Low-thermal-conductive reaction-bonded silicon nitride (RBSN), with a thermal conductivity of 8.08 W/(m·K), was developed by the addition of mullite micro-powder as a rare-earth-free oxide sintering agent to silicon nitride. This addition was expected to generate a glassy grain boundary as well as a solid solution of oxygen across the microstructure of the RBSN, which leads to low thermal conductivity. During the fabrication of a heat-insulating component with a hollow structure made of the RBSN via slip casting and subsequent reaction sintering, it was found that the viscosity of aqueous silicon-mullite slurries decreased evidently by the addition of the mullite micro-powder with ammonium polycarboxylate as a dispersant. The viscosity decreased even though the pH (7–8) of the slurries were maintained, and the particle size distributions did not vary significantly. At the effective mullite content when the viscosity decrease was attained, the ratio of the number of particles of mullite to silicon was approximately 1. Therefore, the viscosity decrease in Si slurry (50 vol%) was primarily caused by the steric hindrance due to the adsorption of dispersant by the mullite particles, preventing the direct contact among the silicon particles, rather than by electrostatic repulsions among the silicon particles. This was also explained through the modality of bimodal dispersion. Thus, it was found that the mullite not only acts as a sintering agent but also facilitates viscosity decrease of highly concentrated aqueous Si slurry, which successfully enables the integration of the slip casting and reaction sintering processes.