Characteristics of β-wollastonite derived from rice straw ash and limestone

This study aims to characterise beta-wollastonite that was derived from rice straw ash and limestone and also its bioactivity by soaking in SBF for 1, 3, 5, 7, 14, and 21 days. Rice straw was fired to obtain SiO2 and CaO from calcined limestone. The ratio of CaO:SiO2 was set at 55:45. The precursor mixture was then autoclaved for 8 h at 135 degrees C and sintered for 3 h at 950 degrees C. The chemical composition of the raw materials was obtained using XRF, which was approximately 79.0 wt.% SiO2 and 97.2 wt.% CaO for rice straw ash and limestone respectively. From the phase identification, the rice straw ash has a cristobalite phase, while the limestone has a CaO phase. The beta-wollastonite phase was obtained after both autoclaving and sintering were performed. Chemical analysis for soaked beta-wollastonite showed that the Ca/P molar ratio was 1.66, which is close to that of "calcium deficient" hydroxyapatite (CDHA). FTIR has indicated the occurrence of rapid adsorption during exposure of beta-wollastonite sample to the SBF, which is apparent from the peak of phosphate after one day of soaking. SEM has revealed the formation of hydroxyapatite microstructures on the surface of the immersed beta-wollastonite sample. For heavy metal elemental evaluation, metal panel that included As, Cd, Hg and Pb were selected and both precursor and beta-wollastonite had fulfilled the requirement of ASTM F1538-03 standard specification. In conclusion, beta-wollastonite produced from rice straw ash and limestone has a characteristic of bioactive properties, and is suitable for implant purposes.