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Seven microporous MgO-Al 2 O 3 ceramics with an Al 2 O 3 content of 15-90 wt% were prepared using Al(OH) 3 and calcined magnesite as raw materials. A wet mixing process was employed during sample preparation to transform the calcined magnesite with a larger particle size to smaller Mg(OH) 2 particles. The in situ decomposition synthesis method and the Kirkendall effect were utilized to produce and control the pore structure of the microporous MgO-Al 2 O 3 ceramics. There were two kinds of pores in the microporous MgO-Al 2 O 3 ceramics. The first one resulted from the in situ decomposition of Al(OH) 3 and Mg(OH) 2 particles, which were small and equally distributed. Another one originated from the position of the Mg(OH) 2 particles due to the Kirkendall effect caused by MgO diffusion. They were similar in size to the Mg(OH) 2 pseudomorph particles. Simultaneously, the Al 2 O 3 content affected the packing behavior and the spinel formation, which changed the characteristics of the pores and necks among the particles. These mechanisms also affected the strengths of the microporous MgO-Al 2 O 3 ceramics. Thus, when the Al 2 O 3 content was 45-90 wt%, the microporous MgO-Al 2 O 3 ceramics had a high compressive strength (10.0-18.3 MPa) and apparent porosity (52.2%-58.4%).

Five microporous mullite-corundum refractory aggregates were prepared from Al(OH) 3 and kaolinite gangue through in situ decomposition synthesis technique. The effects of the sintering temperature (1400-1600 • C) and the particle sizes of raw materials (20.6-94.5 μm) on the microstructures and strengths of the aggregates were investigated through X-ray diffractometer, scanning electron microscopy, and energy-dispersive spectrometer etc., to find out the technological conditions to be controlled in industrial production. The higher sintering temperature promoted the reaction between Al(OH) 3 and kaolinite gangue, leading to the development of primary-mullite as well as the generation of secondary-mullite, which promoted the formation of the neck and improved the strength. Meanwhile, the dense mullite layers were formed continuously on the surface of Al(OH) 3 pseudomorphs, making the micropores inside the pseudomorphs become closed pores, which increased the closed porosity of the aggregates. The reduction of the particle sizes of raw materials changed the particle packing behavior, accelerated the rearrangement of the Al(OH) 3 pseudomorph particles during the process of reactive sintering, and then reduced the closed porosity. To realize the industrial production of microporous mullitecorundum refractory aggregate with high strength (103 MPa) and high closed porosity (16.1%), the sintering temperature should be at about 1600 • C, and the median diameter of raw materials should be at 94.5 μm.

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