Physico-Mechanical and Microstructure Characteristics of Porous Mullite Ceramics

Porous mullite ceramics were prepared from mixtures of Egyptian kaolinite-clay and calcined α-alumina powder. The effects of two methods of compaction as well as two types of binder were investigated. The influence of sintering temperature on the physical properties, crystalline phase, compressive strength and microstructure was studied. Mullite bodies processed by uniaxial pressing utilizing 15wt. % water at 1500°C for 2h exhibited the highest compressive strength of 150 MPa and bulk density of 2.1 g/cm3, as well as, an open porosity of 35%. In the present study, the only stoichiometric mullite was achieved by the crystallization of metakaolin. The mullite crystallizing out from the liquid phase was rich in silica, as determined by EDS, while that resulting from alumina grains was rich in alumina. The microstructure showed that primary mullite appeared at 1200°C, beside the unreacted alumina particles. On the other hand, at 1400°C alumina particles started to react with the glassy phase to form parallel laminates of mullite.


Introduction
Ceramic membranes have excellent strength, good corrosion resistance, high chemical and thermal stability, good anti-fouling properties and long life requirements to operate at high temperature, highly corrosive and high-pressure filtration environment, compared to those prepared from polymers.However, the workability of these membranes and the cost of production restrict their application.The utilization of cheaper raw materials may reduce the cost of production of these membranes [1].
Mullite (3Al 2 O 3 •2SiO 2 ) is a potential candidate for porous ceramic membranes with high strength and creep resistance, a low thermal expansion coefficient, and proper chemical and thermal stability.Porous mullite ceramic membranes have been significantly applied in the fields of catalyst carriers, filters and ceramic supports for filtration membranes [2].
There are several methods for the synthesis of mullite ceramics, including reaction sintering, spark plasma and microwave-assisted sintering, gel casting, foaming and freeze-drying, where it is supplied in the form of powder or dense or porous bodies.
Researchers concentrate on the simplicity of preparation methods and low-cost raw materials, such as the use of solid wastes to replace industrial raw materials, and using pressure-less sintering in an ambient atmosphere [3].
High alumina kaolin and coal fly ash are adequate mix for the synthesis of low-cost mullite ceramics [4].Usually pure alumina or its precursors are added to satisfy the mullite composition.
In general, the gradual dissociation of kaolin with temperature gives first meta-kaolinite, which has a spinel structure, and readily transforms into mullite (the primary form) beside amorphous silica at around 980°C.
As the temperature increases, the amount and size of mullite crystals increase, leaving behind excess silica as a glassy phase or free amorphous silica in the matrix [1].
Schuller [10] described two different types of mullite of different morphologies from studies of porcelain and he showed that primary mullite transforms into secondary mullite at 1400°C and because of the complete dissolution of quartz, a texture consisting of glass and mullite needles results.Some studies explained that primary mullite is formed in areas rich in clay, while secondary mullite is formed in areas containing impurities such as potassium, which acts as flux to assist the formation of the liquid phase, allowing the crystallization of longer secondary mullite crystals [12,13].
Chen et.al. [14] prepared mullite by reactive sintering of a mixture of kaolin combined with Al(OH) 3 and AlF 3 as alumina sources to obtain macroporous mullite.Whereas, porous mullite with an interlocked needle-shaped microstructure was synthesized from China clay and aluminium fluoride trihydrate (AlF 3 •3H 2 O), as reported by Rashad and Balasubramanian [1].Also, bauxite and kaolin powders were used as precursors to prepare porous mullite ceramic membranes by Zhu et.al.[2].
In some cases, kaolin clay was used for the preparation of mullite membranes and the remaining free silica was removed from the sintered membrane by a strong alkali solution (20 wt.% NaOH) in an oven at 80°C for 5 h to achieve higher porosity [17].
When kyanite was added to kaolin, bauxite and feldspar to prepare porous mullite, it enhanced both the porosity and thermal stability by lowering the temperature of glass formation and promoting crystallization of mullite [18].
Hou et.al. [19] have studied the effect of MoO 3 on mullite bodies prepared from kaolin and Al 2 O 3 .They showed that MoO 3 addition lowered the secondary mullitization temperature to below 950°C.They reported that the added Al 2 O 3 powders dissolved in the silica-rich liquid phase to form the secondary mullite, where the crystal growth was controlled by diffusion.Therefore, the rate-determining step for crystal growth is the dissolution velocity of alumina into the silica-rich liquid phase.
Hou and Cui et.al. [20] studied the impact of specific sintering additives; TiO 2 , MgO and MoO 3 on mullite bodies, prepared from kaolin and Al 2 O 3 .They confirmed that the mullite transformation temperature was strongly affected by the type of sintering additives, as they lower the melting point of silicon-rich liquid.The addition of MgO and MoO 3 promoted the formation of secondary mullite with a needlelike morphology.However, the addition of TiO 2 retarded the mullite formation, as it participated in the formation of aluminum titanate or entered the mullite structure.
Sainz et.al. [15] investigated the microstructure of mullite prepared through the sintering reaction of kaolinitealumina mixtures.They showed the existence of a bimodal mullite morphology, corresponding to primary (elongated columnar grains) and secondary (equiaxed grains) forms.
Whereas, porous support with 96 wt.% mullite and 65 % open porosity was developed at a relatively lower temperature of 1300°C from clay and aluminium fluoride trihydrate mixture by adding 10 wt.% alumina into this mixture.The addition of alumina enhanced the mullite formation by consuming the excess SiO 2 in the matrix [16].
The objective of this work is to fabricate porous mullite ceramics from cheaper local raw Egyptian kaolin-clay materials by adding calcined alumina.
The effect of the addition of various amounts of either water or PVA on the degree of compaction of the particles and the form of processing on sintering and physicomechanical properties, as well as the microstructure of the prepared mullite bodies, is investigated.

Preparation of Mullite Bodies
The following raw materials were supplied by Middle East Mining Company (Memco) and Alexandria Company for Refractories (ACR) imported calcined alumina from Alcoa; refined local Egyptian kaolin-clay (with grain size ranging from 0.7 to 1 µm ); and α-Al 2 O 3 (purity 99.5%, with grain size ranging from 6 to 7 µm), respectively.The kaolin-clay was not calcined, as preferred by many authors, to make use of the plasticity of the kaolin-clay in the forming process.The ratios corresponding to the stoichiometric composition of mullite, (3Al 2 O 3 •2SiO 2 ), were weighted, mixed and ballmilled for 12 h using a polyethylene bottle with alumina balls.The powder was mixed by using two binders, namely, tape water and polyvinyl alcohol, PVA, [CH 2 CH(OH)] n , with different ratios (5, 10, 15 and 20%).Then processed in the form of pellets of 25 mm in diameter and 15-16 mm in thickness, applying two different shaping methods, i.e., uniaxial pressing at 30 MPa and hand pressing.
The two main batches obtained were designated as W and P, with Nos. 1 and 2 referring to the water and PVA and the processing method.i.e., W1 and P1 were uniaxially pressed at 30 MPa, while W2 and P2 were hand pressed.Moreover, each subgroup was divided into four groups according to the ratio of added water or PVA.The batches were briefly named, as summarized in Table 1.Such as the sample with 10 wt.% of water and uniaxially pressed was designated WP10, while the sample with 15 wt.% of PVA and hand pressed was designated PH15.The processed samples were left to dry for 48 h at room temperature, and then fired for 2 h in an ambient atmosphere in a muffle furnace at the given temperatures; 1150, 1300, 1400, 1500 and 1550°C with a heating/cooling rate of 5°C/ min.The samples prepared by uniaxial pressing utilizing 20 wt% water or PVA failed to keep their shapes.

Characterization
The starting materials were quantitatively analyzed by wavelength dispersive X-ray fluorescence (AXIOS, WD-XRF Sequential Spectrometer, (Panalytical, 2005).The results obtained are shown in Table 2.The particle size of the kaolin and calcined alumina powders was measured by Dynamic Light Scattering (DLS) (Malvern Zeta Sizer Instrument ZS-Nano, UK, λ=532 nm) at a temperature of 25°C.The powder samples were dispersed in ethanol and sonicated before measurement.
The   The microstructure of the fractured surfaces of the selected samples was examined under scanning electron microscopy (QUANTA FEG, 250, Netherlands) after coating with a thin film of gold.

Characterization of the Raw Materials
The chemical composition (XRF) of Egyptian kaolin-clay is shown in Table 2.The ignition loss of kaolin was 11.40 wt.%.The main constituents were alumina (36.98 wt.%, silica (47.53 wt.%) and the rest were impurities.The main phases detected by XRD were kaolinite as the major phase and quartz as a minor phase, whereas α-alumina was the sole phase detected by XRD, as shown in Fig. 1A and B.
The morphology of the kaolinite-clay and alumina particles is shown in Fig. 2. The kaolinite-clay particles were flaky in shape.Some large kaolin flakes were stacked together to form agglomerates (Fig. 2A).The crystals exhibited a hexagonal structure (Fig. 2B).Alumina crystals showed a crystal size range of 0.5-2 μm (Fig. 2B).
The particle size and size distribution of kaolinite-clay and calcined alumina are shown in Fig. 3.The kaolin presented a median diameter of 0.77 μm On the other hand; the calcined alumina exhibited an average diameter of 0.55 μm.

Phase Composition
The X-ray diffraction patterns of mullite bodies fired for two hours at temperatures between 1200 and 1550°C are shown in Fig. 4. The sample sintered at 1200°C contains mullite (3Al 2 O 3 •2SiO 2 , orthorhombic, PDF#15-0776) and corundum (Al 2 O 3 , hexagonal, PDF#10-0173) beside Cristobalite (SiO 2 , tetragonal, PDF#39-1425).These results are in accordance with those found by Deutou et al. [18].The dissociation of kaolin to give mullite and amorphous silica followed the Eqs.(1 and 2): where the free amorphous silica crystallizes later into Illcristobalite.XRD intensities of both mullite and cristobalite (1) (2) Fig. 5 The linear change % versus temperature for mullite bodies Fig. 6 Bulk density versus temperature for mullite bodies phases are more pronounced at higher temperatures, from 1300 to 1400°C, which indicates that the mullitization reaction is proceeding.Most of the corundum did not participate in the reaction with cristobalite to form mullite in this temperature range.Whereas, increasing the sintering temperature to 1500°C showed that the peaks of cristobalite had lessened, indicating its reaction with alumina to form mullite, while part of the cristobalite was consumed in the glassy phase formation.The reaction can be illustrated by the Eq. ( 3) Between 1500 and 1550°C, the cristobalite diffraction peaks had completely vanished and the intensity of the mullite peaks was more pronounced.Still, relics of corundum were recorded as a minor phase.Back in mind, the added calcined alumina and kaolin were in the ratio corresponding to the nominal chemical composition of mullite.This may be attributed to the fact that part of the cristobalite was consumed in the formation of the glassy phase.Accordingly, the mullitization reaction is nearly complete at 1550°C.Several (3) authors [16,[21][22][23][24][25][26][27] have reached the same conclusion and reported that the sintering temperature is between 1500 and 1600°C for the complete mullitization reaction.

Physical Properties of the Formed Mullite Bodies
The results of the physical properties in terms of the change in dimensions of the fired bodies, bulk density and open porosity of the fired ones are affected by the processing mode, sintering temperature and type of plasticizer used.As anticipated, the type and amount of the plasticizer played a conspicuous role in these properties.
The results of the linear change of the fired bodies are demonstrated in Fig. 5.It displays the dimensions shrinkage percent of the bodies at temperatures ranging from 1150 to 1500℃.Shrinkage increases with increasing sintering temperature up to 1400℃, as the densification process is dominant below 1400°C and therefore the shrinkage percent increases with increasing temperature.Thereafter, the recorded shrinkage is almost negligible above 1400°C.This steady state can be attributed to the two adverse reactions simultaneously taking place: first, a decrease in density.Due to the mullite formed, which induces volume expansion and has less density than the alumina precursor, and second, the shrinkage accompanying the sintering process; these two reactions nullify each other.In general, the hand-pressed samples showed a shrinkage percentage larger than the uniaxially pressed ones.The maximum shrinkage was recorded for the hand-pressed bodies processed using PVA.
Figure 6 shows the change in the bulk density of the fired bodies.Sintering was more pronounced at low temperatures range than in the higher ones (1400 to 1500℃).
This might be due to several parameters that affect the bulk density, such as the formation of mullite (theoretical density of 3.17 g/cm 3 ) at the expense of alumina (theoretical density of 3.99 g/cm 3 ) and the mullitization reaction, which is associated with volume expansion.
It is noted that the samples processed by uniaxial pressing showed higher values of bulk density than samples processed by hand pressing; moreover, the uniaxially pressed samples processed with water showed the highest bulk density value.
The recorded range of bulk density at the firing temperature of 1150 to 1500℃ for the uniaxially pressed samples (subgroups W1 and P1) lies between 1.84 and 2.03 g/cm 3 , whereas the hand pressed samples range between 1.35 and 1.63 g/cm 3 (subgroups W2 and P2).
The results of the open porosity of the samples sintered between 1300 and 1500℃ decrease over the total firing range, as shown in Fig. 7.The uniaxially pressed samples (subgroups W1 and P1) show lower values (between 35 and 40 %), whereas the hand-pressed ones (between 50 and 55 %), (subgroups W2 and P2).Meanwhile, at 1500℃, the maximum open porosity (~ 55%) was attained for PH5 samples.The above results demonstrated that the type and amount of the plasticizer, pressing mode, and sintering temperature affected the final product properties and, accordingly, will affect their applications.

Microstructure of the Fired Bodies
Effect of Temperature on the Microstructure of Sintered Samples The effect of sintering temperature between 1200 and 1500°C on the microstructural changes of the uniaxially pressed samples processed using 5 wt.% water, as an example, is demonstrated in Fig. 8.
The matrix of the sample fired at 1200°C is formed of primary nanometric mullite crystals (Fig. 8 A), preserving the hexagonal platy shape of the original structure of the kaolinite mineral [12,28], leaving behind the unreacted α-alumina and cristobalite grains (Fig. 8 A and B).In this case, the formation of mullite took place via the solid state reaction of kaolin.
As the temperature increases to 1300°C, two biomodal characteristic regions appear; one corresponding to the columnar primary mullite crystals, which increase in size compared to mullite crystals at 1200°C, and the other is the unreacted region of α-alumina grains with varying sizes, (Fig. 8 C and D), giving a porous network.There is no evidence of the presence of the glassy phase at 1300°C.
As the temperature increased to 1400°C (Figs. 8 E and F), three distinct polymorphs of mullite crystals were spotted: columnar primary, secondary acircular and platy-like mullite phases.the former columnar mullite, originating from kaolin, that grew in size, occurring in agglomerates; secondary mullite, resulting from the dissolution of the primary mullite phase in the glassy phase, giving acircular shaped crystals; and finally, the platy-like mullite, formed by the  B, D, F, and H) images at higher magnification dissolution-diffusion reaction of alumina and silica.This indicates that α-alumina grains started to react with the siliceous glassy phase to form mullite. Thereby, they lose their original shape.So mullite appeared as platy-like shaped crystals sticking together, forming a bundle of laminated crystals adhering to the relics of alumina grains (Fig. 8 F).These results are consistent with XRD results, where the intensities of the α-alumina peaks started to decrease at 1400°C.At 1500°C, the resulting microstructure appeared more compact with less pore volume (Fig. 8 G).Meanwhile, the aspect ratio of the acircular mullite increases (crystals >5 μm) forming an interlocked structure (Fig. 8 H).
It could be concluded that the fracture behavior of the samples fired at 1200 and 1300°C is mainly transgranular, as there is no glassy phase in those samples, whereas the samples fired at 1400 and 1500°C show mainly an intergranular fracture due to the presence of the glassy phase, as the glassy phase could bond the mullite crystals together enough to undergo the intergranular fracture.
EDS analysis of the different phases formed is shown in Fig. 9.It is worth mentioning that there are two different compositions detected for columnar primary mullite, one satisfying the mullite formula (3Al 2 O 3 .2SiO 2 ) (Fig. 9 A), while the other is slightly higher in silica content (Fig. 9 B).
The secondary mullite that crystallized out of the glassy phase was richer in silica than the primary mullite (Fig. 9  C).The same result was reached by Lee et.al.[13].They demonstrated that the chemical composition of primary mullite is rich in alumina, while that of secondary mullite is rich in silica.
On the other hand, for the platy-like crystal mullite, EDS revealed that its composition was higher in alumina, as expected (Fig. 9D).The EDS of unreacted alumina grains was also shown in Fig. 9E.

Effect of Processing Parameters
The microstructure of the PP5 and PP15 groups and WP5 and WP15 groups sintered for 2 hours at 1500°C is shown in Figs. 10 and   The microstructure of the PH5 and PH20 groups and WH5 and WH20 groups sintered for 2 hours at 1500°C is shown in Figs. 12 and 13, respectively.
The general trend for the samples processed using PVA was characterized by large voids as PVA is a large molecule (Fig. 12A-D), while the microstructure of the samples processed using water was characterized by an interconnected porous structure.In addition, the distribution of pores when water was used was more homogenous (Fig. 13A-D).
Also, the samples processed by uniaxial pressing show a more dense structure than those processed by hand pressing with a given amount of either water or PVA, which is consistent with the bulk density results (see Fig. 6).In summary, the effect of using the uniaxial pressing and water as binder on the densification is more pronounced than the hand pressing and PVA as binder, which corresponds to the good green body packaging.

Mechanical Properties of the Prepared Samples
The compressive strength of the sintered mullite samples at different temperatures is shown in Fig. 14.The main criteria that influence the compressive strength are the type of phases developed, grain size, presence of the glassy phase and porosity [29].The values of the compressive strength increased for all batches over the firing temperatures from 1300 to 1500°C.The size and shape of the crystalline phases developed might have played an important role as these crystalline phases form a mesh [19,20,30].As a result of the increase in aspect ratio of the acircular mullite formed, which forms an interlocked structure, i.e., the microstructure developed is responsible for the figures obtained.
For the WP5 and WP15 batches, the bulk density is maximum, so the expected compressive strength is maximum (154 MPa), as confirmed in Fig. 14.On the other hand, the batches PH5 with a minimum value of bulk density showed a minimum compressive strength of 7.6 MPa.

Mechanism of Mullite Formation
In the kaolinite-alumina mixture, several studies reported that the mechanism of the crystallization of primary mullite from metakaolinite [20,31,32] occurs around 1000°C by instantaneous mullite nucleation after the dehydroxylation of kaolinite reaction.It is accelerated by the almost ideal molecular mixing of alumina and silica polyhedra in the structure.i.e., mullite nucleates by short range diffusion [33].Through the phase boundary, the mullite crystallizes out, attaining a preferred orientation with the c axis parallel to <110> of the Al-Si spinel phase and metakaolinite.The Al-Si spinel phase caused the collapse of the metakaolinite structure, whereas the growth of the mullite Fig. 12 FE-SEM micrographs of samples sintered for 2 h at 1500°C.A) PH5 and C) PH20, (B,D) at higher magnification Fig. 13 FE-SEM micrographs of samples sintered for 2 h at 1500°C.A) WH5 and C) WH20, (B,D) at higher magnification crystals is accelerated by an instantaneous nucleation process through short distance diffusion, thereby preserving the original platy morphology of the kaolin [34].
While the secondary mullite crystallization took place through the dissolution of the primary mullite phase in the glassy phase, giving acircular shaped crystals [35].
While the mullite formed by the diffusion reaction of the siliceous liquid phase into the alumina grains gave platy mullite by the mechanism termed dissolution-diffusion, in some studies this mullite is called tertiary mullite [36].
Tarvornpanich [37] reported that kaolin clay fired for 3 h at 1200°C contained scaly primary and rod-like ~1-2μm long secondary mullite.He explained the formation of primary mullite in the area once rich in clay and the formation of rod-like mullite in the area containing impurities, which act as flux to facilitate the liquid formation on firing, allowing the crystallization of longer crystals.In another study, Percival et.al. [38] declared that, in kaolinite at 950°C, the metakaolinite is completely decomposed.The resulting spinel-type phase, which is associated with amorphous SiO 2 and some poorly crystalline "primary" mullite, is ɤ-Al 2 O 3 (crystalline) rather than an Al-Si spinel.At >1100 °C, secondary mullite therefore forms primarily from the ɤ-Al 2 O 3 /amorphous SiO 2 reaction and the recrystallization of primary mullite, whereas excess amorphous SiO 2 eventually crystallizes as cristobalite.
Aza et.al. [12] stated that the formation of primary mullite, side by side with a cubic phase, Si-Al spinel, and an amorphous silica-rich phase, was reached at ~980-992°C.From~1136°C growth of mullite crystals occurs, and at T > ~1200°C crystallization of high-temperature cristobalite (SiO 2 ) from a Si-rich amorphous phase takes place.Additionally, in the Si-rich amorphous phase formed at kaolinite-muscovite interfaces, secondary mullite crystallization occurs at ~1300°C.The impurities in the starting kaolin can induce a liquid phase during firing.
Several studies attributed the secondary mullite formation in the kaolinite-alumina mixture to the dissolution of alumina by the silica-rich liquid containing impurities, which form a transitory liquid phase from which mullite nucleated and grew by a "solution-precipitation" mechanism, yet it was reported by authors [12, 14, 25-28, 31, 32, 39, 40] as secondary mullite.
In the present study; the secondary mullite arose from the dissolution of the primary mullite phase in the glassy phase, giving acircular shaped crystals.Meanwhile, the only stoichiometric mullite composition was achieved by the crystallization of metakaolin.The mullite crystallizing out of the liquid phase was rich in silica, as determined by EDS, i.e., Al/Si =2.1, while that resulting from alumina grains was rich in alumina, i.e., Al/Si=5.1.The studied system is a non-equilibrium state.This may be attributed to the fact that the siliceous liquid phase present was not enough to dissolve both the primary mullite and the mullite formed by the diffusion reaction of silica into the alumina grains to recrystallize an acircular secondary mullite with a stoichiometric composition.So the results are three distinct polymorphs of mullite in the fired samples, as identified in the microstructure studied.

Conclusion
• The primary mullite crystals preserve the hexagonal platy shape of the original structure of the kaolinite mineral.• In a kaolinite-alumina system, the microstructure of the formed phases is affected by several parameters, such as temperature; i.e., secondary mullite starts to appear at 1400 °C by the dissolution of the primary one in the glassy phase.• The secondary mullite formed in such a condition in this study exhibits acircular shaped.
Fig. 14 The compressive strength versus temperature for mullite bodies • Mullite bodies processed by uniaxial pressing utilizing 15 wt.% water at 1500 °C for 2 h exhibited the highest compressive strength of 154 MPa and bulk density of 2.1 g/cm3, as well as an open porosity of 35%.• The formation of the interlocked mullite crystals and grain growth hinder the densification from 1400 to 1500 °C, while the interlocked structure is reasonable for the high mechanical properties of the fired bodies.• The type and amount of the plasticizer, pressing mode, and sintering temperature can drastically affect the final product properties and, accordingly, their applications.• The results showed that the porous mullite bodies prepared in the present work could potentially be used to make ceramic filters.
X-ray powder diffraction (XRD) patterns of samples were carried out on a Rigaku D/Max-3c X-ray diffractometer utilizing a CuKα radiation and operated at 40 kV and 50 mA.The bulk density and open porosity were measured for all samples by Archimedes' method.The linear change of the samples at different temperatures was calculated by linear change (%) = [(E − E′)/E] × 100, where E refers to the diameter of the sample before firing, while E′ refers to the sample diameter after firing at elevated temperatures.The compressive strength was measured for all samples by (Matest 24030 Brembate Sopra -Italy, which has a maximum load capacity of 2000 KN.The test samples are in pellets

Fig. 2
Fig. 2 FE-SEM images of the A) kaolin clay and B) α-alumina

Table 1
List of prepared samples with different methods

Table 2
The chemical composition of local Egyptian kaolin

11 ,
Fig. 9 EDs spot analysis of the red arrow