Analyses of thermally treated asbestos cement

Status:

completed 12/2004

Aims:

Approximately 65 % to 70 % of the asbestos imported into the Federal Republic of Germany in recent decades was used for the production of asbestos cement, for example in corrugated asbestos cement as roofing and wall cladding, artificial slate, pipes or mouldings. For the production of asbestos cement, chrysotile (white asbestos) was primarily used; crocidolite (blue asbestos) was also used for certain products (such as pressure pipes, certain panels). From the second world war onwards, several million tons of asbestos cement products were therefore placed on the market, primarily between 1955 and 1985.

Since asbestos cement is assumed to have a service life of around 50 to 70 years, it can be assumed that a large part of the products, which were primarily employed in building construction work, have yet to be disposed of. In view of the German legislation governing recycling and waste, the utilization of wastes containing asbestos is considered an alternative to the expensive landfill solution. Processes are conceivable which convert or destroy the asbestos, for example by fine grinding or by thermal or chemical treatment.

For public authorities, purchasers and distributors, this in turn necessitates means for verifying that the conversion or destruction of asbestos in the course of waste treatment is complete. A range of established methods are available for the analysis of asbestos in technical products. These analysis methods must however be examined with regard to whether their results permit clear conclusions even where the asbestos has been converted only partly to other phases by thermal treatment.

The return of the treated materials to the market, for example in the form of additives or for use as a filler in road construction, may lead to employees being subjected to high exposure of asbestos fibres should the asbestos not have been completely destroyed.

Activities/Methods:

The established analytical methods for the measurement of asbestos in technical products were applied to asbestos cement samples which had been processed in a defined manner. A number of archived samples from asbestos cement panels were employed for the comparative studies. The focus of the studies lay upon panels containing chrysotile. The samples were treated thermally by being baked at 1 000 °C. The duration of baking varied between 15 minutes and 30 hours. The samples were prepared according to the analysis method employed. The samples were ground: in the micronizer mill for two minutes for infrared spectrometry and X-ray diffraction analysis, and in the mortar for analysis by phase-contrast microscopy and scanning electron microscopy. In addition, fracture surfaces featuring chrysotile fibre bundles were examined by scanning electron microscope, and fibre bundles isolated from the samples for the purpose were studied by means of phase-contrast microscopy. For the purpose of comparison, samples from the panels were also analyzed which had not been thermally treated.

Results:

The conversion by thermal treatment of chrysotile contained in asbestos cement can be verified by analysis involving phase-contrast microscopy (by changes in the refraction index and in the morphology of the fibres). Infrared spectrometry also reveals conversion, by the absence of the characteristic absorption bands of the chrysotile and by a significant change in the entire IR spectrum of the treated asbestos cement. In order to attain greater certainty, we recommend that both methods be employed in combination. Valid results can be attained however only if the sample represents a homogeneous fragment of a product. In this case, it can be assumed that if the phase-contrast microscopy analysis reveals only fully converted chrysotile, unconverted chrysotile is unlikely to be present in the same sample. Should the sample comprise mixtures of fragments which were treated differently, small asbestos components beneath the detection limits of the methods, in the order of 1% by mass, may remain undetected in the mixture. For validation purposes, supplementary analysis methods should then be used, such as transmission electron microscopy (TEM). Should analysis suggest that conversion has not been completed, the completeness of the thermal treatment can be ascertained as follows. The sample is baked for one hour at 1 000 °C and then analyzed a second time by infrared spectroscopy. Should the IR spectrum of the second analysis reveal significant differences to that of the first analysis, the thermal conversion of the sample under analysis was not complete. Such materials should not be placed on the market.

The above applies to the verification of asbestos conversion in asbestos cement products which have been baked at at least 1 000 °C. The extent to which lower baking temperatures produce comparable analysis results and conclusions should be ascertained if appropriate in a further project.

Further informations:

Last Update: