Green 3-step synthesis of bioactive wollastonite from industrial wastes: effects of sintering temperature, sintering time and milling time

In recent years, environmental problems arising from the gradual depletion of natural resources and the rapid increase in waste generation have brought recycling and waste management into focus. Since wollastonite (CaSiO₃) as a calcium silicate ceramic is a bioactive material used in various fields, its synthetic production attracts attention. Therefore, the present study aims to produce bioactive wollastonite from marble and quartz wastes as industrial wastes with a 3-step technique from the green perspective. In addition, the effects of production parameters including sintering temperature (900 1000, 1100, 1200, and 1300°C), sintering time (2, 6, and 12 h), and milling time (0.5 and 12 h) on the phase and morphological structure, biocompatibility and bioactivity of the obtained synthetic wollastonite were investigated comparatively in the study. Accordingly, raw waste materials were first characterized with X-ray fluorescence (XRF), thermogravimetric analysis (TG/DTG), X-ray diffractometer (XRD), and scanning electron microscopy (SEM), respectively. TG/DTG results were used to optimize sintering temperatures of the CaO:SiO₂ (with 1:1 molar ratio) aqueous mixtures. The resulting powders were also analyzed using XRD, FTIR, and SEM. Structural characterization revealed that the formation of wollastonite (CS) phases and the polymorphic transformation reaction (from β-wollastonite to α-wollastonite) are affected by sintering and milling time as well as sintering temperature. By adjusting the milling and sintering time, a high-temperature phase α-wollastonite can be synthesized at a relatively low temperature of 1000°C, when β-wollastonite begins to transform. The biocompatibility of the wollastonite powder extracts was evaluated on mouse fibroblast, L929 cell lines by MTT assay and the changing in the phase of quartz by temperature, sintering and milling resulted with increased biocompatibility of the wollastonite powders. The obtained in vitro mineralization results after soaking of the wollastonite powders for 1, 3, and 7 days in SBF proved that SW exhibited good bioactivity due to the formation of spherical-shaped carbonated hydroxyapatite.