Surface of Asbestos-cement AC Roof Sheets and Assessment of the Risk of Asbestos Release

1. Surface of Asbestos-cement (AC) Roof Sheets and Assessmentof the Risk of Asbestos Release J. Dyczek AGH University of Science and Technology, Krakow, Poland

2. AC in Poland � 15 000 000 Mg (t) surface corrosion Presence of cracks and other mechanical damage Risk of asbestos contamination is proportional to degree of surface corrosion and presence of cracks

3. Corrosion � AC durability Asbestos is one of the most durable minerals. Asbestos is practically insoluble in water, like quartz The matrix formed by cement (in sheets usually 90%) consists of compounds much more soluble in water than asbestos

4. Corrosion of AC products AC sheets consist usually of 90% cement and 10% asbestos The corrosion process of AC is practically identical to corrosion of concrete and is accelerated by acid rain The corrosion exposes asbestos fibres Corrosion makes sheets weaker and susceptible to cracking that increases risk of environmental contamination by asbestos

5. Corrosion of AC products The matrix in AC products consists mostly of: Ca(OH)2 calcium hydroxide (10 � 12 %) calcium silicate hydrates (60 � 80%) calcium aluminate hydrates (3 � 10%) calcium aluminate sulphate hydrates (0 � 5%) unreacted cement

6. Solubility of calcium hydroxide in water at 0 �C it is 1.3 g CaO/l The solubility in pure water of the other products of cement water reactions is much lower Surfaces of AC products are eroded by rain. Calcium hydroxide is slowly, but continuously washed out The calcium hydroxide wash-out process slows down in time

7. Ca(OH)2 + CO2 = CaCO3 + H2O CaCO3 is less soluble in water then Ca(OH)2 This process leads to a reduction of the matrix porosity and may retard reaction CaCO3 + CO2 + H2O = Ca(HCO3)2 Ca(HCO3)2 is more soluble than CaCO3

8. Calcium silicate hydrates and calcium aluminate hydrates are salts of the strong base and week acid. Increase of rain acidity (presence of ions of carbonates, sulphates or nitrates) increases the wash-out process for all matrix components As the result of chemical reaction of acid (or acids) new compounds are formed, much more soluble and accelerating corrosion

9. Increasing of a cement matrix porosity Reaction products (e.g. sulphates) may have relatively larger volume � result of it is scaling effect Some new compounds like, e.g., CaSO4 and CaSO3 are responsible for expansion The product (sulphate) has larger volume than the substrates.

10. AC product service life AC products durability Is AC product durability similar to concrete

11. AC product service life differs according to environmental influences. In polluted regions, AC roof sheet corrosion is accelerated by acid rain. In the corrosion process the matrix binding asbestos fibres is washed out, leading to unveiling of the fibers (asbestos fibre exposure). Exposed asbestos fibres may cause asbestos dust release.

12. SEM observations of AC product surface Five different groups of AC products were selected unpainted flat roof sheets (pressed, low porosity AC), unpainted corrugated roof sheets, autoclaved facade sheets, unpainted but covered by inorganic coloured layer, corrugated sheets from cooling tower, Element of ventilation chimney collecting also exhaust gases from gas water heaters. Additionally, the surfaces of fresh and old cracks were observed

13. Samples of unpainted flat sheets were taken in Southwest Poland, from the roof of a large, four-storied building.Industrial agglomeration at busy street junction