"The concept of ‘end of waste’ and recycling of hazardous materials: in depth characterization of the product of thermal transformation of cement-asbestos"

Selected samples of asbestos-containing material (ACM) with different Ca/Si ratios have been treated thermally at 1200ºC for 15 min to obtain an ‘end of waste geo-inspired material’. Before and after treatment, micro-Raman spectroscopy allowed the investigation of both powdered and massive samples by directing the laser beam onto crystals with elongated morphology, thin fibres and the matrix. In the raw samples, chrysotile and/or crocid olite were detected. After the thermal treatment, no asbestos phases were identified in the Raman spectr a collected on fibrous or fibre-like morphologies. The scanning electron microscopy/energy dispersi ve spectroscopy investigations confirmed the onset of a pseudomorphic process during annealing, lead ing to the complete transformation of asbestos minerals into non-hazardous magnesium or calcium magnesium silicates such as forsterite, monticellite, akermanite and merwinite. The identification of such mineral assemblages was inspired by the close inspection of a natural counterpart, the high-temperature contact meta morphic imprint due to the intrusion of a sill into carbonate rocks. The process turned out to occur lar gely at the solid state and involved substantial mobilization of Ca and Mg to form a spinel phase (namely MgFe2O4) which was recognized in the matrix and within, or close to elongated morphologies