Use of İzmit Gulf bottom sludge as a raw material component: eco-friendly wall tile glaze composition

Abstract This study investigates the reuse of İzmit Gulf bottom sludge as a silica substitute in black opaque wall tile glazes. The sludge, containing 44.64 wt% SiO₂ and 7.47 wt% Fe₂O₃, was incorporated at 25–100 wt% replacement rates. Viscosity measurements showed that higher sludge levels significantly increased viscosity from 460 to 920 mPa·s. Thermal analyses revealed that sintering and softening temperatures rose from 734 °C to 798 °C (STD) to 798 °C and 832 °C (S4), demonstrating improved refractory behavior. Microhardness values peaked at ~ 720 Hv in the S1 composition (25 wt% sludge), exceeding both the reference glaze (~ 650 Hv) and higher sludge samples, which showed reduced hardness due to porosity. Gloss decreased progressively from 92 (STD) to 75 (S4), while color parameters remained stable. Leaching tests confirmed that all heavy metals stayed within regulatory limits, although chromium release increased to 0.03 mg/L in S4 but remained below the 0.05 mg/L threshold. Therefore, S1 composition offered the most balanced performance, combining improved hardness, acceptable gloss, and environmental compliance. The findings demonstrate that İzmit Gulf sludge can be valorized as a sustainable glaze raw material when used in controlled proportions, supporting resource conservation and waste minimization in ceramic production.