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

Our plain-English summary of The transformation sequence of cement-asbestos slates up to 1200 degrees C and safe recycling of the reaction product in stoneware tile mixtures, by A.F. Gualtieri, C. Cavenati, I. Zanatto, et al (Journal of Hazardous Materials, 2008).

The study

• Authors: A.F. Gualtieri, C. Cavenati, I. Zanatto, M. Meloni, G. Elmi, M.L. Gualtieri
• Journal: Journal of Hazardous Materials
• Published: 2008;152(2):563-70
• Cited by: 23 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Cement-asbestos is the main asbestos containing material still found in most of the European countries such as Italy. Man- and weathering-induced degradation 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 alternative solution to the disposal in dumping sites is the direct temperature-induced transformation of the cement-asbestos slates into non-hazardous mineral phases. This patented process avoids the stage of mechanical milling of the material before the treatment, which improves the reactivity of the materials but may be critical for the dispersion of asbestos fibres in working and life environment. For the first time, this paper reports the description of the reaction path taking place during the firing of cement-asbestos slates up to the complete transformation temperature, 1200 degrees C. The reaction sequence was investigated using different experimental techniques such as optical and electron microscopy, in situ and ex situ quali-quantitative X-ray powder diffraction. The understanding of the complex reaction path is of basic importance for the optimization of industrial heating processes leading 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 asbestos containing materials must be entirely asbestos-free, and should not contain more than 0.1 wt% fraction of the carcinogenic substances such as cristobalite. Moreover, if fibrous phases other than asbestos (with length to diameter ratio >3) are found, they must have a geometrical diameter larger than 3 microm. We have demonstrated that using an interplay of different experimental techniques, it is possible to safely verify the complete transformation 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 with the exception of having a larger content of aluminium, iron and magnesium. This product can be safely recycled for the production of stoneware tile mixtures. The addition of 3-5 mass% of CATP does not bear significant variations to the standard parameters of white porcelain tile mixtures.

In plain English

This was a materials-science laboratory study, not a study of people or health. The authors heated commercial cement-asbestos roofing slates, whose minerals included clinochrysotile (a form of chrysotile asbestos), in an industrial kiln at up to 1200 degrees C. They tracked the mineral changes with X-ray diffraction and with optical and electron microscopy (SEM and TEM). Chrysotile broke down between 700 and 800 degrees C into other minerals. X-ray diffraction alone could not confirm that all asbestos was gone. Its detection limit was about 0.5 to 1 mass%, and a peak from a newly formed mineral overlapped chrysotile's main peak. Electron microscopy found the 1200-degree product entirely asbestos-free, with former fibres recrystallized into roughly spherical crystals. No cristobalite was found within the limits of X-ray diffraction. In porcelain stoneware tile mixtures, adding 5 or 10 mass% of the product worsened several properties at the lower firing temperatures. Adding 3 mass% gave results similar to the standard mixture, but whiteness was worse in every mixture containing the product. The authors concluded the product could be safely recycled in tiles at about 3 to 5 mass%.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Experimental materials-science study: thermal treatment of cement-asbestos slates in an industrial kiln, mineral analysis of samples fired at various temperatures, and laboratory tests of ceramic tile mixtures containing the fired product
• Who was studied: Commercial cement-asbestos slates (each 122 cm long, 57 cm wide, 1 cm thick). The number of slates is not stated. The study was carried out in Italy.
• Exposure: Heating of the slates from 25 to 1200 degrees C. The slates contained clinochrysotile (chrysotile asbestos) together with calcite, quartz and minor gypsum, illite, kaolinite and portlandite. The 1200-degree product was then added to porcelain stoneware tile mixtures at 3, 5 and 10 mass%.
• Main results:
  • In the heated slates, chrysotile decomposed between 700 and 800 degrees C, crystallizing promptly into forsterite and enstatite.
  • X-ray powder diffraction alone could not verify complete transformation. Its detection limit was around 0.5 to 1 mass%, and a peak of the newly formed mineral brownmillerite overlapped the main chrysotile peak in the 1200-degree sample.
  • Under electron microscopy, chrysotile bundles treated at 1200 degrees C had fully recrystallized into newly formed crystals. Their cleavage ran perpendicular to the fibre axis instead of parallel to it.
  • SEM/TEM analysis of what the authors call a statistically representative number of specimens (number not given) showed the 1200-degree product was entirely asbestos-free.
  • Within the limits of X-ray diffraction, the 1200-degree product contained no residual or newly formed cristobalite.
  • Phase composition of the 1200-degree product, in mass%: ferrite 1.0, larnite 11.0, akermanite 46.1, merwinite 13.7, amorphous phase 28.3.
  • Tile mixtures with 5 and 10 mass% of the product, fired at 1200 and 1210 degrees C, had higher water absorption, smaller linear shrinkage and lower density than the standard mixture. At 1220 degrees C the properties were nearly equivalent, especially for the 5 mass% mixture.
  • The tile mixture with 3 mass% of the product had water absorption, linear shrinkage and density similar to the standard mixture.
  • Resistance to staining was unacceptable for the 5 and 10 mass% mixtures fired at 1200 and 1210 degrees C. It was fairly good for all mixtures fired at 1220 degrees C and for the 3 mass% mixture.
  • Colour, especially whiteness, was worse in all mixtures containing the product than in the standard mixture, at every firing temperature. The authors attributed this to iron in the product.
• The authors' conclusion: The authors concluded that the 1200-degree product had no residual asbestos and could be safely recycled into stoneware tile mixtures. They put the maximum admissible amount at around 3 to 5 mass%.
• Limitations the authors state:
  • The authors stated that X-ray powder diffraction results need confirmation by electron microscopy, because of its detection limit and overlapping peaks.
  • The authors stated that phase contrast optical microscopy has low resolution and limited use for telling whether a fibre has been transformed.

Topics

Medical terms in this study, explained in our Lexicon: Abatement, Carcinogenic, Asbestos.

Related studies

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• Crystal chemistry of the high temperature product of transformation of cement-asbestos — A. Viani, A.F. Gualtieri, et al. (2013)
• In vitro biodurability of the product of thermal transformation of cement-asbestos — A.F. Gualtieri, A. Viani, et al. (2012)
• The thermal transformation of Man Made Vitreous Fibers (MMVF) and safe recycling as secondary raw materials (SRM) — A.F. Gualtieri, E. Foresti, et al. (2009)

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