Advanced silica sol-bonded cordierite castables: A comparative study of cement-free versus conventional CAC-bonded systems

The production of monolithic refractory components for thermal units operating at moderate temperatures but exposed to frequent and severe thermal cycling remains challenging due to the long drying times and high energy consumption associated with conventional calcium aluminate cement–bonded castables. This study addresses these limitations through the development and evaluation of a cement-free cordierite-based refractory castable bonded with silica sol. The objective is to assess the feasibility of replacing calcium aluminate cement with silica sol and to compare the structural, thermal, and mechanical properties of the proposed material with those of a conventional cordierite refractory castable. The investigated materials were prepared using identical cordierite aggregates and different binders. Phase composition was analysed by X-ray diffraction (XRD), thermal behaviour was investigated using thermogravimetry and differential thermal analysis (TG/DTA), and dimensional changes were evaluated via dilatometry. Mechanical performance was assessed by cold compressive strength measurements after drying at 110°C and firing at 1200°C. Additionally, the temperature dependence of Young’s modulus was determined via the impulse excitation technique (IET) during one complete thermal cycle (i.e. heating and cooling) with a maximum temperature of 1200°C. The silica sol–bonded castable exhibited a significantly higher cold compressive strength after firing at 1200°C, reaching 42.3 MPa, compared to only 8.9 MPa for the calcium aluminate cement–bonded material. The Young’s modulus values were 28–62 % higher. The relative length change at 1200°C was 0.51% for the silica sol system and 0.47 % for the cement-based castable, indicating comparable thermal deformation behaviour. These results confirm that the cement-free cordierite castable combines superior mechanical strength and higher Young’s modulus with acceptable thermal expansion characteristics, making it a promising material for refractory components subjected to intensive thermal cycling.