Physicochemical and Optical Characteristics of Borate-Based Glasses Synthesized from Rice Husk Ash via a Sustainable Route

The sustainable utilization of agricultural waste has gained importance as an effective strategy to reduce environmental impacts and conserve natural resources. Rice husk ash (RHA), an abundant agricultural by-product rich in silica, is a promising sustainable source of silica for glass systems. In this study, silica was extracted from RHA via an optimized alkali-acid precipitation process and characterized by XRD, FTIR, SEM and BET. The extracted silica exhibited a non-crystalline nature with interconnected Si-O-Si networks, irregular agglomerated morphology, a specific surface area of 81.632 m2.g–1, a pore volume of 1.123 cm3.g–1, and an average pore diameter of 1.921 nm. The silica was subsequently incorporated into an alkali borate glass system by melt quenching. XRD confirmed vitrification, yielding an amorphous structure, while FTIR, Raman and SEM analyses revealed the evolution of the glass network and structural modifications. Thermal analysis identified characteristic glass transition, crystallization and melting behaviour, confirming vitrification. Progressive network modification increased the density, optical band gap and metallization criterion, while decreasing molar volume, Urbach energy, and refractive index. These findings establish processing-structure-property relationships and substantiate the feasibility of utilizing RHA-derived silica as a sustainable alternative to conventional silica for borate glass materials.