Transformations through pseudomorphosis of asbestos minerals in thermally processed asbestos-containing materials investigated through SEM/EDS and micro-Raman spectroscopy: implications for recycling of hazardous wastes

Asbestos-containing materials, extensively used in the past in many European countries, are now considered hazardous wastes of great concern. It has been proved that inertization can be obtained via thermal treatment above 1100 °C. This solution relies upon the scientific evidence that all asbestos minerals at high temperature transform into stable crystalline silicates via a solid state recrystallization process [1]. Asbestos fibres preserve the same external crystal habit although a complete modification of the structure at a molecular scale occurred. This phenomenon is called pseudomorphosis. With increasing the temperature of the thermal treatment above 650-750 °C, the transformation sequence of chrysotile asbestos predicts the crystallization of forsterite (Mg2SiO4) and enstatite (MgSiO3 ) [1]. In a system high in Ca, such as cement-asbestos, crystallization of cement phases such as larnite (Ca2SiO4), ferrite (ideally Ca4Al2Fe2O10), and Al-,Ca-,Mg-rich silicates, such as akermanite (ideally Ca2MgSi2O7) and merwinite (ideally Ca3MgSi2O8), occurs. In this work, analytical and spectroscopic techniques coupled with microscopy allowed for the study of individual residual pseudo-morphosed fibre bundles, in cement-asbestos samples heat treated at 1200 °C. Phases detected were mainly monticellite (CaMgSiO4) or akermanite. They likely formed through the reactions: CaO + MgSiO3 (en) -> CaMgSiO4 (mtc), and CaMgSiO4 (mtc) + CaO + SiO2 -> Ca2MgSiO7 (ake). This suggests that, although transformation reactions occurred largely at the solid state, a substantial mobilisation of Ca and Mg resulted. Such a process is essential for the attainment of the bulk mineralogical composition predicted by the phase diagrams in the system CaO-MgO-SiO2 [2]; however, because of crystallization under non equilibrium conditions, departures from the expected bulk phase composition are still observed. This study contributes to the definition of factors conditioning the recycling of transformed cement-asbestos as secondary raw material [2-3].