support. After drying, the samples were cut in pellet. Once the green tapes were driedwere atdried room at temperatureroom temperature (12 h), (12 specimens h), specimens of 30 mm of 30 diameter mm diameter (disks) were(disks) collected were collected using a circularusing a punch.circular These punch. samples These samples were sintered were sintered in an electric in an furnace electric atfurnace a heating at a rateheating of 5 ◦rateC/min of 5 up °C/min to 1000 up◦ C,to and1000 1100 °C, and◦C, dwelling1100 °C (, time dwelling of 12 mintime at of the 12 maximummin at the temperature.maximum temperature). A cooling rate A ofcooling 5 ◦C/ minrate wasof 5 used°C/min to decreasewas used toto roomdecrease temperature. to room temperature. Figure3 illustrated Figure 3 the illustrated shape and the color shape of and kaolin–talc-based color of kaolin– tapestalc-based after sinteringtapes after at sintering 1000 and at 1100 1000◦C. and 1100 °C.

FigureFigure 3.3. Samples after heatheat treatmentstreatments atat 10001000 andand 11001100 ◦°CC (soaking(soaking timetime ofof 1212 min).min). 2.4. Characterization Techniques 2.4. Characterization Techniques 2.4.1. Thermal Analysis 2.4.1. Thermal Analysis Differential thermal and gravimetric analyses (DTA/DTG) were measured using SETSYS 2400 DTA-TGADifferential equipment thermal from and SETARAM gravimetric (Caluire, analyses France). (DTA/DTG) The analysiswere measured was carried using out SETSYS under 2400 the DTA-TGA equipment from SETARAM (Caluire, France). The analysis was carried out under the temperature range 25–1300 ◦C at a heating rate of 5 ◦C/min using dried-air atmosphere (for all experiments,temperature Ptrange crucibles 25–1300 were °C used). at a Theheating thermal rate behavior of 5 °C/min helped using to determine dried-air the atmosphere most appropriate (for all thermalexperiments, cycle duringPt crucibles sintering were in orderused). to avoidThe th crackermal formation behavior within helped sintered to determine products. the most appropriate thermal cycle during sintering in order to avoid crack formation within sintered 2.4.2.products. Apparent Porosity During this study, the apparent porosity of green and sintered specimens (ISO 11272:2017) was 2.4.2. Apparent Porosity calculated using equation (Equation (1)). However, the bulk density (ρbulk) was determined from weight and dimensionalDuring this study, measurements; the apparent apparent porosity density of green (ρapparent and sintered) was estimated specimens using (ISO a11272:2017) Micromeritics was AccuPyccalculated II 1340using helium-pycnometer equation (Equation (Norccross, (1)). However, GA, USA).the bulk density (ρbulk) was determined from weight and dimensional measurements; apparent density (ρapparent) was estimated using a ! ρbulk Micromeritics AccuPyc II 1340 helium-pycnometerP = 1 (Norccross, GA, USA). (1) − ρapparent =(1− )  (1) 2.4.3. X-Ray Diffraction X-ray diffraction measurement of the samples was performed using a Bruker D5000 X-ray powder 2.4.3. X-Ray Diffraction diffractometer (Bruker, Karlsruhe, Germany) equipped with a crystal monochromator (Cu-Kα radiation equalX-ray to 1.5406 diffraction Å) at room measurement temperature. of Thethe measurementssamples was wereperformed achieved using at 40 a kVBruker and 30D5000 mA in X-ray a 2θ rangepowder from diffractometer 2◦ to 70◦, with (Bruker, a 2θ scanning Karlsruhe, rate ofGe 0.5rmany)◦/min andequipped a step ofwith 0.02 ◦a. crystal monochromator (Cu-Kα radiation equal to 1.5406 Å) at room temperature. The measurements were achieved at 40 2.4.4.kV and Scanning 30 mA in Electron a 2θ range Microscopy from 2° to 70°, with a 2θ scanning rate of 0.5°/min and a step of 0.02°. In order to observe the microstructure of the samples, scanning electron microscope (SEM) observations2.4.4. Scanning (Stereosacan Electron Microscopy Cambridge S260 SEM microscope, Oxford Instruments, Abingdon, UK), In order to observe the microstructure of the samples, scanning electron microscope (SEM) observations (Stereosacan Cambridge S260 SEM microscope, Oxford Instruments, Abingdon, UK), were performed on the bottom and upper faces as well as on the sides of tapes. To prepare samples, the upper and bottom faces of specimens were set on carbon adhesive patches to stick to the sample holder. Fractures of tapes were set vertically on carbon paste to observe slices. Silver lacquer was

Minerals 2020, 10, 1080 5 of 12 were performed on the bottom and upper faces as well as on the sides of tapes. To prepare samples, the upper and bottom faces of specimens were set on carbon adhesive patches to stick to the sample holder. Fractures of tapes were set vertically on carbon paste to observe slices. Silver lacquer was evenly applied on sample surfaces. In all cases, a 15 mm gold layer was deposited in order to improve the electrical conduction at the surface of samples (to improve the quality of the images).

2.4.5. Biaxial Flexural Strength The mechanical properties of each green and sintered tape with different talc amounts were assessed by the measurement of the biaxial flexural strength through universal machine Lloyd Easy Test EZ 20 (Largo, FL, USA) equipped with a piston-ring configuration. The crosshead velocity and the preload were fixed at 1 mm min 1, 0.2 N, respectively. A high sensitivity sensor (100 N) was chosen · − because load submitted to specimens has to represent at least 1% of the maximum range. Samples used for biaxial bending tests were shaped as 30 mm in diameter disks. For the piston on the three ball test, the modified Kirstein and Woolley’s [22] equation for the maximum biaxial flexure strength (in MPa) was:

2 ! 2 3PR(1 + ν) A 1 ν B A σR = (1 + 2 ln + − 1 ) (2) 4πe2 × B 1 + ν × − 2A2 × C2

In this formula, PR is the ultimate sustained load, A is the diameter of the ring equal to 28 mm of the support ring, B is diameter of the upper piston equal to 5 mm, C is the specimen diameter (mm), e is the specimen thickness (mm), and ν is Poisson’s ratio (0.25). For all tested specimens, diameter (C) was voluntarily chosen as 30 mm. For each set of specimens, 5 values of stress to rupture were measured, and the results presented are the mean values of each series of 5 measurements.

3. Results and Discussion

3.1. XRD, Thermal Behavior and Microstructure of Green and Sintered Specimens In order to the determine DTA and TG curves for the diﬀerent Mg-enriched kaolin samples, the ﬁnal properties of samples after sintering at 1000 and 1100 ◦C were characterized. The typical behavior of the prepared tapes are presented on Figure4. Indeed, four main transformations were observed in all our samples, namely:

The dehydration of the green tapes in the region between 70 and 120 C (endothermic peak); • ◦ The burning of organics used as binders and plasticizer in the range 150 to 400 C (consecutive • ◦ exothermic peaks associated with signiﬁcant mass loss); The dehydroxylation of kaolinite between 400 and 650 C, characterized by an endothermic peak • ◦ which appeared proportional to the kaolin content in the mixture (also the related mass loss); The dehydroxylation of talc in the range 850 to 1000 C, characterized by an endothermic peak • ◦ (proportional to the talc content as the mass loss) very close to the exothermic peak associated to the structural reorganization of the metakaolinite.

Considering these thermal analyses results, it appeared that above 1100 ◦C, an important melting occurred within our kaolin–talc tapes; thus, it was decided to conduct the heat treatments at 1000 and 1100 ◦C in line with the characterization of apparent porosity and ﬂexural strength. First, the crystalline phase of green and sintered tapes were analyzed using XRD measurements. The obtained XRD patterns are presented on Figures5 and6. Minerals 2020, 10, x FOR PEER REVIEW 6 of 12

Minerals 2020, 10, 1080 6 of 12 Minerals 2020, 10, x FOR PEER REVIEW 6 of 12

Figure 4. DTA (a) and TG (b) curves of three kaolin–talc tapes (under air up to 1200 °C), K50T50, K70T30, and K90T10.

FigureFigure 4. 4. DTADTA ((aa)) andand TGTG ((bb)) curvescurves ofof threethree kaolin–talckaolin–talc tapes (under (under air air up up to to 1200 1200 °C),◦C), K50T50, K50T50, First,K70T30,K70T30, the and and crystalline K90T10. K90T10. phase of green and sintered tapes were analyzed using XRD measurements. The obtained XRD patterns are presented on Figures 5 and 6. First, the crystalline phase of green and sintered tapes were analyzed using XRD measurements. The obtained 1100 XRD°C patterns∗ are(011) presented on Figures •5 andMullite 6. ∗ Quartz 1100 °C ∗(011) • Mullite

(100) ∗ Quartz (120) (110)

∗ (230) (220) (112) (111)

• (102) (201)

∗ (320) (110) •• ∗ •• ∗ ∗ • (100) (120) (110)

∗ (230) (220) (112) (111)

• (102) (201)

∗ (320)

(110) • • •• ∗ • ∗ ∗ 1000 °C (011)∗ ο Muscovite

1000 °C (011)∗ ο Muscovite (201) (111) (100) ∗ (120) (114) (110) (320) (102) (230) (112) (110) • (002) ∗ ο • • ο ∗ ∗ •• ∗ (201) (111) (100) ∗ (120) (114) (110) (320) (102) (230) (112) (110) • (002) ∗ ο • • ο ∗ ∗ •• ∗ Green ∗ (101)  Geothite ♦ Kaolinite (002) (-206) Magnesite Green (101)  ♦ Kaolinite (001) ♦ ∗ Geothite♦ ⊕ Rutile (002) ♦ (002) (-206) Magnesite ♦ ⊕ ο (001)(004) (021) ♦ (104) (110) Rutile (021)

(004) (211) ο (110) (006) (211) (002) ♦ ♦ (210) ♦ ⊕ ♦ ⊕ ο (004) (021) (104) (110) ⊕ ♦ (021)

(004) (211) ο (110) (006) (211) ♦ ♦ (210) ⊕  ♦ ⊕ ♦ ⊕ 5 1015202530354045505560 2θ (°) 5 1015202530354045505560 θ (°) FigureFigure 5. 5. XRDXRD patterns patterns of of K94M6 K94M6 (green (green tapes2 tapes and and tapes tapes sintered sintered at at 1000 1000 and and 1100 1100 °C).◦C). Figure 5. XRD patterns of K94M6 (green tapes and tapes sintered at 1000 and 1100 °C).

Minerals 2020, 10, 1080 7 of 12 Minerals 2020, 10, x FOR PEER REVIEW 7 of 12

(a)

 (220) K50T50 (220) • Mullite K50T50 • Mullite ∗ Quartz ∗ Quartz Z Cristobalite Z Cristobalite

Spinel (011) Spinel (210) (011)  (311) (111) (210) (111) ∗ (101) ∗ ♦ (101) (100) • (100) • ♦ Cordierite (110) • (220) • (130) (220) (130)

(110) Z (121) Z (121) (102) ∗ ∗ (320)

(320) • • • (102) • • • ∗ • • ∗ • •

K90T10 K90T10

(120) •

(011) (011) (210) ∗ ∗ •

(110) (111) (121) • (220)(220) • • (100) • ∗  (320)

(101) (130) • (220) • (102)  Z ∗

10 15 20 25 30 35 40 45 50 55 60 10 15 20 25 30 35 40 45 50 55 60 2θ (°) 2θ (°) (b) (c)

FigureFigure 6.6. XRDXRD patternspatterns of of K90T10 K90T10 and and K50T50 K50T50 tapes: tapes: (a )(a green) green samples, samples, (b) ( samplesb) samples sintered sintered at 1000 at 1000◦C, and°C, and (c) samples (c) samples sintered sintered at 1100 at 1100◦C. °C.

ForFor the the samples samples K94M6, K94M6, the mainthe main crystallized crystallized phases appearedphases appeared to be few quartzto be (JCPDSfew quartz 01-070-7344) (JCPDS and01-070-7344) mullite (JCPDS and mullite 00-015-0776). (JCPDS Cordierite 00-015-0776). phase Co (JCPDSrdierite 00-012-0303) phase (JCPDS seemed 00-012-0303) to be present seemed through to be thepresent very smallthrough diﬀ ractionthe very peak small located diffraction at 15◦ andpeak 29.7 located◦ (2θ). at The 15° reaction and 29.7° sequence (2θ). The could reaction be summarized sequence regardingcould be summarized the Equations regarding (3)–(7), with the Equations (3)(3)–(7), and (4)with for Equations the formation (3) and of primary(4) for the and formation secondary of mullite,primary and and Equations secondary (5) mullite, and (7) and for the Equations crystallization (5) and of (7) cordierite for the crystallization [23–25]. Equation of cordierite (5) characterized [23–25]. theEquation decarbonation (5) characterized of magnesite the decarbonation prior to its reaction of magnesite with alumina prior to to its form reaction a spinel with phase. alumina Note to form that thea spinel competition phase. Note between that the competition crystallization between of mullite the crystallization and the formation of mullite of cordierite and the tendedformation to be of proﬁtablecordierite totended mullite to untilbe profitable temperature to mullite close tountil 1200 temperature◦C. close to 1200 °C. → 2(Al2(AlSi2SiO2O)5) 2Al 2AlO2O33SiO·3SiO2 ++ SiO2 (amorphous)(amorphous) (3) (3) 2 2 5 → 2 3· 2 2 3(Al2Si2O5) → 2(3Al2O3·2SiO2) + 5 SiO2 (amorphous) (4) 3(Al2Si2O5) 2(3Al2O3 2SiO2) + 5 SiO2 (amorphous) (4) → MgCO3 ·→ MgO + CO2 (5) MgCO3 MgO + CO2 (5) 2(MgO·Al2O3) + 5(SiO→2) → (2MgO·2Al2O3·5SiO2) (6) 2(MgO Al2O3) + 5(SiO2) (2MgO 2Al2O3 5SiO2) (6) 5(3Al2O3·2SiO2)· + 15 MgO → 2(2MgO·2Al→ 2O· 3·SiO2) +· 11(MgO·Al2O3) (7) In the case of kaolin–talc mixtures, we detected the presence of spinel and cristobalite (due to the structural reorganization of metakaolinite) after sintering at 1000 °C. In addition, the sintering of

Minerals 2020, 10, 1080 8 of 12

5(3Al O 2SiO ) + 15 MgO 2(2MgO 2Al O SiO ) + 11(MgO Al O ) (7) 2 3· 2 → · 2 3· 2 · 2 3 MineralsIn 2020 the, 10 case, x FOR of kaolin–talcPEER REVIEW mixtures, we detected the presence of spinel and cristobalite (due8 of 12 to the structural reorganization of metakaolinite) after sintering at 1000 ◦C. In addition, the sintering of thesethese samples samples at 11001100 ◦°CC tendedtendedto to enhance enhance the the crystallization crystallization of of mullite mullite (JCPDS (JCPDS 00-015-0776) 00-015-0776) and and the theformation formation of cordieriteof cordierite (JCPDS (JCPDS 00-012-0303) 00-012-0303) as expected.as expected. The The observation observation of theirof their microstructure microstructure by bySEM SEM (Figure (Figure7) showed 7) showed a signiﬁcant a significant change change in the in shapethe shape of the of clay the platelets,clay platelets, which which appeared appeared with withrounded rounded corners corners due thedue formation the formation of transitory of transito liquidry liquid phases phases in agreement in agreement with thewith ternary the ternary phase phasediagram diagram SiO2-Al SiO2O2-Al3-MgO.2O3-MgO.

FigureFigure 7. 7. TypicalTypical SEM SEM images images of of the the ka kaolin–talcolin–talc tapes tapes K90T10 K90T10 before before and and after after sintering sintering at at 1100 1100 °C.◦C.

3.2. Mechanical Properties and Porosity 3.2. Mechanical Properties and Porosity AfterAfter sintering sintering of of the the various various specimens, specimens, the the ob obtainedtained biaxial biaxial flexural ﬂexural strength strength and and apparent apparent porosityporosity are are presented presented in in Figures Figures 88 andand9 9.. InIn thethe greengreen tapes,tapes, thethe additionaddition ofof talctalc toto kaolinkaolin tendedtended toto decreasedecrease the stress toto rupture,rupture, whilewhile the the apparent apparent porosity porosity was was close close to to 40% 40% in allin all cases cases (see (see Figure Figure9b). 9b). A similar trend was observed with the sample KT2M6 compared to KT2. This behavior seemed to be related to the higher plasticity of the kaolin used in this study compared to the talc material or to the magnesite. However, according to our previous study [1], it was indicated that the mechanical properties of K94M6 tapes were strongly affected by their microstructure, and as a result, biaxial flexural strength increased when porosity decreased [26]. Consequently, it appears in Figure 8 that

Minerals 2020, 10, 1080 9 of 12

A similar trend was observed with the sample KT2M6 compared to KT2. This behavior seemed to be related to the higher plasticity of the kaolin used in this study compared to the talc material or to the magnesite. However, according to our previous study [1], it was indicated that the mechanical propertiesMinerals 2020, of 10, K94M6 x FOR PEER tapes REVIEW were strongly affected by their microstructure, and as a result, biaxial9 of 12 flexural strength increased when porosity decreased [26]. Consequently, it appears in Figure8 that the stressthe stress to rupture to rupture values decreasedvalues decreased while sintering while atsintering 1000 or at 1100 1000◦C. Moreover,or 1100 °C. the Moreover, decarbonation the ofdecarbonation magnesite contributed of magnesite to increasingcontributed the to apparentincreasing porosity the apparent of K94M6 porosity samples of K94M6 after sintering samples up after to 1100sintering◦C, thus up to to 1100 observed °C, thus decrease to observed the biaxial decrease flexural the strengthbiaxial flexural down tostrength 10 MPa. down to 10 MPa.

Stress to rupture (MPa) 60 Porosity (%) 60

50 50

40 40

30 30 Prosity (%)

20 20 Stress to rupture (MPa) to rupture Stress

10 10

0 0 Green 1000 °C 1100 °C Temperature (°C) Figure 8.8.Mechanical Mechanical properties properties and and apparent apparent porosity porosity of KT2- of kaolinKT2- mixedkaolin with mixed 6 mass% with of6 magnesite.mass% of magnesite. Upon sintering at 1000 ◦C, the sample K70T30 exhibited the best stress to rupture (20 MPa) compared to the other samples sintered at the same temperature. The latter samples exhibited higher apparent porosity, justifying the observed behavior. Moreover, for the sample containing magnesite (K94M6), the level of apparent porosity was higher due to the decarbonation during sintering that significantly contributed to increasing the remaining porosity [17,27]. Sintering at 1000 ◦C tended to promote the decrease in viscosity of the amorphous phase (Arrhenius type dependence of viscosity with temperature), and therefore, promoted the initiation of viscous flow sintering of the studied samples. Consequently, an increase in the stress to rupture and a decrease in the apparent porosity were noted for most specimens. K50T50 samples despite very a low decrease in their apparent porosity exhibited the lowest stress to rupture values. The observation of the microstructure of K50T50 samples regarding the top surface (upper side) and bottom surface (in contact with the tape carrier) allowed observing a significant heterogeneous distribution of pores, grains and amorphous phase within these samples (Figure 10). It seemed likely that the sintering did not progress uniformly in the considered samples and could explain the lower mechanical resistance collected for K50T50. In the case of the K94M6, it a high apparent porosity (close to 50%) was observed in agreement with the low stress to rupture. Such trend seemed consistent with the importance of the source of magnesium while using mixtures with kaolin in order to promote the formation of cordierite and enhance the properties of use. Indeed, it was necessary to perform the sintering of kaolin-magnesite mixture at a higher temperature (1200 ◦C) in order to obtain equivalent properties of use obtained while using kaolin–talc mixtures. Moreover, the use of 10 to 30 mass % of talc in kaolin–talc mixtures allowed the enhancement of the stress to rupture and apparent porosity after sintering at only 1100 ◦C.

Figure 9. Comparative values of (a) the stress to rupture and (b) the apparent porosity of kaolin–talc tapes before and after sintering at 1000 and 1100 °C.

Minerals 2020, 10, x FOR PEER REVIEW 9 of 12

the stress to rupture values decreased while sintering at 1000 or 1100 °C. Moreover, the decarbonation of magnesite contributed to increasing the apparent porosity of K94M6 samples after sintering up to 1100 °C, thus to observed decrease the biaxial flexural strength down to 10 MPa.

Stress to rupture (MPa) 60 Porosity (%) 60

50 50

40 40

30 30 Prosity (%)

20 20 Stress to rupture (MPa) to rupture Stress

10 10

0 0 Green 1000 °C 1100 °C Temperature (°C)

MineralsFigure2020, 108., 1080Mechanical properties and apparent porosity of KT2- kaolin mixed with 6 mass% 10of of 12 magnesite.

Minerals 2020, 10, x FOR PEER REVIEW 10 of 12

Upon sintering at 1000 °C, the sample K70T30 exhibited the best stress to rupture (20 MPa) compared to the other samples sintered at the same temperature. The latter samples exhibited higher apparent porosity, justifying the observed behavior. Moreover, for the sample containing magnesite (K94M6), the level of apparent porosity was higher due to the decarbonation during sintering that significantly contributed to increasing the remaining porosity [17,27]. Sintering at 1000 °C tended to promote the decrease in viscosity of the amorphous phase (Arrhenius type dependence of viscosity with temperature), and therefore, promoted the initiation of viscous flow sintering of the studied samples. Consequently, an increase in the stress to rupture and a decrease in the apparent porosity were noted for most specimens. K50T50 samples despite very a low decrease in their apparent porosity exhibited the lowest stress to rupture values. The observation of the microstructure of K50T50 samples regarding the top surface (upper side) and bottom surface (in contact with the tape carrier) allowed observing a significant heterogeneous distribution of pores, grains and amorphous phase within these samples (Figure 10). It seemed likely that the sintering did not progress uniformly in the considered samples and could explain the lower mechanical resistance collected for K50T50. In the case of the K94M6, it a high apparent porosity (close to 50%) was observed in agreement with the low stress to rupture. Such trend seemed consistent with the importance of the source of magnesium while using mixtures with kaolin in order to promote the formation of cordierite and enhance the properties of use. Indeed, it was necessary to perform the sintering of kaolin-magnesite mixture at a higher temperature (1200 °C) in orderFigure Figureto obtain 9.9. ComparativeComparative equivalent valuesvalues properties ofof ((aa)) thetheof stressusestress obtained toto rupturerupture while andand ( (busingb)) thethe apparent apparentkaolin–talc porosity porosity mixtures. of of kaolin–talc kaolin–talc Moreover, the usetapes of 10before to 30 and mass after % sintering of talc atin 1000 kaolin–talc and 1100 mixtures °C. allowed the enhancement of the stress to tapes before and after sintering at 1000 and 1100 ◦C. rupture and apparent porosity after sintering at only 1100 °C.

FigureFigure 10. SEMSEM images images of of K50T50 K50T50 tapes tapes sintered sintered at at 1100 °C.◦C.

4. Conclusions The present study aimed to characterize the final properties of kaolin–talc mixtures (10 to 50 mass% of talc) shaped by tape casting comparatively to those of K94M6 (kaolin-magnesite mixture with 6 mass% of magnesite) obtained in a previous study in order to understand the effect of magnesium source regarding such cordierite-mullite ceramics. The starting mixtures were prepared as indicated in part I of this paper, namely optimization of slurries dispersion and stability. A 0.2 mass% of dolaflux B was used as the dispersant for kaolin–talc mixtures. The sintering of samples at 1000 °C led to porous and weak products (low stress to rupture values) compared to a sintering at 1100 °C. Moreover, compared to the K94M6 sample, the apparent porosity of kaolin–talc mixtures was lowered, while their stress to rupture was increased after sintering at 1100 °C. The XRD analyses of these samples indicated the presence of quartz and mullite within K94M6 tapes. In addition, mullite, quartz and cordierite (with well-defined peaks) appeared

Minerals 2020, 10, 1080 11 of 12

4. Conclusions The present study aimed to characterize the final properties of kaolin–talc mixtures (10 to 50 mass% of talc) shaped by tape casting comparatively to those of K94M6 (kaolin-magnesite mixture with 6 mass% of magnesite) obtained in a previous study in order to understand the effect of magnesium source regarding such cordierite-mullite ceramics. The starting mixtures were prepared as indicated in part I of this paper, namely optimization of slurries dispersion and stability. A 0.2 mass% of dolaflux B was used as the dispersant for kaolin–talc mixtures. The sintering of samples at 1000 ◦C led to porous and weak products (low stress to rupture values) compared to a sintering at 1100 ◦C. Moreover, compared to the K94M6 sample, the apparent porosity of kaolin–talc mixtures was lowered, while their stress to rupture was increased after sintering at 1100 ◦C. The XRD analyses of these samples indicated the presence of quartz and mullite within K94M6 tapes. In addition, mullite, quartz and cordierite (with well-defined peaks) appeared in kaolin–talc mixtures. The microstructure of K50T50 (kaolin with 50 mass% of talc) after firing at 1100 ◦C exhibited heterogeneous densification and distribution of pores and amorphous phase. The results highlighted the interesting properties obtained while using a phyllosilicate such as talc as a magnesium source to produce mullite-cordierite ceramics at alower temperature with enhanced densification and mechanical properties. Nevertheless, investigations should continue regarding the influence of the heating cycle (heating rate and soaking time), the purity of kaolin and talc materials and the related thermal properties.

Author Contributions: Conceptualization, F.Z. and G.L.-N.; methodology, F.Z. and G.L.-N.; software, A.H., I.D.; validation, G.L.-N. and F.Z.; formal analysis, A.H. and G.L.-N.; investigation, A.H., I.D., N.T.-D.; resources, C.P., G.L.-N. and F.Z.; data curation, A.H., I.D., and G.L.-N.; writing—original draft preparation, A.H. and G.L.-N.; writing—review and editing, A.H. and G.L.-N.; visualization, G.L.-N.; supervision, F.Z. and G.L.-N.; project administration, F.Z., G.L.-N., N.T.-D. and C.P.; funding acquisition, F.Z. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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