Thermo-Physical and Mechanical Characterisation of Pumice-Kaolin Composite Refractories with Multi-Objective Optimisation for Thermal Insulation Applications

Pumice-kaolin composites were produced from Nigerian raw materials to examine how pumice content (25, 50 and 75 wt%) and pumice particle size (75, 150 and 300 μm) affect the balance between thermal insulation and strength in fired refractories. A two-factor Taguchi L9 design was used, and all specimens were fired at 1200 °C for 1 h. Bulk density ranged from 1.44 to 2.00 g/cm3, as-fired flexural strength from 0.18 to 0.95 MPa and apparent thermal conductivity at a nominal 1000 °C from 1.33 to 1.68 W/m·K. Two formulations failed an apparent specific gravity check and were excluded from the analyses that use porosity or bulk density. No test cone deformed up to a kiln temperature of 1200 °C. Particle size governed thermal conductivity (85.8% contribution, p = 0.001). Grey relational analysis of the seven retained runs gave the highest grade to 25 wt% pumice with 75 μm particles (0.734; first in ten of thirteen sensitivity scenarios), and factor-level analysis placed the preferred region at 25 wt% pumice with 75 to 150 μm particles. The composites do not meet ASTM C155 limits for insulating firebrick, and their use as backup lining requires confirmation by replicated, load and service tests.