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Abstract

This research aimed to assess how coarse bauxite affects the microstructure and surface-related characteristics of refractory made using Iraqi flint, bauxite and kaolin raw materials. The study focused on determining how porosity, mullite morphological features and grain packing occur with the addition of coarse bauxite (≤ 1.18 mm) to flint refractories and what effect those features have on the mechanical properties of hardness and surface roughness. The originality of this research lay in the deliberate introduction of coarse bauxite as an alumina source. As such, the addition resulted in a poorly bonded mullite phase; a microstructural characteristic not examined in detail in flint systems nor previously reported in refractories produced from Iraqi raw materials. A distinct correlation of structure and property was observed in all samples (10–40 wt% coarse bauxite). Porosity reduction in the 10% and particularly the 20% samples resulted in greater densification and subsequently smoother surfaces, along with the highest measured hardness. Higher additions of coarse bauxite (30–40%) led to the development of larger, less densely interconnected mullite, which in turn created surface roughening and reduced hardness. While the 40% sample had the lowest measured hardness of all compositions, it was a functionally critical condition due to the rough surface texture and open mullite network developing at the furnace hot face, enhancing thermal stability through crack deflection, increased thermal shock resistance, and improved mechanical keying for protective coatings. While 20% bauxite delivers optimal mechanical strength, the 40% offers a thermally resilient, rough surface essential for demanding furnace environments.

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