CRYSTAL CHEMISTRY OF CEMENT-ASBESTOS AND ITS HIGH TEMPERATURE PRODUCTS

Asbestos-containing materials (ACM) are an example of hazardous waste which has become matter of greatconcern due to its widespread diffusion. Cement asbestos (CA) are the most represented ACM. Recently, anindustrial process for the thermal destruction of CA wastes was developed [1] in compliance with the Europeandirectives and Italian legislation. Sealed packages of CA slates undergo prolonged annealing in the temperaturerange 1200-1300 C, during which both serpentine and amphibole asbestos minerals are completely transformedinto newly-formed silicates.The potential of this product as secondary raw material relies on the effectiveness and reproducibility of theinertization process at the industrial scale, and on the choice of suitable recycling solutions. At the scope, is of greatinterest to investigate how the high temperature transformed product is affected by the chemistry and mineralogyof the starting CA material. Literature data on CA slates are scarce and incomplete, and a comprehensive picturewith emphasis on areal distribution and compositional variability at a large scale is lacking.In this work, 27 CA samples coming from different localities in Italy, and their high-temperature inertizationproducts were characterized with a combination of analytical techniques, including XRF, XRPD, SEM/EDS, FTIRand micro-Raman. Raw materials revealed a complex mineralogy comprising cement hydrated phases, a residualnon-hydrated component, and a relevant fraction attributable to various processes of alteration. The industrialinertization process was reproduced at the laboratory scale by heat treating small chunks of cement-asbestosat 1200 C. A series of solid state reactions leading to global structural changes of the matrix with completetransformation of asbestos minerals was observed. Chemical gradients due to limited ionic diffusion testifiedrecrystallization under non-equilibrium conditions. This didn’t prevented the use of the CaO-SiO2-MgO phasediagram in order to relate the mineralogy of thermally treated samples with their chemistry. Effects of annealingtime and temperature on the crystallization kinetics were investigated with further thermal treatments. With theaid of thermodynamic calculations both factors were considered to act in favour of equilibrium. Three classes ofheat-treated CA, showing distinct chemical and mineralogical fingerprints, were identified. XRF data allow forthe content of CA packages, and thus, the corresponding heat-treated products, to be quickly classified. Analysescould be carried out indifferently before or after the thermal treatment. This result is of importance in view of thepotential recycling applications. Classes of transformed product can be selected and eventually mixed in functionof the solution adopted. This is the case of larnite-rich products, already recognized as larnite-rich cements highin magnesium, potential constituent of green cements, and tested as substitute for cement in commercial concrete[2].