Colloidal silica-bonded refractory castables have been developed recently. It was found that colloidal silica is one of the best binders and can substitute other binders such as cement in No Cement Castable (NCC) and Ultra Low Cement Castable (ULCC) refractories. Also composition of colloidal silica with appropriate additives resulted in a gel form which makes the initial strength. Moreover, the nano-size silica particles are extremely reactive in high alumina castables and may encourage mullite formation in the microstructure. In the current study, four castables were prepared. The sample containing 6wt % microsilica was a reference, then microsilica was replaced by different amounts of colloidal silica (2.5, 5, 7.5 wt %). Silica and water content was kept constant. It’s concluded that the castables containing the optimum amount of silica sol shows a remarkable increase in both castable fluidity and mechanical strength (CCS and MOR) in dried and sintered states. It was also found that nanosilica particles increase the rate of needle-shaped mullite formation during sintering at 1400°C. According to FTIR results, the addition of Calcium Aluminate Cement (CAC) to the silica sol may be responsible for the increment of siloxane bridges (Si-O-Si).
