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C.C. Arruda*, P.H.L. Cardoso*, I.M.M. Dias*, R. Salomão*

Hydrotalcite (Mg6Al2(OH)16(CO3)·4H 2O): A Potentially Useful Raw Material for Refractories

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The corresponding author, Hydrotalcite also known as aluminum- layered double hydro- hydrotalcite, nanopar- Prof. Dr. Rafael Salomão, re- xide, was broadly investigated in the past decade due to its many appli- ticles, porous ceramics,

ceived his Ph.D. in Materials cations in catalysis, adsorptive flotation and flame retardant in polymers. spinel, MgO, Al(OH)3 Science and Engineering from In refractory castables, hydrotalcite was observed to be a by-product of INTERCERAM 62 (2013) [3] the Federal University of São the hydroxylation reaction between magnesia sinter and hydratable alu- Carlos (Brazil, 2005). Since mina or calcium aluminate cement. It can be synthesized using copre- 2010 he has been a lecturer cipitation reactions from Al-Mg soluble salts. Novel methodologies were and researcher at the São recently developed to produce large quantities of hydrotalcite starting Carlos School of Engineering (EESC), Materials from caustic magnesia and aluminum hydroxide under controlled temper-

Engineering Department, University of São Paulo ature. Considering its chemical composition (Mg6Al2(OH)16(CO3)·4H 2O),

(Brazil). He has authored/co-authored more which is roughly 60 mass-% MgO and 40 mass-% Al2O3, and the many than 60 technical papers. The main topics of advances made in its synthesis, hydrotalcite has great potential for use Prof. Salomão’s research group (Integrated in refractories, especially those involving in situ generation of spinel

Solutions in Manufacture and Ceramic Materi- (MgAl2O4). This paper critically reviews the occurrence of hydrotalcite in als, SIMMaC) are advanced refractory ceramics, refractories, the most effective synthesis methods, and presents some ceramic-polymer nanocomposites and nano- potential uses in refractory applications. porous ceramics. E-Mail: [email protected]

1 Introduction applications are: oil adsorbents, supports presence of anions and water molecules be- The term “anionic clays” is used to designate for catalysts, sorption agents for industrial tween the lamellae, which leads to stacking natural or synthetic layered double hydro- wastewater containing anionic surfactants, of the double hydroxide layers with a slight- xides (LDHs) containing anionic species in colourings and acid herbicides, sensors in ly ordered interlayer domain. In the LDH their interlayer domain [1–3]. This denomi- electrochemistry, and flame retardant in case, the lamellae are not only held together nation arose in parallel with the expression polymer nanocomposites [5, 8, 9]. by hydrogen bonds, as is the case for , “cationic clays” used for materials that are The LDHs can be represented by the general but by electrostatic attraction between the comprised of negatively charged layers, such formula: positively charged layers and the interlayer 3+ as Al2Si2O9H4 (with Al cations occupying anions. A schematic representation of the 2+ 3+ x+ m- the sites of silicon or divalent cations such as [M (1–x) M x (OH)2] ·(A x/m)·nH2O (1) structure of an LDH is shown in Fig. 1. Mg2+, or Ca2+ taking the place of aluminum) Among the many LDHs, those known as 2+ which have interlayer cations to neutralize where M is a divalent metallic cation (for Mg-Al hydrotalcites (Mg6Al2(OH)16(CO3)·4H2O) the aluminosilicate charges [4–6]. Synthesis example, Mg2+ or Ca2+), M3+ is a trivalent are the most studied and employed in tech- of LDHs began around 1930, using precipi- metallic cation (such as Al3+, Cr3+ or Fe3+), nological applications due to their easy syn- tation reactions of dilute metal salt solu- Am– is an intercalated anion with charge thesis and wide range of properties [1, 2, 5]. 2– tions in alkaline media. After the Second m– (usually (CO3) ) and 0.20 Յ ϫ Յ 0.33. Recent studies have found that this type of World War, many researchers studied as- The of an LDH is often hydrotalcite also has great potential for ap- pects related to their structure, synthesis described by reference to the structure of plication in refractories [10–15]. In refrac- and properties [7]. Currently, several types brucite (Mg(OH)2). In brucite, magnesium tory castables, hydrotalcite was discovered of LDHs are used in various applications cations are located at the center of octa- to be a by-product of the hydroxylation re- depending on their composition, crystallin- hedra, which have hydroxyl anions on their action between magnesia sinter and hydrat- ity, thermal stability and other physico- vertices. These octahedra share their edges able alumina or calcium aluminate cement. chemical properties. Some examples of forming flat and neutral layers, which are The small amount of hydrotalcite formed at held together by hydrogen bonds. In this the surface of the magnesia particles be- type of structure, when divalent cations are haves as a protective layer minimizing the isomorphically substituted by trivalent ones, deleterious effects of the hydroxylation * Materials Engineering Department, São Carlos School of Engineering, Avenida Trabalhador São-carlense the layers generate a positive residual charge. [12, 13]. Hydrotalcite can also be used in 400, São Carlos-SP (Brazil) Electroneutrality of the system requires the manufacturing porous ceramics due to ex-  ,17(5&(5$0²5()5$&725,(60$18$/,

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tensive weight loss [14] and the formation 1 of highly refractory phases (in particular,

spinel MgAl2O4) [15] which accompanies its thermal decomposition. Because Mg-Al hydrotalcite does not exist in significant quantities in nature [1], it must be produced for applications in refractories at competitive costs and with controlled characteristics (mainly, particle size distri- bution, specific surface area and crystallin- ity). For the production of Mg-Al hydro- talcite, several factors must be considered, including the degree of substitution of diva- lent cations for trivalent ones, the nature of the interlayer anion, the pH of the synthesis medium and, in some cases, the reaction at- mosphere [3]. Furthermore, to obtain abun- dant crystalline material, parameters such as the concentration of the reactants, their rate of addition, degree of agitation, the pH of Fig. 1 • General structure of hydrotalcite [2, 4] the resulting suspension and the mixture temperature must be controlled. In order to understand the many production possibili- containing salts of the divalent and trivalent therefore precipitates (Table 1, c–e). These

ties, the following section presents a critical cations (MgCl2 and AlCl3, for instance) is reactions are performed under strong stir- view of the recent literature about hydrotal- added to an alkaline solution containing the ring and require a further purification step

cite synthesis. anion that will be interleaved (NaHCO3, for to remove any remaining counter instance) (Table 1, a–b) [16]. In the copre- (Table 1, f–g). In addition to the reactants’ 2 Methods of synthesis and cipitation at constant pH approach, the concentrations, the speed of addition, the modification dissolved di- and trivalent cations, the inter- final pH of the suspension, the degree of 2.1 Coprecipitation methods layer anion, and alkaline solution are all agitation (usually vigorous) and the mixture Coprecipitation based methods are the most combined at the same time. This second temperature must be carefully controlled. used routines for preparation of hydrotal- method requires the use of a more sophisti- Coprecipitation is usually carried out at rel- cite (Table 1) [1, 4, 7, 8]. Numerous studies cated experimental apparatus, but it results atively low temperatures (up to only 35 °C) employing this method can be found in the in particles with greater uniformity [17]. As to prevent the formation of other phases literature. They take two different forms: co- pH is increased, the Mg2+ and Al3+ ions react such as simple hydroxides. - 2– precipitation at variable pH and at constant with OH and (CO3) generating hydro- One of the inconveniencies of the coprecipi- pH. In the variable pH method, a solution talcite, which has very low solubility and tation method is that the instantaneous

Table 1 • Compilation of the coprecipitation methods Steps Observations

2+ – a) Acid (pH ~ 4-6) aqueous solution of bivalent (6MgCl2 A 6Mg + 12Cl ) 2+ 3+ 3+ – Excess of Mg or Al ions: precipitation of hydrotal- and trivalent (2AlCl3 A 2Al + 6Cl ) cations (Mg:Al ratio of (3-2):1) cite plus Mg(OH) or Al(OH) , respectively ? 2 3 b) 6Mg2+ + 2Al3+ + 18Cl– + nH O (aq.) (aq.) (aq.) 2 pH Յ 7: stable solution ?

2– c) pH increase (by addition of NaHCO3, NaOH or NH4OH) (CO3) ions can also be supplied by CO2 dissolution in ? water during the mixing d) 6Mg2+ +2Al3+ + 18Cl– + mNa+ + m(CO2– ) ) + mOH– (aq.) (aq.) (aq.) (aq.) 3 (aq. pH ~ 10–12: metastable solution ? e) Mg Al (OH) (CO )·4H O + 18NaCl 6 2 16 3 2 (precipitated) (dissolved) pH ~ 10–12: hydrotalcite suspension ? f) Purification (by centrifugation or dialysis) Withdrawal of dissolved NaCl from the suspension ? g) Mg Al (OH) (CO )·4H O 6 2 16 3 2 (aqueous suspension) Particles: small, irregular and with low crystallinity ? h) Hydrothermal treatment (autoclave) and controlled drying (lyophilization) Increase crystallinity, narrow particle size distribution ? and avoid agglomerates formation

i) Mg6Al2(OH)16(CO3)·4H2O(dried) Dried hydrotalcite particles ,17(5&(5$0²5()5$&725,(60$18$/, 

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value of pH may vary at different locations 2 Fig. 2 • Nanoparticles within the suspension, no matter how fast of hydrotalcite prepared the agitation. Because of this, the method by cohydroxylation of often results in large agglomerations of pri- caustic magnesia (MgO) mary particles in stable aggregates with wide and aluminum hydroxide particle size distribution. This morphology (Al(OH)3) after 3 h at generally has very low specific surface area 150 °C in hydrothermal condition, see [13] for and almost no porosity. Moreover, once ag- further details gregates are formed, they are very stable and resist attempts at disintegration even under ultrasonic treatment. As an alternative route for better coprecipitation, urea (CH4N2O) added to the initial system can form a ho- mogeneous and stable solution with the metal salts at low temperatures. When the temperature is raised to approximately 90 °C, the urea decomposes slowly as pH is increased homogeneously. This process shifts the pH of each part of the solution 2+ 3+ – 2– equally, avoiding the hydrotalcite particle ing point, which increases solubility. In Mg + Al + OH + (CO3) A agglomeration [19]. this condition, ions can diffuse over long Mg6Al2(OH)16(CO3)·4H2O (4) Coprecipitated Mg-Al hydrotalcites and distances and choose more favourable crys- other LDHs have numerous possible appli- tal planes and positions for precipitation In this system, two important benefits re- cations. These include use as agents for ad- and growth. The kinetics of nucleation and lated to the combination of MgO and hy- sorption of chemical species (phosphate crystal growth is modified; fewer crystalliza- dratable alumina can be observed: ions, sulfate and sulfide diluted in water, tion cores are formed (because dissolution carbon dioxide, carboxyl groups, hydroxyl, is preferred over precipitation) and they 1) a thin hydrotalcite layer forms over the phenyl, carbonyl and phosphorus), in catal- tend to grow more expressively, since there surface of the MgO sinter particles that ysis (hydrotalcite composed of Mg and Al are few sites for precipitation. hinders its hydroxylation, and can be used as catalysts in the transesterifi- 2) after reacting with hydratable alumina, cation reaction of sunflower oil and oil used 2.3 Cohydroxilation of MgO and Al(OH)3 significantly stronger structures are at- in the presence of methanol), and many The residual presence of counter ions is tained. other possibilities [5, 6, 9]. an important drawback of coprecipitation methods. After precipitation of hydrotalcite, Exploiting this behaviour, a recent work – – + + 2.2 Modification by hydrothermal reaction salt anions and cations (Cl , (NO3) , Na , K , describes a novel method to produce large Hydrothermal treatments using autoclaves for example) remain dissolved in the sus- quantities of hydrotalcite from caustic expose a suspension of hydrotalcite parti- pension. If the hydrotalcite will be used in MgO and Al(OH)3. Caustic MgO is a high cles to an elevated temperature for a given refractory applications, further purification surface area reactive form of MgO that re- time under the action of water vapour steps (centrifugation or ultrafiltration) acts easily with water, releasing a large pressure and heating. This mechanism im- would be required, reducing the output of quantity of Mg2+ions while increasing the proves the crystallinity of the synthesized the process significantly and increasing its pH of the suspension above 11 [21, 22, 23]. samples, refines their geometry and nar- cost (Table 1, e–f). Due to this difficulty, re- Al(OH)3, on the other hand, is an ampho- rows their size distribution. Some papers in cent works have investigated cleaner and teric compound that has great solubility in – the literature have studied the effects of more effective alternative routes to produce water, forming Al(OH)4 ions in pH above different hydrothermal treatment condi- hydrotalcite [13, 20]. 9. Combining these raw materials in suita- tions on the structural properties of syn- As an alternative to coprecipitation, in the ble temperature conditions (above 50 °C thetic hydrotalcite [13, 18], After synthesis method known as cohydroxylation, forma- and up to hydrothermal conditions of by coprecipitation, samples were subjected tion of hydrotalcite occurs without the pres- 150 °C) causes for mation of hydrotalcite to different hydrothermal treatment condi- ence of counter ions. Hydrotalcite normally nanoparticles (D50~200 nm) without need tions in a stainless steel autoclave coated occurs in refractory castables as a by-prod- of further purification steps [13]. Figure 2 with PTFE (Table 1, h–i). The hydrotalcite uct of the reaction between Mg2+ (from the shows hydrotalcite nanoparticles formed crystallization was significantly affected by hydroxylation-dissolution reaction of MgO by this technique. variations in hydrothermal treatment tem- sinter), Al3+ (from hydratable alumina hy- As will be seen in the next section, hydrotal- 2- perature between 70 °C and 140 °C for 0 to dration) and (CO3) (from CO2 dissolved in cite synthesized by this method is suitable 11 hours. water during mixing) ions, according to the for use in refractory formulations. The As a general rule, after increasing the hydro- general cohydroxylation expressions: process provides: thermal treatment temperature, there was + an increase in the average particle size of MgO + H2O A MgOH (aqueous) + 1) high output and competitive cost, hydro talcite and a decrease in the specific OH– A Mg2+ + OH– (2) 2) purity level compatible with the surface area. This behaviour can be under- requirements of the refractory industry, – – stood from the fact that, at high pressure, Al(OH)3 + nOH A Al(OH) 4 (aqueous) + and water can be heated beyond its normal boil- nOH– A Al3+ + nOH– (3) 3) stable suspension of very thin particles.  ,17(5&(5$0²5()5$&725,(60$18$/,

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3 Fig. 3 • Effect of Mg-Al 4 Conclusions hydrotalcite addition to Magnesium-aluminum hydrotalcite the apparent porosity of (Mg6Al2(OH)16(CO3)·4H2O) is a potentially a dense Al2O3 matrix, see useful raw material for refractories. Its [15] for further details chemical composition (close to spinel,

MgAl2O4) and the many possible synthesis methods favour its use in different appli- cations in this field. In situ generation of hydro talcite as a byproduct of the hydroxy- lation reaction of MgO and hydratable alu- mina is a well-known anti-hydration tech- nique for castables containing MgO. It pro- vides a protective coating on the surface of the MgO particles, halting hydroxylation and its deleterious effects. Preformed hy- drotalcite can also be used as a porogenic agent in dense high alumina matrixes due to its large mass loss after decomposition and because of the formation of spinel. The high refractoriness of this phase allows high po- rosity to be achieved even at temperatures in 3 Hydrotalcite as a raw material served) with high surface area greater than the range of 1500 °C. Finally, considering its for porous ceramics 100 m2/g. The combination of these two many technological applications in other Due to their unique combination of high mechanisms generates structures with po- fields (such as catalysis, water treatment and refractoriness and low thermal conductivity, rosity of about 50–60 % after sintering at nanocomposites) and the extent of recent porous ceramics are appropriate for appli- 1100–1200 °C. However, when the sintering research about its synthesis and characteri- cations that involve contact with high tem- temperature exceeds this range, a significant zation, hydrotalcite is clearly an up-and- perature (above 600 °C) fluids, such as hot reduction in porosity is observed (Fig. 3) coming trend in materials science and en- air and liquid metal filtering and thermal [15, 25, 26]. To avoid this densification and gineering. insulation. This technological importance porosity loss, a recent work replaced the in-

motivated the development of many pro- serted Al(OH)3 by an equivalent volume of Acknowledgments duction methods. One of the most explored Mg-Al hydrotalcite [15]. The authors would like to acknowledge in literature is based on decomposition of Comparing the two systems, at lower tem- the Brazilian Research Foundations FAPESP,

Al(OH)3 previously inserted into a dense peratures (up to 1100 °C), the porosity at- CNPq and CAPES for supporting this re-

Al2O3 matrix. Pore generation occurs during tained is similar (Fig. 3). However, as firing search. the material’s initial heating and is caused temperature increases to 1500 °C the sam-

by two factors: volume shrinkage accompa- ples with Al(OH)3 undergo a significant re- nying dehydroxylation and the large quan- duc tion of porosity. With hydrotalcite, on References tity of defects and irregularities created by the other hand, porosity remains practically [1] Miyata, S., Kumura, T.: Synthesis of new hydrotal- cite-like compounds and their physicochemical freshly decomposed material (alumina tran- the same as is observed at 1100 °C. The lower properties. Chemistry Letters 8 (1973) 843–848 sition phases) [15, 24, 25, 26]. densification of samples containing hydro- [2] Reichle, W.T.: Anionic Clay Minerals, Chemtech 16 Compared to alternative methods that use talcite can be explained by the formation of (1986) 58–63 [3] Reichle, W.T.: Synthesis of anionic clay-minerals foams and organic particles, using Al(OH)3 Mg-Al spinel (MgAl2O4). Because spinel is (mixed metal-hydroxides, hydrotalcite). Solid State as a porogenic agent does not release toxic an extensive solid solution, it typically forms Ionics 22 (l986) [1] 135–141 volatiles, generates high refractoriness phas- a microstructure consisting of large grains, [4] Vaccari, A.: Preparation and catalytic properties of cationic and anionic clays. Catalysis Today 41 es (alpha-alumina) and can easily be in- which naturally decreases the driving force (1998) 53–71 corporated into various ceramic matrices, for sintering and densification [27]. It should [5] Vaccari, A.: Clays and Catalysis: a Promising Future. including insulating refractory concrete. be noted, however, that improved high tem- Applied Clay Science 14 (1999) 161–198 [6] Vaccari, A.: Layered double hydroxides: present and Despite these advantages, an important dif- perature porosity was obtained when the future. Applied Clay Science 22 (2002) [1–2] 75–76 ficulty can be introduced by the generated total sample composition reached the [7] Sato, T., Fujita, H., Endo, T., Shimada, M.: Synthe- sis of hydrotalcite-like compounds and their physi- transition phases. They have a strong ten- Al2O3:MgO ratio suitable for formation of co-chemical properties. Reactivity of Solids 5 dency to promote sintering with the dense stoichiometric spinel (1:1 mole or ~30 mass-% (1988) 219–228 phase of the matrix. During its decompo- MgO or ~71 vol.-% of hydrotalcite). Lower [8] Cavani, F., Trifiro, F. Vaccari, A.: Hydrotalcite-type sition at about 300–450 °C, aluminum hy- porosity levels were observed for composi- anionic clays: Preparation, properties and applica- tions. Catalysis Today 11 (1991) 173 droxide undergoes a volumetric contraction tions with excess Al2O3 or MgO. This de- [9] Evans, D.G., Duan, X.: Preparation of layered dou- of about 60 %, generating pores with di- pendence of porosity on sample composi- ble hydroxides and their applications as additives mensions of 100 nm to 10 µm, depending tion stems from the fact that up to 1500 °C, in polymers, as precursors to magnetic materials and in biology and medicine. Chemical Communica- on initial granulometry. Furthermore, after excesses of Al2O3 or MgO precipitate at grain tions (2006) 485–496 this reaction, defect-rich transition com- boundaries, restricting their growth and [10] Ghanbari, K., Sharp, J.H., Lee, W.E.: Hydration of pounds are formed. This material is very favouring densification [15, 27]. Because of refractory oxides in castable bond systems-I: alu- mina, magnesia, and alumina-magnesia mixtures. porous (in the literature, internal pores on this, spinel solid solution forms under a Journal of the European Ceramic Society 22 (2002) the order of 50 to 200 nm have been ob- narrow range of composition conditions. [4] 495–503 ,17(5&(5$0²5()5$&725,(60$18$/, 

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[11] Ye, G., Troczynski, T.: Hydration of hydratable alu- [17] Ogawa, M., Kaiho, H.: Homogeneous precipitation [23] Salomão, R., Bittencourt, L.R.M., Pandolfelli, V.C.: mina in the presence of various forms of MgO,. Ce- of uniform hydrotalcite particles. Langmuir 18 A novel approach for magnesia hydration assess- ramics International 32 (2006) 257–262 (2002) 4240–4242 ment in refractory castables. Ceramics Internation- [12] Salomão, R., Pandolfelli, V.C.: The role of hydraulic [18] Sharma, S.K., Kushwaha, P.K., Srivastava, V.K., al 33 (2007) [5] 803–810 binders on magnesia containing refractory casta- Bhatt, S.D., Jasra, V.R.: Effect of hydrothermal con- [24] Mista, W., Wrzyszcz, J.: Rehydration of transition bles: calcium aluminate cement and hydratable alu- ditions on structural and textural properties of aluminas obtained by flash calcination of gibbsite. mina. Ceramics International 35 (2009) [8] 3117– synthetic hydrotalcites of varying Mg/Al Ratio. In- Thermochimica Acta 331 (1999) [1] 67–72 3124 dustrial and Engineering Chemistry Research 46 [25] Deng, Z., Fukasawa, T., Ando, M.: High-surface-ar- [13] Salomão, R., Milena, L.M., Wakamatsu, M.H., Pan- (2007) 4856–4865 ea alumina ceramics fabricated by the decomposi-

dolfelli, V.C.: Hydrotalcite synthesis via co-precipi- [18] Zeng, H., Deng, X., Wang, Y., Liao, K.: Preparation tion of Al(OH)3. Journal of the American Ceramic

tation reactions using MgO and Al(OH)3 precursors. of Mg-Al hydrotalcite by urea method and its cata- Society 84 (2001) [3] 485–491 Ceramics International 37 (2011) 3063–3070 lytic activity for transesterification. AIChE Journal [26] Deng, Z., Fukasawa, T., Ando, M.: Microestructure [14] Pesic, L., Salipurovic, S., Markovic, V., Vucelic, D., 55 (2009) [5] 1229–1235 and mechanical properties of porous alumina ce- Kagunya, W., Jones, W.: Thermal characteristics of [20] Valente, J.S., Cantu, M.S., Figueiras, F.: A simple ramics fabricated by the decomposition of hydrox- a synthetic hydrotalcite-like material. Journal Ma- environmentally friendly method to prepare versa- ide. Journal of the American Ceramic Society 84 terial Chemical 2 (1992) [10] 1069–1073 tile hydrotalcite-like compounds. Chemistry of Ma- (2001) [11] 2638–2644 [15] Salomão, R., Villas-Bôas, M.O.C., Pandolfelli, V.C.: terials 20 (2008) 1230–1232 [27] Bayley, J.T., Russel Jr., R.: Sintered spinel ceramics. Alumina-spinel porous ceramics for high tempera- [21] Rocha, S.D.F., Mansur, M.B., Ciminelli, V.S.T.: Kinet- American Ceramic Society Bulletin 47 (1968) [11] ture applications. Ceramics International 37 (2011) ics and mechanistic analysis of caustic magnesia 1025–1029 1393–1399 hydration. Journal of Chemical Technology and Bio- [16] Kelkar, C.P., Schultz, A.A., Cullo, L.A.: Synthesis of technology 79 (2004) 816–821 hydrotalcite-like materials with a sheet-like mor- [22] Amaral, L.F., Oliveira, I.R., Salomão, R., Frollini, E., phology. In: Synthesis of porous materials – zeo- Pandolfelli, V.C.: Temperature and common-ion ef- lites, clays and nanostructures. Marcel Dekker Inc., fect on (MgO) hydration. Ceram- New York (USA), 1996. ISBN-13: 978-0824797591 ics International 36 (2008) [3] 1047–1054 Received: 04.07.2013

56th INTERNATIONAL COLLOQUIUM ON REFRACTORIES 2013

September 25th and 26th, 2013 . EUROGRESS, Aachen, Germany

Conference Topic Refractories for Industrials

tGlass   tChemistrytQuality management tCement/ tRefractory raw materialstWear and Corrossion /plaster tShaped and unshaped tRecycling tCeramics refractories tEnvironmental tIncineration tProcessing and refractory Protection lining service

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