Phase Composition and Pore Evolution of Porous Periclase- spinel Ceramics Prepared from Magnesite and Al(OH)3

Porous periclase-spinel ceramics with 30.5-43.6% apparent porosities were prepared from magnesite and Al(OH)3 via an in-situ decomposition pore-forming technique. The reaction process of the pseudomorphs of magnesite and Al(OH)3 was analyzed through X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and a microscopy measurement method, and the effects of the sintering temperature and Al(OH)3 content on the phase composition and microstructure were investigated. It is found that the decompositions of magnesite and Al(OH)3 can create a large amount of micro-sized pores in the pseudomorphs of magnesite and Al(OH)3, and that sintering temperature promotes the reaction between the pseudomorphs of magnesite and Al(OH)3 by the interdiffusion of Mg and Al through liquid. The Al(OH)3 content affects the formed spinel and the liquid phase between periclase particles, which influences the pore size and porosity.


Introduction
Magnesia-chrome refractories have been widely used in the burning zone of the cement kiln in China, due to their excellent high temperature properties.However, Cr 6+ leaching from magnesia-chrome refractories may result in serious environmental pollution, which limits the application of the magnesia-chrome refractories.Periclase-spinel refractory is a kind of chrome-free refractory to substitute the magnesia-chrome materials, but the shell temperature at the external surface of the cement kiln could exceed 300-400°C in service leading to severe heat loss because the periclase-spinel refractories are manufactured by using dense aggregates as main raw materials [1].In order to save energy, it is necessary to develop a kind of lightweight periclase-spinel aggregate to substitute the dense one, which could be obtained by crushing the porous periclase-spinel ceramics [1].
Porous spinel ceramics can be prepared through an extrusion molding method, a foaming technique and a template method, etc. [2][3][4][5] However, the porous periclase-spinel ceramics prepared through the mentioned methods were not suitable as the lightweight refractory aggregates because of their large pore size, complex production processing and high cost.In-situ decomposition pore-forming (ISDP) technique is an alternative and environmentally friendly method to prepare the lightweight aggregates, which could utilize the decomposition of raw materials to create micro-sized pores [6].For example, the porous corundum-mullite, cordierite and cordierite-mullite ceramics with high porosities, micro-sized pores and high strengths have been prepared through the ISDP technique [7][8][9][10].
In our early works, the porous spinel ceramics have been prepared from magnesite and Al(OH) 3 via the ISDP technique, and the effects of the Al(OH) 3 content (22.7-88.9wt%)and the amount of TiO 2 added on the phase composition and microstructure have been investigated [1,11].But until now, the reaction and sintering process of the pseudomorphs of magnesite and Al(OH) 3 has not been investigated, and the effect of the reaction and sintering process on the pore structure has not been understood.In the present work, the reaction and sintering process was analyzed through preparing the porous periclase-spinel ceramics from magnesite (83.5-100wt%) and Al(OH) 3 (0-16.5wt%)at different sintering temperature, and the effects of the sintering temperature and Al(OH) 3 content on the phase composition and microstructure were investigated.

Experimental procedure
Magnesite and Al(OH) 3 were used as raw materials.Their chemical compositions are listed in Tab.I.The average particle sizes of magnesite and Al(OH) 3 , measured by using a laser particle size analysis (Mastersizer2000), were 15.063μm and 88.898μm, respectively.Four powder mixtures, containing 0wt%, 5wt%, 10.7wt% and 16.5wt% Al(OH) 3 , were denoted as A 0 , A 5 , A The above starting powder mixtures were initially mixed for 2 hours in polyurethane pots using alumina balls, and then were pressed into cylinders (50 mm in height and 50 mm in diameter) under a pressure of 100 MPa.The four kinds of green compacts were dried at 110°C and then heated at 1600°C for 3 hours in an electric furnace.Additionally, compacts A 0 and A 16.5 were also heated at 1000°C and 1400°C for 3 hours to study the effect of sintering temperature.
The apparent porosities were measured based on the Archimedes' principle using kerosene as the medium.The phases transformations were characterized by X-ray diffractometry (XRD, Philips Xpert TMP) with Cu Kα radiation (λ= 1.54187 Å).The XRD patterns were recorded in the range of 10-90° with a scanning speed of 2° per minute.The relative phase contents of specimens and the lattice constants of spinel were calculated by the X'Pert Highscore Plus software (Version 2.2.1).The microstructures and compositions of the specimens were measured by a scanning electron microscopy (SEM) with EDAX (Philips XL30).The pore size distributions (PSDs), the average pore sizes (APZs) and the percentages of pore area of the magnesite and Al(OH) 3 pseudomorphs were measured through by a microscopy measurement method [12] through an optical microscope (Axioskop 40).In the microscopy measurement, the pore sizes were calculated based on an assumption of a round pore model, one value was obtained through analysis of 10 images, and the standard deviation of the measurement was ±15%.

Apparent porosity and bulk density
The bulk densities (BDs) and apparent porosities (APs) of the specimens A 0 and A 16.5 sintered at different temperatures are shown in Fig. 1.With an increase of temperature from 1000°C to 1600°C, the AP of specimen A 0 decreases from 63.5% to 30.5%, and the BD increases from 1.25g/cm 3 to 2.49g/cm 3 .When the sintering temperature is 1000°C, the APs and BDs of the specimens A 0 and A 16.5 are very close, respectively.With increasing the sintering temperature to 1400°C and 1600°C, the APs of specimens A 16.5 are remarkably higher than those of specimens A 0 .The BDs and APs of specimens sintered at 1600°C are shown in Fig. 2. Specimens A 0 and A 5 have similar BDs and APs, which are 2.49-2.48g/cm 3 and 30.5-30.8%,respectively.Whereas, for specimens A 10.7 and A 16.5 , their APs and BDs change greatly.The AP and BD of specimen A 10.7 are 36.0%and 2.28 g/cm 3 respectively, and the AP and BD of specimen A 16.5 are 43.6% and 2.01 g/cm 3 respectively.

Phase composition
The XRD patterns of specimens A 16.5 sintered at different temperatures are shown in Fig. 3, and the relative phase contents of the specimens are listed in Tab.II.When the sintering temperature is 1000°C, the phases of specimen A 16.5 are periclase in majority, and minor spinel and corundum.When the sintering temperature increases to 1400°C, the corundum phase disappears, and the phases are periclase and spinel; with a further increase of sintering temperature to 1600°C, the relative ratio of the phases negligibly changes.The XRD patterns of specimens sintered at 1600°C are shown in Fig. 4, and the relative phase contents of the specimens are listed in Tab.III.The phase of specimen A 0 is only periclase, and those of other specimens are periclase and spinel.From specimen A 5 to specimen A 16.5 , the relative content of spinel increases gradually.

Pore characteristics
The pore size distributions (PSDs) of specimens A 0 and A 16.5 sintered at different temperatures are shown in Fig. 5. Unimodal PSDs are observed in specimens A 0 and A 16.5 sintered at 1000°C and 1600°C, whereas bimodal PSDs are observed in specimens A 0 and A 16.5 sintered at 1400°C.Two kinds of pores exist in the specimens: one is the small pores with pore size less than 10μm, and the other is the large pores with pore size more than 10μm.When the sintering temperature is 1000°C, the small pores are dominant in specimens A 0 and A 16.5 and their average pore sizes (APZs) are 1.42μm and 1.73μm, respectively.With an increase of sintering temperature, the peak value of the small pore decreases, whereas, that of the large pore increases and the curve of PSD moves rightward.When the sintering temperature is 1600°C, the APZs of specimens A 0 and A 16.5 are 16.56μm and 75.3μm, respectively.The PSDs of specimens sintered at 1600°C are shown in Fig. 6.Unimodal PSDs are observed in all the four specimens.For specimens A 0 , A 5 and A 10.7 , the APZs are mainly in the range of 20-50μm; and from specimen A 0 to specimen A 10.7 , the peak value of PSD increases slightly.For specimen A 16.5 , the curve of PSD moves rightward greatly, and the peak value of PSD increases sharply.The APZs of specimens A 0 , A 5 , A 10.7 and A 16.5 are 16.5μm, 18.7μm, 16.5μm and 75.3μm, respectively.Fig. 6.Pore size distributions of specimens sintered at 1600°C.

Microstructure
The microstructures and EDS results of specimens A 16.5 sintered at different temperatures are shown in Fig. 7.When the sintering temperature is 1000°C, the Al(OH) 3 and magnesite pseudomorphs with micro-sized pores exist in the specimen, and the reaction between the pseudomorphs does not take place obviously (Fig. 7(1)).With an increase of temperature to 1400°C, magnesite pseudomorphs become dense, and most Al(OH) 3 pseudomorphs are destroyed and transferred into spinel through the reaction with MgO.But in this specimen sintered at 1400°C, an incompletely destroyed Al(OH) 3 pseudomorph is observed and shown in Fig. 7(2).In this pseudomorph, the Al 2 O 3 particle with a large amount of micro-sized pores is in the center, the porous spinel is at the circumference, and the dense ring is between the center and the circumference.The dense ring consists of MgO, Al 2 O 3 , SiO 2 and CaO, which maybe spinel and liquid phase at high temperature.With a further increase of temperature to 1600°C, porous spinel becomes dense, and some spinel rings with a large pore are observed in Fig. 7(3).The microstructures of specimens sintered at 1600°C are observed in the experiment, and those of specimens A 0 and A 10.7 are shown in Fig. 8.It can be seen from Fig. 7(3) and Fig. 8, specimen A 0 consists of dense periclase particles, glass phase and pores; whereas, for the specimens A 10.7 and A 16.5 , the pore area in the microstructure increases obviously, and the particle sizes of spinel are larger than those of periclase.It should be noted that, in specimen A 0 , all glass phases are distributed between periclase particles, however, in specimens A 10.7 and A 16.5 , most of glass phases are distributed in the spinel particles, and little are found between periclase particles.

Discussion
On the base of the above experimental results, the reaction and sintering process of magnesite and Al(OH) 3 can be deduced, as shown in Fig. 9. Firstly, there are many point contacts or small area contacts on every particles of magnesite and Al(OH) 3 (Fig. 9(1)).At elevated temperature, magnesite and Al(OH) 3 decompose at 300ºC and 670ºC, respectively, and the pseudomorphs with a large amount of micro-sized pores are formed (Fig. 9(2)).When the sintering temperature is higher than the melting point of the glass phase between periclase particles, the rearrangement of periclase particles and the penetration of liquid into Al(OH) 3 pseudomorphs occur (Fig. 9(3)).With an increase of the sintering temperature, Al 2 O 3 and MgO dissolve into liquid, and then spinel precipitates (Fig. 9(4)).The interdiffusion of Al 3+ and Mg 2+ through liquid phase promotes the reaction between the pseudomorphs of magnesite and Al(OH) 3 ; simultaneously, the rearrangements of particles in the two pseudomorphs are also activated (Fig. 9(4)).Especially in Al(OH) 3 pseudomorphs, the capillary force makes Al 2 O 3 grains in the center rearrangement, and leaves a pore behind (Fig. 9(4)).In this stage, the volume of small pores decreases, and the pores located at particle intersections grow (Fig. 5).Finally, with a further increase of sintering temperature, the reaction between the two pseudomorphs almost finishes, and the liquid phase sintering dominates in this stage, which makes the particles more dense (Fig. 9(5)) and the pores growth (Fig. 5).The reaction and sintering process explains why most liquid phases (Figs.7(3), 8(2)) and a pore (Figs.7(3), 9(5)) are in the spinel particle.
Obviously, the phase compositions and pore structures of porous periclase-spinel ceramics via the ISDP technique depend on the four factors as following: 1) The porosity of green compact, which depends on the packing behavior of raw materials and the molding pressure of specimen; 2) The decompositions of Al(OH) 3 and magnesite, which can create a large amount of micro-sized pores; 3) The volume expansion, which derives from the formation of spinel; 4) The sintering conditions.
According to the packing behavior of bimodal powder mixtures [13], the increase in the Al(OH) 3 content in the specimen should be associated with a decrease in the porosity of green compact, which is contrary to the results shown in Fig. 2. Additionally, the percentages of pore area of the magnesite and Al(OH) 3 pseudomorphs in the microstructure of specimen A 16.5 sintered at 1000ºC are 51.4% and 53.7%, respectively, which means the decompositions of magnesite and Al(OH) 3 can create similar porosities, and thus, the factors 3 and 4 should play more important role in affecting the pore evolution.
The expansion caused from the formation of spinel has two reverse effects on the porosity and the pore structure: on the one hand, the formed spinel fills in the pores between periclase particles, which would reduce the pore size and porosity; on the other hand, the formed spinel broadens the distance between periclase particles, which would increase the pore size and porosity.In present work, the particle size of Al(OH) 3 is almost six times bigger than magnesite, resulting that the particle size of spinel is bigger than that of periclase, thus the formed spinel broadens the distance between periclase particles.The lattice constant of spinel and the expansion of specimen caused by the formation of spinel (1600ºC) are listed in Tab.IV.For specimen A 5 , the expansion caused by the spinel formation is only 0.83 vol%, which maybe equal to the densification caused by the packing behavior, so the apparent porosity changes little comparing with the specimen A 0 .For specimens A 10.7 and A 16.5 , the expansions caused by the formation of spinel increase to 1.95 vol% and 2.47 vol%, which maybe higher than the densification caused by the packing behavior, so the apparent porosities increase obviously.

Tab. IV Lattice constant of spinel and expansion of specimen caused by formation of spinel (1600ºC).
Lattice constant of spinel (Å) Expansion of specimen (vol%) After the reaction, the liquid phase sintering dominates the final process.For the specimen A 16.5 , the reaction almost finishes at 1400 ºC (Fig. 3).With an increase of sintering temperature to 1600 ºC, the phase composition changes little, whereas, the liquid phase sintering is promoted, which results in the decrease of the apparent porosity.When the sintering temperature is 1600 ºC, from specimen A 0 to specimen A 16.5 , the amount of liquid phases between periclase particles decreases because of the penetration of liquid phase into the Al(OH) 3 pseudomorphs, which decreases the sintering degree of periclase particles, and then increase the pore sizes and porosities.

Conclusion
(1) The decompositions of magnesite and Al(OH) 3 create a large amount of microsized pores in the pseudomorphs of magnesite and Al(OH) 3 .The sintering temperature promotes the reaction between the pseudomorphs of magnesite and Al(OH) 3 by the interdiffusion of Mg 2+ and Al 3+ through liquid.With an increase of sintering temperature, the Al(OH) 3 pseudomorph is destroyed, and the periclase particle becomes dense, which results that the small pores disappears and the large pores grow.
(2) When the sintering temperature is 1600°C, with an increase of Al(OH) 3 content from 0 to 16.5wt%, on the one hand, the relative content of the formed spinel increases from 0 to 32wt%, which broadens the distance between periclase particles; on the other hand, the amount of liquid phases between periclase particles decreases because of the penetration of liquid phase into the Al(OH) 3 pseudomorphs, which decreases the sintering degree of periclase particles.The two aspects both make the pore size and porosity increase.

Fig. 1 .
Fig. 1.Bulk densities and apparent porosities of specimens A 0 and A 16.5 sintered at different temperatures.

Fig. 5 .
Fig. 5. Pore size distributions of specimens A 0 and A 16.5 sintered at different temperatures.

Fig. 7 .
Fig. 7. Microstructures and EDS results of specimens A 16.5 sintered at different temperatures.
Relative phase contents of specimens sintered at 1600°C (wt%).