Due to its hydration, the use of fine magnesia in refractory castables is a key concern. Nevertheless, recent studies pointed out that the volume expansion associated with this reaction can be used to develop strength in cement-free castables, resulting in magnesia based binder. Considering this aspect, the objective of the present work was to evaluate the potential of MgO and MgO-SiO2 as binders for refractory castables. With this purpose, three different sources of magnesia were used, one dead-burned and two caustic ones. For the MgO-SiO2 system different amounts of fumed silica (1wt.%, 2wt.% and 4wt.%) were added to the castable compositions in order to avoid the damages caused by magnesia hydration. The results attained pointed out that for the sinter calcined above 1500 °C, the low CaO/SiO2 ratio is one of the important issues to inhibit the deleterious effects associated to the hydration. For the caustic magnesia (calcined below 1200 °C) the free SiO2 and CaO contents are fundamental for the binding potential and final physical integrity of the product. Furthermore, the addition of fumed silica was presented as an efficient mechanism to control the hydration of different MgO sources; however, high levels of fumed silica can result in a marked castable flow reduction.
magnesia hydration; magnesia based binder; refractory castables
