Development and Properties of New Mullite Based Refractory Grog

ma e ials A icle De elopmen and P ope ies o New Mulli e Based Re ac o y G og Da id Zemánek 1,2 , Ka el Lang 2, Lukáš T dík2, Dalibo Všianský3, Lenka Ne ˇ i o á1,2, Pe Š u sa 2,3, Pa el Ko ᡠ3, Lucie Ke šne o á3and Ka el D oˇ ák1,*   Ci a ion: Zemánek, D.; Lang, K.; T dík, L.; Všianský, D.; Ne ˇ i o á, L.; Š u sa, P.; Ko ᡠ, P.; Ke šne o á, L.; D oˇ ák, K. De elopmen and P ope ies o New Mulli e Based Re ac o y G og. Ma e ials 2021,14, 779. h ps://doi.o g/10.3390/ma14040779 Academic Edi o : Mangiala di Te esa Recei ed: 21 Decembe 2020 Accep ed: 26 Janua y 2021 Published: 7 Feb ua y 2021 Publishe ’s No e: MDPI s ays neu al wi h ega d o ju isdic ional claims in published maps and ins i u ional a il- ia ions. Copy igh : © 2021 by he au ho s. Licensee MDPI, Basel, Swi ze land. This a icle is an open access a icle dis ibu ed unde he e ms and condi ions o he C ea i e Commons A ibu ion (CC BY) license (h ps:// c ea i ecommons.o g/licenses/by/ 4.0/). 1Facul y o Ci il Enginee ing, B no Uni e si y o Technology, Ve eˇ í331/95, 602 00 B no, Czech Republic; [email p o ec ed].cz (D.Z.); [email p o ec ed].cz (L.N.) 2P-D Re ac o ies CZ JSC, Nád ažní218, 679 63 VelkéOpa o ice, Czech Republic; Ka el.Lang@pd-g oup.com (K.L.); Lukas.T [email p o ec ed] (L.T.); pe [email p o ec ed] (P.Š.) 3Depa men o Geological Sciences, Facul y o Science, Masa yk Uni e si y, Ko lᡠská267/2, 602 00 B no, Czech Republic; [email p o ec ed] (D.V.); Pa el.Ko a @pd-g oup.com (P.K.); Lucie.Ke sne o a@pd-g oup.com (L.K.) *Co espondence: [email p o ec ed].cz; Tel.: +420-54114-7511 (ex . 8067) Abs ac : The p esen ed s udy is ocused on op imiza ion and cha ac e iza ion o a high-alumina e ac o y agg ega e based on na u al aw ma e ials—kaolins, clays one, and mulli e dus by-p oduc (used o inc ease he alumina and mulli e con en s, espec i ely). In o al, ou indi idual o mulas wi h he Al 2 O 3 con en s be ween 45 and 50 w .% we e designed; he samples we e subsequen ly i ed, bo h in a labo a o y o en and an indus ial unnel u nace. The e ec s o epea ed i ing we e examined du ing indus ial pilo es s. Mine al and chemical composi ions and mic os uc u es, o bo h he aw ma e ials and designed agg ega es, we e ho oughly in es iga ed by he means o X- ay luo escence spec oscopy, powde X- ay di ac ion, and op ical and scanning elec on mic oscopies. Po osi y, mine al composi ion, and mulli e c ys al-size de elopmen du ing he i ing p ocess we e also s udied. Based on he acqui ed esul s, he o mula wi h he pe spec i e o be used as a new mulli e g og, ea u ing simila p ope ies as he a ailable comme cial p oduc s, howe e , wi h educed p oduc ion expenses, was selec ed. The quali y o g og de e mines o a la ge ex en he p ope ies o he inal p oduc . Hence, op imiza ion o agg ega es o speci ic e ac o ies is o a g ea impo ance. The p oduc ion o enginee ed agg ega es p o ides he oppo uni y o u ilize indus ial by-p oduc s. Keywo ds: mulli e; e ac o y; high-alumina g og; kaolin; clays one 1. In oduc ion Al hough nume ous scien i ic pape s dealing wi h ce amics and e ac o ies ha e been published, ew au ho s ha e ocused on e ac o y g og so a . Mo eo e , in e ac o ies enginee ing, he esea ch has mos ly been ocused on he ma ix; he a en ion has shi ed o agg ega es only ecen ly [ 1 ]. Agg ega es a e an indispensable pa o e ac o ies, hey enhance he o e all p ope ies, such as olume s abili y du ing i ing and high empe a u e beha iou , and ep esen 50–100% o he aw mix o mos inal p oduc s. The p oduc ion o e ac o ies is, among o he ac o s, con olled by he a ailabili y o g og [ 2 , 3 ]. Enginee ing o agg ega es (i.e., he en i e p oduc ion sys em, including design, cus omiza ion, p ope ies es ing, e alua ion o pe o mance o e ac o y p oduc s, and hei applica ion o desi ed pu poses) is expec ed o b ing new b eak h oughs in e ac o ies echnologies. Design and cus omiza ion o agg ega es comp ise designing o shape, su ace p ope ies, chemical and phase composi ion, as well as op imizing mic os uc u e [ 1 ]. In he pas , e ac o y ma e ials a end-o -li e we e dumped in land ills and new b icks we e made o new aw ma e ials. Such was ing had a huge impac on he en i onmen , especially because o he con inuous mining o aw ma e ials [ 4 , 5 ]. This a i ude has changed no only o ecological, bu also o economic easons [6,7]. Ma e ials 2021,14, 779. h ps://doi.o g/10.3390/ma14040779 h ps://www.mdpi.com/jou nal/ma e ials Ma e ials 2021,14, 779 2 o 15 The gene al si ua ion in he e ac o y indus y has changed signi ican ly du ing he las decades (especially in ecen yea s), which also a ec ed he p oduc ion o i ed clay. Along wi h d op in demand o s anda d i ed clay caused by changes in he s eel manu ac- u ing echnology, he p oduc ion o i ed clays one has dec eased. The eno mous changes wo ldwide we e p ima ily gi en by he es ablishmen o new ecological egula ions in China, exhaus ion o clays one deposi s in Eu ope (e.g., F ance, Poland, Czech Republic), and lack o o he aw ma e ials used o g og p oduc ion, such as magnesi e, bauxi e, and all ypes o aluminium oxides. Limi ed clays one esou ces led o he heap u iliza ion and b ique ing echnology applica ion, bu he esul s o hese app oaches we e unsa is ac o y ega ding shaping and i ing. These we e he impulses beyond he de elopmen o high alumina e ac o y g og [8–10]. Mode n empe a u e-s able componen s ho -p oduced om me allic ma e ials based on elemen s such as i anium [ 11 ], a e-ea h me als [ 12 ], o ungs en [ 13 ] in oduce he necessi y o de elop du able empe a u e- esis an appliances o hei p oduc ion and p ocessing. To esis high empe a u es and main ain he shape and unc ionali y, he in e io o i ually any u nace is made o e ac o y ma e ials, which ha e become sophis- ica ed p oduc s wi h ca e ully designed composi ions. The ce amic indus y cons an ly de elops me hods o lowe he cos and inc ease he quali y and inal p ope ies o he ma e ials [14,15]. Classi ica ion o e ac o ies acco ding o he chemical composi ion is based on he anion: ca ion a io and consis s o h ee ypes o ma e ials: acidic, basic, and neu al. The anion: ca ion a io highe han 1.5:1.0 indica es acidic e ac o y ma e ials, o example SiO 2 con aining one ca ion and wo anions. The neu al a io is equal o 1.5:1.0, o example Al 2 O 3 , and 1.0:1.0 indica es basic e ac o ies, o example MgO. This classi ica ion is widely used in he me allu gic p oduc ion because he e ac o ies ha e o be compa ible wi h alloys p ocessing. Ano he classi ica ion can be, o example, by me hods o ins alla ion— shaped and unshaped, by me hods o manu ac u e— used and sin e ed, and by po osi y con en —po ous and dense [14,16]. Each ype o e ac o y ma e ial, including g og, is applied in di e en indus ial sec- o s gi en by hei cha ac e is ic p ope ies. Aluminosilica e e ac o ies, mulli e ma e ials, and i eclay belong among acidic up o neu al e ac o ies. They a e usually applied in elec ic u naces, coke o ens, annealing u naces, e c. They con ain silica and be ween 40 o 90 w .% o alumina. The dominan aw ma e ials o hese ypes o e ac o ies a e clays [4,14,17]. The inal in e media e phase o he sin e ing p ocess is mulli e. The ea lies syn hesis o mulli e was done by i ing kaolini e. Du ing hea ing o kaolini e, X- ay amo phous me akaolini e is p oduced a i s . Subsequen ly, so called spinel phase and γ -alumina a e o med, and inally, abo e 1000 ◦ C, mulli e and amo phous silica a e p oduced. To a oid he o ma ion o amo phous silica, addi ional alumina has o be added o he ini ial aw ma e ial [ 18 ]. The chemical composi ion o mulli e in e ac o ies is usually close o 3Al 2 O 3· 2SiO 2 , which co esponds o he Al 2 O 3 con en o nea ly 72 w .%. The idealized o mula o he mos commonly p oduced mulli e is 3Al 2 O 3· 2SiO 2 , howe e , he a io o alumina and silica may a y om 2:1 o 3:2. Among he speci ic p ope ies o mulli e a e high he mal esis ance up o 1700 ◦ C in ai a mosphe e, low he mal expansion coe icien 6 × 10 −6 K −1 and conduc ibili y 4–6 W/(m K) a 100–1400 ◦ C, high c eep and co osion esis ance, and, las bu no leas , a ou able physical and mechanical beha iou [ 18 , 19 ]. The i ing p ocess is ealized in sha , unnel, o o a ing kilns a he maximum empe a u e usually no exceeding 1350 ◦ C. In p inciple, a i icial mulli e can be ob ained in wo ways. The majo i y o mulli e in indus ial condi ions is p oduced ia solid-s a e syn hesis by i ing clay, Al 2 SiO 5 mine als (andalusi e, sillimani e, and kyani e), bauxi e, and o he solid aw ma e ials. Ano he way is he sol-gel syn hesis. Solu ion-sol-gel-de i ed mulli es a e cha ac e ized as chemical-mulli es. They a e syn hesized by chemical eac ion, py olysis, and mulli iza ion [ 18 , 20 ]. Each o hese ways o mulli e o ma ion p o ides mulli e wi h di e en p ope ies [16]. Ma e ials 2021,14, 779 3 o 15 The e a e wo ways o indus ial p oduc ion o ma e ials wi h high mulli e con en (abo e ~50% o mulli e): sin e ing (>1500 ◦ C), and using (>1830 ◦ C, in some cases e en >2000 ◦ C). The e m sin e -mulli e e e s o mulli es syn hesized by hea ing o empe a- u es below he mel ing poin ( o c ys allize and densi y he mulli e). Fused-mulli es a e p epa ed by hea ing alumina and silica mix u es o empe a u es abo e he mel ing poin , ollowed by cooling, du ing which mulli es c ys allize. Mulli es p oduced by using ha e highe Al 2 O 3 :SiO 2 a ios han hose p oduced by sin e ing om iden ical ini ial ma e ials. The amoun o alumina in sin e -mulli es usually does no exceed 77 w .% [ 3 , 21 ]. Mulli e con en s in i ed clays o clays ones a e usually below 65 w .% o he whole ma e ial, including he amo phous phase. Ex eme mulli e con en s occu in so called used mul- li es, which may be used as componen s o high alumina g ogs o enhance hei posi i e p ope ies. F om he chemical poin o iew, posi i e pe o mance o mulli e based g og is de e mined by high con en o alumina and low con en s o CaO, MgO, Fe 2 O 3 , and alkalis. The p esence o de imen al elemen s, mainly alkalis, may signi ican ly dec ease he o e all hea a igue esis ance [18,22]. The pape p esen s he enginee ing, cha ac e iza ion, and manu ac u ing possibili ies o new mulli e based agg ega es wi h Al 2 O 3 con en be ween 45 and 50 w .%. The main objec i e o he esea ch is o p esen new agg ega es ea u ing p ope ies simila o hose o comme cially a ailable agg ega es, howe e , his solu ion is much cheape since we use clays one and kaolins mixed wi h used mulli e by-p oduc . 2. Ma e ials and Me hods 2.1. Raw Ma e ials and Mixes Raw mixes wi h he con en s o Al 2 O 3 be ween 45 and 50 w .% we e designed and p e- pa ed in indus ial condi ions (P-D Re ac o ies CZ JSC, VelkéOpa o ice, Czech Republic). To each such high alumina con en s, suppo ing aw ma e ials we e necessa y o be added o he na u al clays one and loa ed kaolins (Tables 1and 2). Fo economic easons, MOTIM Whi e Fused Mulli e p oduced by Elec oco undum L d., Mosonmagya ó á , Hunga y, was chosen (0–1 mm ac ion; see Table 3 o he pa icle size dis ibu ion). MOTIM mulli e is a by-p oduc om mechanical p ocessing o used mulli e blocks designed o glass indus y applica ions. The e o e, i can be conside ed as a seconda y aw ma e ial. The a io o 85% clays one–kaolin and 15% MOTIM mulli e was used o each he Al 2 O 3 con en o a leas 45%. Fo he chemical and phase composi ions o he aw ma e ial see Table 4in he Resul s sec ion. Table 1. Lis o aw ma e ials. Raw Ma e ial Clays one Kaolin Mulli e G og W Supe KN-83 GP3 DS1 MOTIM Si e/ Kaznˇejo si e, Czech Republic/ Kosya in, Uk aine/ Kosya in, Uk aine/ Chlumˇcany si e, Czech Republic/ Elec oco undum L d. P oduce Ke amos company SOKA company SOKA company Sedleckýkaolin, a.s. company Mosonmagya ó á , Hunga y Ma e ials 2021,14, 779 4 o 15 Table 2. Calcula ed chemical composi ions o aw mixes (85% clay, 15% MOTIM mulli e) based on WDXRF analysis in w .%, ounded o wo decimal places. Fo mula Designa ion Raw Ma e ials Al2O3SiO2Fe2O3TiO2CaO MgO K2O Na2O WW supe clays one + MOTIM 48.62 47.51 0.88 1.37 0.37 0.15 0.49 0.25 K KN83 kaolin + MOTIM 49.74 47.82 0.40 1.23 0.30 0.07 0.31 0.14 G GP3 kaolin + MOTIM 48.53 48.86 0.76 0.67 0.26 0.20 2.89 0.10 D DS1 kaolin + MOTIM 45.20 50.19 0.54 0.56 0.32 0.20 0.71 0.38 Table 3. Pa icle size dis ibu ion o MOTIM mulli e, 0–1 ac ion. Sie e Opening [mm] >1 0.5–1.0 0.2–0.5 0.2–0.09 <0.09 w .% 0 44 34.6 15 6.4 Table 4. Chemical and phase composi ion o aw ma e ials in w .%, ounded o wo and one decimal places espec i ely. Raw Ma e ial/Composi ion Clays one Kaolin Mulli e G og W Supe KN-83 GP3 DS1 MOTIM Loss o Igni ion (1000 ◦C) −14.24 −13.47 −13.15 −12.46 Chemical Composi ion Al2O342.15 43.53 42.04 37.93 76.38 SiO253.19 53.57 54.86 56.49 23.40 Al2O3:SiO2 a io 0.79 0.81 0.77 0.67 3.26 Fe2O31.03 0.43 0.87 0.61 0.01 TiO21.68 1.50 0.81 0.68 0.01 CaO 0.38 0.29 0.24 0.12 0.01 MgO 0.17 0.07 0.23 0.24 – K2O 0.85 0.36 0.57 3.53 0.08 Na2O 0.45 0.15 0.28 0.1 0.1 Phase Composi ion and Gene al Fo mula Kaolini e (Al2Si2O5(OH)4) 80.2 93.3 89.7 69.9 – Illi e (K0.65Al2(AlSi3O10)(OH)2) 11.7 4.1 4.7 6.2 – Smec i e (no gene al o mula a ailable) 2.8 – – Ana ase (TiO2) 2.5 0.8 0.7 0.6 – Qua z (SiO2) 2.8 1.8 4.3 10.8 0.1 Feldspa s (K x Na y Ca 1−(x+y) Al 2−(x+y) Si 2+(x+y) O 8 ) - - 0.6 12.5 – Mulli e (Al(4 + 2x)Si(2 −2x)O(10 −x) whe e x = 0.17 o 0.59) – – – – 80.5 Co undum (Al2O3) – – – – 0.2 C is obali e (SiO2) – – – – Amo phous phase NA NA NA NA 19.2 Fo he pilo indus ial expe imen s, p essed b ique es om he aw mixes wi h he app oxima e dimensions o 7 cm × 5 cm × 3 cm we e p epa ed. The b ique ing echnology enables p epa a ion o bodies o op imized sizes and posi i ely impac s educ ion in was ing o aw ma e ials ( he ese es o kaolins and clays ones a e limi ed). Fo labo a o y expe imen s, he mixes we e addi ionally homogenized wi h a wheel mixe o 10 min. Ma e ials 2021,14, 779 5 o 15 Be o e p essing wi h he p essu e o 10 MPa in o cylinde bodies wi h 5 cm in diame e , 10% o wa e was added o he d y mixes. Th ee i ing expe imen s we e used: 1. Fi ing in labo a o y o en—hea ing 10 ◦ C/min o he maximum empe a u es o 1250, 1450, and 1550 ◦C, ollowed by 5 h soaking a he maximum empe a u e 2. One cycle o i ing in indus ial unnel kiln: 1480 ◦C wi h 5 h soaking 3. Two cycles o i ing in an indus ial kiln wi h he egime iden ical as lis ed abo e (ad 2) 2.2. Analy ical Me hods Chemical composi ion analysis o he aw ma e ials was pe o med by wa eleng h- dispe si e X- ay spec oscopy (WDXRF) using SPECTROSCAN MAKC-GV ins umen (Spec on Company, S . Pe e sbu g, Russia) equipped wi h QUANTITATIVE ANALYSIS 4.0 so wa e. Samples we e analysed in o ms o used beads. Powde X- ay di ac ion analysis o he aw ma e ials and enginee ed agg ega es was conduc ed on Panaly ical Empy ean di ac ome e (Mal e n Panaly ical Company, Almelo, The Ne he lands) equipped wi h Cu-anode, 1-D posi ion sensi i e de ec o a con en ion B agg–B en ano e lec ion geome y. The se ing we e s ep size–0.013 2 θ , ime pe s ep— 188 s, and angula ange 5–80 2 θ . Con en s o he amo phous phase we e quan i ied using he addi ion o 10 w .% luo i e (CaF 2 ) as an in e nal s anda d. Quan i a i e phase analysis was done ia he Rie eld me hod using Panaly ical High Sco e 3.0 plus so wa e. Pola izing ligh mic oscopy (PLM) examina ion was pe o med on 30 µ m hick pol- ished hin sec ions using an Olympus BX 51 mic oscope (Olympus Company, Tokyo, Japan). Scanning elec on mic oscopy wi h X- ay mic oanalysis (SEM/EDS) was conduc ed on gold-coa ed mechanically b oken specimens ( o mo phological analyses) and on polished ca bon-coa ed hin sec ions ( o chemical mic oanalyses) using TESCAN MIRA 3 (Tescan Company, B no, Czech Republic) ins umen wi h he accele a ing ol age o 30 kV. De e mina ions o bulk densi y, appa en densi y, appa en po osi y, and wa e ab- so p ion we e done by he well-known hyd os a ic g a i y me hod (e.g., ˇ CSN EN 993-1 s anda d [ 23 ]). Appa en po osi y and po e size dis ibu ion was de e mined by me cu y po osime y using The mo Finnigan POROTEC Pascal 140–240 ins umen (The moFishe Scien i ic, Wal ham, MA, USA). 3. Resul s and Discussion 3.1. Cha ac e iza ion o Raw Ma e ials The alumina con en (be ween 42–44 w .%) and alumina:silica a io (0.77–0.81) was simila o he clays one (W-supe ) and wo o he kaolin samples (KN-83 and GP3). In DS1 kaolin, bo h he pa ame e s we e signi ican ly lowe (37.93 and 0.67%). The Al 2 O 3 :SiO 2 a io in MOTIM mulli e g og (3.26) was much close o he mulli e alue o 2:1 (3.39), han 3:2 (2.54), which co esponds o he p oduc ion by using. The Fe 2 O 3 con en was be ween 1.03 (clays one) and 0.43 (KN-83), while in he MOTIM mulli e i eached only 0.01%. The CaO con en s we e below 0.4% in all he clay and kaolin samples. The con en s o alkalis in all he aw ma e ials we e ela i ely low wi h he excep ion o DS1 kaolin, whe e he con en o K2O eached 3.53%. The con en s o mine als p esen in he aw ma e ials e lec ed hei chemical com- posi ions: he highes kaolini e con en (o e 93%) was iden i ied in he KN-83 sample, and he lowes one (below 70%) in he DS1 kaolin. The mos di e se associa ion o clay mine als was ound he W-supe clays one, whe e, besides kaolini e and illi e, smec i e was also p esen . The highes con en s o “impu i ies”, qua z and eldspa s, we e ound in he DS1 kaolin. The high pe cen age o eldspa s co esponds o he high con en s o po assium in his sample. Chemical and phase composi ions o he used aw ma e ials a e gi en in Table 4. Mo phology o he aw ma e ials was obse ed using SEM. The size o kaolini e pseudohexagonal pla ele s no mal o (0 0 1) was appa en ly he la ges (up o 5 µ m) in Ma e ials 2021,14, 779 6 o 15 he DS1 kaolin, whe e also he c ys al size was he mos uni o m. The smalles kaolini e c ys als (<1 µ m) we e obse ed in W-supe clays one, and GP3 kaolin samples. I can be assumed ha kaolini e c ys al size, i s dis ibu ion and, hence, speci ic su ace may a ec eac i i y. Fo compa ison o mic os uc u es see Appendix A. The SEM obse a ion o MOTIM mulli e (Figu e A5) co esponds o he sie e analysis esul s. The pa icle size o he MOTIM suppo ing aw ma e ial was below 1 mm wi h he mean alue o 0.28 µm (Table 3). 3.2. Tailo ed Agg ega es 3.2.1. Phase Composi ion The bulk phase composi ion, including amo phous (“glassy“) phase, o all he ag- g ega es p epa ed in bo h he labo a o y and indus ial condi ions was analysed by XRD (Table 5, Appendix B). The c is obali e and amo phous phase con en s o well homogenized samples i ed in a labo a o y o en di e ed signi ican ly (Figu e 1). In agg ega e samples K and D, c is obali e did no appea a any o he i ing empe a u es (1250, 1450, and 1550 ◦ C). The con en o he amo phous phase was ela i ely high al eady a 1250 ◦ C and inc eased negligibly wi h inc easing empe a u e. Fo sample D, such a se e e mel ing al eady a lowe empe a u es can be explained by he abundance o po assium in he o mula (2.89%, Table 2). Howe e , o sample K, he chemical composi ion did no p o ide any clea explana ion o he cou se o he mel ing p ocess. In samples W and G, he con en s o he amo phous phase and c is obali e co ela ed nega i ely wi h each o he . C is obali e mel ed below 1450 ◦ C in sample W, and abo e his empe a u e in sample G. Residual qua z in he amoun s abo e 1% emained in samples W and G a 1450 ◦ C. On he o he hand, qua z did no ans o m o c is obali e bu mel ed di ec ly in samples K and D. Ma e ials 2021, 14, x FOR PEER REVIEW 7 o 15 Figu e 1. C is obali e and amo phous phase con en s in labo a o y- i ed samples; no c is obali e was de ec ed in K-L and D-L samples. Figu e 2. Con en s o mulli e in syn he ised samples and e e ence ma e ial; L =l abo a o y i ing, T,TT = one and wo cycles o i ing in indus ial unnel kiln. Table 5. Phase composi ion o p epa ed g og samples (L = labo a o y o en, T = indus ial unnel kiln one i ing cycle, TT = indus- ial unnel kiln wo i ing cycles) and e e ence ma e ial – Re e ence ma e ial. The o mulas wi h simila composi ion as he e e - ence ma e ial a e highligh ed by ed colou . Sample Fi ing Mulli e Co undum C is obali eQua z Amo phous Phase Designa ion Tempe a u e[°C] WL 1250 74.5 0.0 16.1 4.0 5.5 1450 75.6 0.0 7.4 3.8 13.1 1550 71.2 0.0 0.5 0 29.7 W−T 1550 58.5 0.5 20.3 0.9 19.8 W−TT 1550 54.9 0.7 14.6 0.4 29.4 G−L 1250 69.5 0.0 19.6 3.0 7.9 1450 70.7 0.0 24.8 1.0 3.5 1550 71.7 0.0 0.6 0.0 27.7 G−T 1550 56.9 0.1 21.8 0.1 21.1 G−TT 1550 55.9 0.1 23.2 0.0 20.8 K−L 1250 59.1 0.0 0.0 3.9 36.8 1450 57.2 0.0 0.0 0.0 42.8 1550 54.9 0.0 0.0 0.0 45.1 Figu e 1. C is obali e and amo phous phase con en s in labo a o y- i ed samples; no c is obali e was de ec ed in K-L and D-L samples. Ma e ials 2021,14, 779 7 o 15 Table 5. Phase composi ion o p epa ed g og samples (L = labo a o y o en, T = indus ial unnel kiln one i ing cycle, TT = indus ial unnel kiln wo i ing cycles) and e e ence ma e ial–Re e ence ma e ial. The o mulas wi h simila composi ion as he e e ence ma e ial a e highligh ed by ed colou . Sample Fi ing Mulli e Co undum C is obali e Qua z Amo phous Phase Designa ion Tempe a u e [◦C] WL 1250 74.5 0.0 16.1 4.0 5.5 1450 75.6 0.0 7.4 3.8 13.1 1550 71.2 0.0 0.5 0 29.7 W-T 1550 58.5 0.5 20.3 0.9 19.8 W-TT 1550 54.9 0.7 14.6 0.4 29.4 G-L 1250 69.5 0.0 19.6 3.0 7.9 1450 70.7 0.0 24.8 1.0 3.5 1550 71.7 0.0 0.6 0.0 27.7 G-T 1550 56.9 0.1 21.8 0.1 21.1 G-TT 1550 55.9 0.1 23.2 0.0 20.8 K-L 1250 59.1 0.0 0.0 3.9 36.8 1450 57.2 0.0 0.0 0.0 42.8 1550 54.9 0.0 0.0 0.0 45.1 K-T 1550 67.2 0.0 31.1 0.1 1.6 K-TT 1550 66.5 0.0 24.4 0.0 9.1 D-L 1250 56.6 0.0 0.0 4.0 39.4 1450 55.3 0.0 0.0 0.7 44.2 1550 54.9 0.0 0.0 0.6 44.5 D-T 1550 51.2 0.0 1.6 0.0 47.2 D-TT 1550 49.5 0.0 1.8 0.1 48.6 Re e ence Ma e ial 70.1 2.4 21.1 0.3 6.0 Mulli e con en s did no signi ican ly change be ween 1250 and 1550 ◦ C in he labo a o y-p epa ed samples (Figu e 2). In he samples i ed a 1250 ◦ C, bo h he s uc u es o mulli e—close o 3:2 and 2:1—we e p esen . A highe empe a u es, he 3:2 s uc u e became dominan . Based on he mulli e peak p o iles, i is appa en ha he c ys allini y o mulli e inc eased be ween 1250 and 1450 ◦ C. The mulli e peaks’ alues o ull wid h a hal maximum (FWHM) we e signi ican ly highe in he samples i ed a 1250 ◦ C compa ed o hose exposed o highe empe a u es. A e i ing a an indus ial unnel kiln, mulli e con en was simila as in analogous labo a o y-p epa ed sample o sample G. In samples W and D, he acqui ed mulli e con en s we e lowe , and in sample K i was highe (Figu e 2). Mulli e con en s in he indus ial samples can be a ec ed by inhomogenei y o he aw mix. The highe mulli e con en in sample K co ela ed wi h he low amo phous phase con en (below 10%), in compa ison o he same o mula i ed in labo a o y condi ions (o e 36–44%). I may be assumed ha seconda y mulli e o med by c ys alliza ion o he mel du ing slow cooling in he unnel kiln. In all he o mulas, mulli e con en sligh ly dec eased a e he second cycle o indus ial i ing, which may be a ibu ed o co osion by mel . Fi eclay e ac o ies a e usually no exposed o such high empe a u es (1550 ◦ C) du ing hei li ecycle. The common maximum exposi ion empe a u e does no exceed 1400 ◦ C. The e o e, he isk o he occu ence o co osion by mel du ing hei wo king li e ime is limi ed. Ma e ials 2021,14, 779 8 o 15 Ma e ials 2021, 14, x FOR PEER REVIEW 7 o 15 Figu e 1. C is obali e and amo phous phase con en s in labo a o y- i ed samples; no c is obali e was de ec ed in K-L and D-L samples. Figu e 2. Con en s o mulli e in syn he ised samples and e e ence ma e ial; L =l abo a o y i ing, T,TT = one and wo cycles o i ing in indus ial unnel kiln. Table 5. Phase composi ion o p epa ed g og samples (L = labo a o y o en, T = indus ial unnel kiln one i ing cycle, TT = indus- ial unnel kiln wo i ing cycles) and e e ence ma e ial – Re e ence ma e ial. The o mulas wi h simila composi ion as he e e - ence ma e ial a e highligh ed by ed colou . Sample Fi ing Mulli e Co undum C is obali eQua z Amo phous Phase Designa ion Tempe a u e[°C] WL 1250 74.5 0.0 16.1 4.0 5.5 1450 75.6 0.0 7.4 3.8 13.1 1550 71.2 0.0 0.5 0 29.7 W−T 1550 58.5 0.5 20.3 0.9 19.8 W−TT 1550 54.9 0.7 14.6 0.4 29.4 G−L 1250 69.5 0.0 19.6 3.0 7.9 1450 70.7 0.0 24.8 1.0 3.5 1550 71.7 0.0 0.6 0.0 27.7 G−T 1550 56.9 0.1 21.8 0.1 21.1 G−TT 1550 55.9 0.1 23.2 0.0 20.8 K−L 1250 59.1 0.0 0.0 3.9 36.8 1450 57.2 0.0 0.0 0.0 42.8 1550 54.9 0.0 0.0 0.0 45.1 Figu e 2. Con en s o mulli e in syn he ised samples and e e ence ma e ial; L = l abo a o y i ing, T,TT = one and wo cycles o i ing in indus ial unnel kiln. Table 5shows he e ec o he i ing empe a u e on he phase composi ion o labo a o y-p epa ed samples, he compa ison wi h indus ial samples p oduced by one and wo i ing cycles, and he e e ence ma e ial. The phase composi ion o sample K i ed in he indus ial unnel kiln was he closes o he comme cial e ac o y mulli e agg ega e used as he e e ence ma e ial. In labo a o y condi ions, simila composi ion was acqui ed o he sample wi h o mula G (Table 5). The he e ogeneous mic os uc u e o he indus ially p epa ed samples was obse ed by pola izing ligh mic oscopy (PLM). Physical-mechanical p ope ies o he composi e p oduc s a e usually s ongly a ec ed by he in e aces o di e en used ma e ials. Fi ed clay nodules, locally o he size o e 1 cm, we e su ounded by mix u es o well-dispe sed clay binde and MOTIM mulli e pa icles in all he examined samples ( o examples see Figu e 3). The in e aces o hese egions we e ela i ely sha p (Figu e 4) wi h no subs an ial po osi y possibly dec easing he physical-mechanical p ope ies o he samples. G ey colou ing o he nodules nea he in e aces was caused by mig a ion o i on oxides owa ds hei cen al pa s. In some cases, i on oxides o med isola ed ci cula spo s in he i ed clay nodules. The ims o MOTIM mulli e did no exhibi any se e e co osion (Figu e 3). Ma e ials 2021, 14, x FOR PEER REVIEW 8 o 15 K−T 1550 67.2 0.0 31.1 0.1 1.6 K−TT 1550 66.5 0.0 24.4 0.0 9.1 D−L 1250 56.6 0.0 0.0 4.0 39.4 1450 55.3 0.0 0.0 0.7 44.2 1550 54.9 0.0 0.0 0.6 44.5 D−T 1550 51.2 0.0 1.6 0.0 47.2 D−TT 1550 49.5 0.0 1.8 0.1 48.6 Re e ence Ma e ial 70.1 2.4 21.1 0.3 6.0 The he e ogeneous mic os uc u e o he indus ially p epa ed samples was ob- se ed by pola izing ligh mic oscopy (PLM). Physical-mechanical p ope ies o he com- posi e p oduc s a e usually s ongly a ec ed by he in e aces o di e en used ma e ials. Fi ed clay nodules, locally o he size o e 1 cm, we e su ounded by mix u es o well- dispe sed clay binde and MOTIM mulli e pa icles in all he examined samples ( o ex- amples see Figu e 3). The in e aces o hese egions we e ela i ely sha p (Figu e 4) wi h no subs an ial po osi y possibly dec easing he physical-mechanical p ope ies o he samples. G ey colou ing o he nodules nea he in e aces was caused by mig a ion o i on oxides owa ds hei cen al pa s. In some cases, i on oxides o med isola ed ci cula spo s in he i ed clay nodules. The ims o MOTIM mulli e did no exhibi any se e e co osion (Figu e 3). Figu e 3. Sample K-T– i ed clay nodules su ounded by mix u e o i ed clay and mulli e; PPL = plane pola ized ligh , XPL = c ossed pola ized ligh . Figu e 4. Sample G-T–in e ace o la ge, i ed clay nodule and mix u e o i ed clay and agmen s o used mulli e. 3.2.2. Mulli e C ys al Size De elopmen The de elopmen o mulli e c ys als‘ mean size along c c ys allog aphic axis wi h i ing empe a u e (5 h soaking) in he clay pa o he g og was obse ed and calcula ed using SEM (Figu e 5; mulli e was iden i ied by EDS analysis). Simila end was obse ed in all he samples (Table 6). The slowes g owing o mulli e c ys als was obse ed in DS1 kaolin, which was he mos alkali- ich one. Howe e , he alues we e simila o each o he Figu e 3. Sample K-T– i ed clay nodules su ounded by mix u e o i ed clay and mulli e; PPL = plane pola ized ligh , XPL = c ossed pola ized ligh . Ma e ials 2021,14, 779 9 o 15 Ma e ials 2021, 14, x FOR PEER REVIEW 8 o 15 K−T 1550 67.2 0.0 31.1 0.1 1.6 K−TT 1550 66.5 0.0 24.4 0.0 9.1 D−L 1250 56.6 0.0 0.0 4.0 39.4 1450 55.3 0.0 0.0 0.7 44.2 1550 54.9 0.0 0.0 0.6 44.5 D−T 1550 51.2 0.0 1.6 0.0 47.2 D−TT 1550 49.5 0.0 1.8 0.1 48.6 Re e ence Ma e ial 70.1 2.4 21.1 0.3 6.0 The he e ogeneous mic os uc u e o he indus ially p epa ed samples was ob- se ed by pola izing ligh mic oscopy (PLM). Physical-mechanical p ope ies o he com- posi e p oduc s a e usually s ongly a ec ed by he in e aces o di e en used ma e ials. Fi ed clay nodules, locally o he size o e 1 cm, we e su ounded by mix u es o well- dispe sed clay binde and MOTIM mulli e pa icles in all he examined samples ( o ex- amples see Figu e 3). The in e aces o hese egions we e ela i ely sha p (Figu e 4) wi h no subs an ial po osi y possibly dec easing he physical-mechanical p ope ies o he samples. G ey colou ing o he nodules nea he in e aces was caused by mig a ion o i on oxides owa ds hei cen al pa s. In some cases, i on oxides o med isola ed ci cula spo s in he i ed clay nodules. The ims o MOTIM mulli e did no exhibi any se e e co osion (Figu e 3). Figu e 3. Sample K-T– i ed clay nodules su ounded by mix u e o i ed clay and mulli e; PPL = plane pola ized ligh , XPL = c ossed pola ized ligh . Figu e 4. Sample G-T–in e ace o la ge, i ed clay nodule and mix u e o i ed clay and agmen s o used mulli e. 3.2.2. Mulli e C ys al Size De elopmen The de elopmen o mulli e c ys als‘ mean size along c c ys allog aphic axis wi h i ing empe a u e (5 h soaking) in he clay pa o he g og was obse ed and calcula ed using SEM (Figu e 5; mulli e was iden i ied by EDS analysis). Simila end was obse ed in all he samples (Table 6). The slowes g owing o mulli e c ys als was obse ed in DS1 kaolin, which was he mos alkali- ich one. Howe e , he alues we e simila o each o he Figu e 4. Sample G-T–in e ace o la ge, i ed clay nodule and mix u e o i ed clay and agmen s o used mulli e. 3.2.2. Mulli e C ys al Size De elopmen The de elopmen o mulli e c ys als‘ mean size along c c ys allog aphic axis wi h i ing empe a u e (5 h soaking) in he clay pa o he g og was obse ed and calcula ed using SEM (Figu e 5; mulli e was iden i ied by EDS analysis). Simila end was obse ed in all he samples (Table 6). The slowes g owing o mulli e c ys als was obse ed in DS1 kaolin, which was he mos alkali- ich one. Howe e , he alues we e simila o each o he (in compa ison o comme cial samples, o which hey a e app oxima ely by one o de o magni ude la ge ). Ma e ials 2021, 14, x FOR PEER REVIEW 9 o 15 (in compa ison o comme cial samples, o which hey a e app oxima ely by one o de o magni ude la ge ). Figu e 5. Compa ison o mulli e c ys als size in ma ices o K and G samples a e wo cycles o indus ial i ing wi h he e e ence ma e ial ( i ing empe a u e 1480 °C/5h soaking). Table 6. Mean size o mulli e c ys als [μm] on ma ix o labo a o y-p oduced samples, MOTIM mulli e and e e ence ma e ial based on SEM obse a ion. Fi ing Tempe a u e [°C] / Sample 1250 1450 1550 W-L 0.2 0.6 4.0 G-L 0.2 1.0 6.0 K-L 0.5 2.0 10.0 D-L 0.1 0.4 2.0 Comme cial Mulli e Based P oduc s Re e ence Ma e ial 3.0 MOTIM 283.0 3.2.3. Po e Size Analysis The e ec o i ing empe a u e on po e-size dis ibu ion de elopmen in labo a o y- p epa ed samples is shown in Figu es 6 and 7. The o al olume o open po es wi h he diame e s below 1 μm was signi ican ly lowe in all he samples i ed a 1450 °C (in com- pa ison o hose i ed a 1250 °C). The o al olume o small po es also dec eased be ween 1450 and 1550 °C in he G-L and W-L samples. In he K-L and D-L samples, sligh inc ease in open po osi y be ween 1450 and 1550 °C can be obse ed. The labo a o y-p epa ed g og samples based on kaolins ea u ed lowe o al olumes o po es wi h he diame e s be ween 1 and 40 μm han he sample based on clays one (W-L), his po e size o which was dominan . A e i ing a 1250 °C, all he kaolin-based samples displayed simila po e- size dis ibu ions wi h app oxima ely 85 ol.% o po es below 1 μm in diame e . The low- es o al po osi y a e all he i ing egimes was obse ed in D-L sample. The inc ease in he i ing empe a u e om 1250 o 1450 °C led o signi ican clinke ing o K-L and D-L samples, which mani es ed by he dominance o small po es (below 1 μm). Figu e 5. Compa ison o mulli e c ys als size in ma ices o K and G samples a e wo cycles o indus ial i ing wi h he e e ence ma e ial ( i ing empe a u e 1480 ◦C/5h soaking). Table 6. Mean size o mulli e c ys als [ µ m] on ma ix o labo a o y-p oduced samples, MOTIM mulli e and e e ence ma e ial based on SEM obse a ion. Fi ing Tempe a u e [◦C]/Sample 1250 1450 1550 W-L 0.2 0.6 4.0 G-L 0.2 1.0 6.0 K-L 0.5 2.0 10.0 D-L 0.1 0.4 2.0 Comme cial Mulli e Based P oduc s Re e ence Ma e ial 3.0 MOTIM 283.0 3.2.3. Po e Size Analysis The e ec o i ing empe a u e on po e-size dis ibu ion de elopmen in labo a o y- p epa ed samples is shown in Figu es 6and 7. The o al olume o open po es wi h he diame e s below 1 µ m was signi ican ly lowe in all he samples i ed a 1450 ◦ C (in compa ison o hose i ed a 1250 ◦ C). The o al olume o small po es also dec eased