The transformation sequence of cement-asbestos slates up to 1200 °C and safe recycling of the reaction product in stoneware tile mixtures

Cement–asbestos is the main asbestos containing material still found in most of the European countries such as Italy. Man- and weatheringinduceddegradation of the cement–asbestos slates makes them a source of dispersion of asbestos fibres and represents a priority cause of concern.This concern is the main prompt for the actual policy of abatement and disposal of asbestos containing materials in controlled wastes. An alternativesolution to the disposal in dumping sites is the direct temperature-induced transformation of the cement–asbestos slates into non-hazardous mineralphases. This patented process avoids the stage of mechanical milling of the material before the treatment, which improves the reactivity of thematerials but may be critical for the dispersion of asbestos fibres inworking and life environment. For the first time, this paper reports the descriptionof the reaction path taking place during the firing of cement–asbestos slates up to the complete transformation temperature, 1200 ◦C. The reactionsequence was investigated using different experimental techniques such as optical and electron microscopy, in situ and ex situ quali-quantitativeX-ray powder diffraction. The understanding of the complex reaction path is of basic importance for the optimization of industrial heating processesleading to a safe recycling of the transformed product.For the recycling of asbestos containing materials, the Italian laws require that the product of the crystal chemical transformation of asbestoscontaining materials must be entirely asbestos-free, and should not contain more than 0.1 wt% fraction of the carcinogenic substances such ascristobalite. Moreover, if fibrous phases other than asbestos (with length to diameter ratio >3) are found, they must have a geometrical diameterlarger than 3m. We have demonstrated that using an interplay of different experimental techniques, it is possible to safely verify the completetransformation of asbestos minerals in this temperature-induced process.The product of transformation of cement–asbestos (CATP) has a phase composition similar to that of a natural or a low temperature clinker withthe exception of having a larger content of aluminium, iron and magnesium. This product can be safely recycled for the production of stonewaretile mixtures. The addition of 3–5 mass% of CATP does not bear significant variations to the standard parameters of white porcelain tile mixtures.