Controle da hidratação do óxido de magnésio: fundamentos físico-químicos e aplicação tecnológica
Santos Junior, Tiago dos
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Magnesium oxide has been proposed as an alternative binder to calcium aluminate cement in refractory castables with reduced amount of CaO. The binding effect is generated by the MgO hydration, resulting in brucite [Mg(OH)2]. The formation of this hydroxide is followed by volumetric changes which, depending on its magnitude, can induce cracks in the material. However, if the hydration reactions are controlled, it would be possible to obtain in situ Mg(OH)2, which could fill in the pores and increase the mechanical strength of the refractory. Considering this possibility and based on a kinetic model that describes MgO hydration as a nucleation and growth process, in this work the control of the kinetic of Mg(OH)2 formation in aluminous castables, was studied. Considering the bar resonance as the main characterisation technique for measuring the in situ elastic modulus, the effect of particle size, MgO reactivity, temperature, time of curing and drying on the kinetic of strengthening of castables containing MgO in the matrix, were evaluated. When finer or less reactive MgO were used, the brucite’s nuclei were more dispersedly formed, favouring the growth of Mg(OH)2 crystals. In order to increase the nuclei density and to limit the growth of the formed crystals, more nucleation sites were activated. Propionic, acetic and formic acids were selected for this purpose. Among those, formic acid activated a greater number of nucleation sites, controlling brucite formation and inhibiting the samples’ cracking. Due to the high degree of hydration, explosion of the samples during the first heating of the castables, was detected. Nevertheless, this drawback was eliminated by the addition of hydratable alumina or a microsilica-based additive to the designed compositions. As a consequence, aluminous castables bonded with MgO and formic acid were successfully prepared, leading to a novel processing route not yet found in the literature.
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