Potential of alternative waste materials: rice husk ash and waste glass cullet with boric acid addition for low-fired unglazed tiles

This study aimed to investigate the potential of utilizing waste materials by adding boric acid (H3BO3) for producing low-fired unglazed tiles under low temperatures. Eighteen formulations containing rice husk ash (RHA), green glass cullet (GGC), and local kaolin clay (LKC) were constructed and divided into three groups with different RHA contents of 0, 10, and 20 wt%. Boric acid was also added with three amounts of 0, 2, and 3 wt% in mixtures. Specimens of these mixtures were produced by uniaxial pressing at 10 MPa and then fired at 900 º С for 1 h. The results showed that the formula of group B contained 10 wt% RHA, 60 wt% GGC, and 30 wt% LKC with the addition of 2 wt% boric acid. Moreover, the formula of group C contained 20 wt% RHA, 50 wt% GGC, and 30 wt% LKC by adding 3 wt% boric acid. Both formulas can achieve the ISO 13006 standard of ceramic tiles in terms of modulus of rupture and water absorption. Characterization of these formulas was investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), and the CIELAB colorimetric coordinates. SEM results confirmed that the glassy-phase and needle-like wollastonite crystals contributed to the development of the strength and dense microstructure of fired specimens. For XRD patterns, crystalline phases, e.g., nepheline, wollastonite-1A, and calcium silicate can improve the mechanical properties of ceramic bodies. It was concluded that reutilizing RHA and GGC wastes by adding boric acid is feasible to produce eco-friendly unglazed tiles at low sintering temperature.