Citric acid role and its migration effects in nano-bonded refractory castables

In recent years there has been great interest in the use of colloidal silica suspensions as a binder source for refractory castables. However, the impact of additives on the overall performance of these materials is an important subject that has not been well explored and understood yet. Thus, this work investigates the migration effects of citric acid in colloidal silica-bonded high-alumina castables. Measuring the flowability of designed compositions over time has revealed that citric acid works as a retarder of MgO (the colloidal silica setting agent), resulting in enough workability (up to 90 min). However, depending on the added amount of this additive, it may migrate to the sample׳s surface during curing, decreasing the castable׳s permeability. FTIR spectroscopy and permeability measurements were used as the tools for investigating these effects. The results were evaluated by considering the adsorbability of citrate ions on alumina and magnesia surfaces in the pH range of the alumina-colloidal silica system. Finally, technical solutions were suggested to optimize the workability of high-alumina colloidal silica-bonded refractory castables.