Alkali-vapor attack and spalling of silica-mullite bricks in the transition zone

Silica-mullite bricks are built on mullite and silicon carbide. A dense glaze can form on the surface at high temperature; the bricks are strong and relatively low in conductivity, and have long been used as the working lining in the transition zone of cement rotary kilns, and as hanging brick in the kiln hood. Once alternative fuels and waste are fed to the kiln, the alkali-vapor load rises, and the path of damage becomes quite clear.

Liquid and alkali vapor formed by reaction between the cement burden and the brick enter mainly through the pores. Ca²⁺ and K⁺ in the liquid keep a high chemical potential and dissolve Al₂O₃ and SiO₂ at the liquid front, so the attack can keep moving inward. On cooling, crystals such as gehlenite, potassium feldspar and potassium chloride precipitate, and the original structure is pulled apart; the difference in thermal expansion between the dense hot end and the altered layer then grows, and cracks, spalls and fractures follow.

Aluminosilicate raw materials do not all answer alkali vapor in the same way. Bauxite with corundum and mullite as its main phases tends to form kaliophilite and to crack; once the structure loosens, gaseous alkali moves in more easily. Mullite-rich material can form leucite and a transient liquid at the surface, and the attack then stays in the skin. A grog with a high glass content forms more liquid with the alkali, and is more readily broken apart. If the alkali load in the transition zone is heavy, a glaze alone is not enough: porosity and phase composition have to be considered together.

Hanging brick in the kiln hood sits where the gas turns and radiation is more intense; thermal shock and alkali are present there as well. The working lining of the transition zone and the hanging brick of the hood belong in the same picture of gas chemistry and thermal shock.