Elaboration and Characterization of Mullite Refractory Products from Moroccan Andalusite
Enrichment of schist mined in Morocco resulted in an extract essentially made of andalusite, quartz and muscovite, which was investigated with the aim to synthesize mullite refractory materials. X-ray diffraction analysis was used to assess phase transformations upon heating at different temperatures. Mullite formation starts as early as 1200℃ and seems almost total at 1450℃. This process correlates with a weak shrinkage (3.5% at 1450℃) and the formation of an important glass phase. Green compacts obtained by unidirectional pressing the extract powder were heated under air in the temperature range 1200℃ - 1600℃. Chemical and mechanical properties of these materials were investigated and correlated with their microstructure.
Mullitization of Andalusite in Refractory Bricks
Mullite formation was studied in andalusite bricks fired in the range 1300-1600°C Mullitization of andalusite grains retains the grain shape and bulk chemistry. An andalusite monocrystal is transformed into a composite made of a mullite monocrystal (≃ 80 %)with a capillary network filled with a silica rich glass. The capillaries form interconnected tubes parallel to the c axis, common to the mullite crystal and the parent andalusite grain. Heating andalusite crystals reveals that only ≃10-15% of the glass is expelled out of the host grain. This observation is supported by volume balance calculations In spite of a high amount of glass, andalusite based refractories exhibit an excellent behaviour in industrial use resulting from the trapping of most of the glass in the mullite.
Mechanical behaviour and durability of high volume fly ash cementitious composites
The purpose of this research is to separate different morphological particles of ASTM class F fly ash, and study their effect on mechanical behaviour and durability of high volume cementitious mixtures. In this research wet separation of raw fly ash is carried out, which resulted in three layers of different morphological particles. The first layer of particles float, comprise of about 1-5% of fly ash, is identified as cenospheres or hollow spheres. The second layer of particles is measured to be 55-60% of raw fly ash and consisting of porous spherical and rounded particles rich in Si and Al. The third layer particles is measured to be about 35-40% of raw fly ash. High volume fly ash cementitious composites containing second or third layer particles are tested under compression and bending, highlighting a higher strength and ductility in comparison to cementitious ones containing raw fly ash particles. KEYWORDS. Cementitious matrix; Raw fly ash; Second layer particles; Third layer particles.