Production of ceramics involves multiple stages, among which sintering is critical for achieving desired properties. The sintering environment, particularly the sintering atmosphere, influences the densification and phase transformation of ceramics. Understanding the phase transformation and densification is essential for optimizing the microstructure and performance of the final ceramic products. This study investigated the effect of sintering atmosphere on the densification and phase transformation of the binary composition of red clay and black cinder. Red clay and black cinder are iron-rich silicates abundant in Lanao del Norte, Philippines. They were mixed in varying proportions, dry-pressed, and sintered at 1200°C under oxidizing (air) or inert (argon) atmospheres. Densification was evaluated through water absorption (WA) and apparent porosity (AP), while mechanical strength was evaluated via the Modulus of Rupture (MOR) test. Phase composition and microstructure were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM).

Argon-sintered samples exhibited enhanced densification, with water absorption and apparent porosity of 0.011% and 0.02%, respectively, compared to 0.25% WA and 0.43% AP in air-sintered samples. Correspondingly, the MOR increased by an average of 25%, reaching up to 47.57 ± 4.18 MPa under argon versus 36.77 ± 2.15 MPa in air. XRD results showed distinct phase differences between air- and argon-sintered samples. In air, the phases present were tridymite, mullite (Al₆Si₂O₁₃), and cristobalite (SiO₂ polymorph), while in argon, cristobalite was absent, with new observed phases of hercynite (FeAl₂O₄) and bytownite (CaAl₂Si₂O₈). SEM observations further confirmed a denser microstructure in argon-sintered samples. In conclusion, sintering atmosphere affects the densification and phase transformation of red clay–black cinder ceramics. Argon-sintered samples demonstrate enhanced densification and higher strength, suggesting potential applications in high-strength ceramics and refractories for preliminary kiln linings in basic environments, with performance under actual operating conditions requiring further assessment.