Phosphate chemical binder as an anti-hydration additive for Al2O33MgO refractory castables

Even though many investigations have focused on describing the kinetics of both MgO hydration and Mg(OH)2 decomposition, few studies have addressed the understanding of the mechanisms to control or to modify the reaction of magnesia with water. Silica is the most applied anti-hydration additive in MgO-containing castables. The use of silico-phosphates (SiO2 rich and presenting higher reactivity and solubility) seems to be a promising alternative to induce a faster generation of magnesium–silicate–hydrated gels on the MgO grain surface, inhibiting brucite formation. In the present work, the performance of a commercial phosphate chemical binder (combined or not with silica fume) in Al2O33MgO refractory castables and MgO aqueous suspensions was evaluated. Thermodynamic simulations, thermogravimetric measurements, X-ray diffraction, splitting tensile and hot elastic modulus tests were carried out to understand the additive performance and the characterization of the castables properties. According to the simulated Pourbaix diagrams, two different gels [Mg3Si2O5(OH)4 or Mg3Si4O10(OH)2] can be formed in the structure of the samples (depending on the Mg:Si molar ratio), halting the MgO hydration. Due to the high amount of MgO (6 wt%) contained in the designed Al2O33MgO castables, the chrysotile-like phase [Mg3Si2O5(OH)4] should be the main compound formed during the refractories' processing. Moreover, the blend of silica fume and the phosphate-based additive consisted of the most effective route to stop the Mg(OH)2 formation so that castables with high flowability, mechanical strength levels and elastic modulus values can be prepared.