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Hydration mechanisms of magnesia-based refractory bricks Zhou, Shuxin

Abstract

Hydration of magnesia-based refractory bricks could occur during storage, during drying after installation, or in service, and the hydration would cause damage to refractory bricks and furnace linings. In order to understand the hydration mechanisms of magnesia-based refractories, three types of bricks were chosen: magnesia, magnesia-spinel and magnesiachrome bricks, and hydration tests were performed at 60 to 130°C in 98% relative humidity, water, and steam. The variation of the modulus of elasticity (MOE), determined by the impulse excitation technique (IET), as well as apparent porosity, air permeability, pore size and pore size distribution were correlated with the hydration data. The phase compositions and microstructure modifications were also studied on selected specimens by XRD and SEM/EDS. Based on the experimental results, a hydration model of "cylindrical pore model" was established and a hydration mechanism was suggested. The hydration takes place in three stages. In the first stage, which is controlled by chemical reaction, a film of brucite forms and MOE quickly increases. During the second stage, which is controlled by diffusion, the MOE gradually reaches a maximum value followed by a slow, decrease due to the formation of cracks on the film and weakening of grain boundaries until the MOE reaches the initial value. At this point, the third stage, corresponding to "dusting", starts to take place until the brick disintegrates. The results also indicate that the hydration rate increases with rising temperature and the CaO/SiO₂ ratio. The variations in permeability and porosity are opposite to that in MOE. A nondestructive method - IET to assess the hydration degree of magnesia-based refractory bricks was proposed.

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