Global Asbestos Congress 2004
Surface of Asbestos-cement (AC) Roof Sheets and Assessment of the Risk of Asbestos Release
Technology Building Materials Department, Krakow Technical University of Mining and Metallurgy, Poland.
A number of samples of asbestos-cement roof sheets were taken from roofs of buildings in a small village, different towns and industrial centres.
Sample surfaces were observed in a SEM. It was found that surface corrosion depends on air pollution and is faster on AC roof plates in industrial centres and smallest on samples of AC plates from roofs in some small towns. Observations suggest that AC plate breaking causes a serious risk of releasing asbestos fibres and that on new surfaces of a broken plate there are a number of asbestos fibres sticking out, partly damaged, and any mechanical effect may produce respirable fibres.
In an assessment of the risk of asbestos fibre release from materials containing asbestos, especially from AC building materials, an important role is played by the assessment of the surface corrosion. Presence of cracks and other mechanical damage should also be taken into consideration as important factors.
AC was, and in some countries still is, frequently and on a relatively large scale used in the form of flat and corrugated roof and facade sheets. AC sheets in many countries were produced and used for about a hundred years. For example, in Poland there are 1,300,000,000 m2 roofs covered by AC sheets, i.e., 15,000,000 tons. Additionally, in cooling towers and other cooling installations in industry we have over 100,000 tons of AC sheets.
Usually on roofs, in normal condition of roof exploitation, mechanical stress appears rather occasionally and is rather moderate. As long as the sheet surface corrosion is small enough, and asbestos fibres are not exposed, mechanically provoked asbestos release may be discounted. Risk of asbestos contamination is proportional to degree of surface corrosion and presence of cracks. Significant increase of risk of asbestos fibres release follows any attempt at a plate's replacement or other roof mending.
A number of measurements of asbestos fibre concentration in air executed in Poland confirm the above statements. In these circumstances the corrosion process in AC sheets should be taken as the most important factor in estimating AC sheet durability, and observations of AC sheet surfaces may help us to understand and to assess the risk of environmental contamination by asbestos fibres.
The presence of so many AC sheets around us causes problems of AC durability and AC corrosion to be rather important.
AC products consist of 84 to 92 % of Portland cement and, respectively, 16 to 8% of asbestos. Asbestos is one of the most durable minerals. Its resistance to water, acids and other aggressive chemicals is very high. Asbestos is practically insoluble in water, like quartz or BaSO4.
The matrix formed by cement (in sheets usually 90%) consists of compounds much more soluble in water than asbestos; especially more soluble in acids, even in weak acids, like the acid present in acid rain. The matrix in AC materials is identical to the cement paste in concrete, chemically and physically.
It is well known that acid rain accelerates concrete corrosion.
Mostly, AC products are formed from a water suspension of asbestos and cement by separating solids, cement grains and asbestos fibres from the water and simultaneously shaping. Freshly formed products are wet. So, all physical and chemical processes are very similar to fresh concrete. Cement reacts with water, and in consequence we observe the process of binding and hardening. Cement water reactions in so called green products and later on, in the following days and months, are identical to those in concrete.
Therefore the corrosion process of AC is practically identical to corrosion of concrete and is accelerated by acid rain.
It should be mentioned, that concrete is formed in rather big elements like plates, beams or pillars or even bigger like elements of civil engineering structures. The dimensions of concrete elements are measured in meters - often many meters. AC sheets are rather thin. AC sheet thicknesses usually range from 4mm to 8mm. Then, corrosion may not only expose fibres but also make sheets weaker and susceptible to cracking, that increases risk of environmental contamination by asbestos.
This paper presents observations, using a scanning electron microscope [SEM], of fragments of AC sheets collected from roofs, cooling towers and from ventilation installations.
2. AC corrosion
As it was earlier mentioned, AC sheets consist usually of 90% cement and 10% asbestos1. As a result of physical processes and chemical reactions of the cement a matrix is formed. Fibres of asbestos are matrix reinforcing. AC material is a composite reinforced by asbestos fibres.
In the matrix we may recognise physical processes like solvation, precipitation, crystal growth and recrystallization, and chemical processes - a number of different chemical reactions of cement with water. The matrix in AC products consists mostly of calcium hydroxide (10 - 12 %), calcium silicate hydrates (60 - 80%), calcium aluminate hydrates (3 - 10%), calcium aluminate sulphate hydrates (0 - 5%) and unreacted cement (10 - 25%).
Among these components, the solubility of calcium hydroxide in water is relatively high; even at 0 ēC it is 1.3 g CaO/l. The solubilities in pure water of the other products of cement water reactions are much lower. Thus, surfaces of AC products are eroded by rain, mostly because calcium hydroxide is slowly, but continuously washed out. The calcium hydroxide wash-out process slows down in time, because on the surface, part of the calcium hydroxide reacts with carbonate ions forming calcium carbonate CaCO3
Ca(OH)2 + CO2 = CaCO3 + H2O (1)
CaCO3 is less soluble in water then Ca(OH)2
Unfortunately in higher concentration of CO2 CaCO3 goes in to Ca(HCO3)2
CaCO3 + CO2 + H2O = Ca(HCO3)2 (2)
Ca(HCO3)2 is more soluble than CaCO3
Similarly to reaction (1) CaCO3 is formed not only from calcium hydroxide, but also from calcium aluminate and calcium silicate hydrates. This process leads to a reduction of the matrix porosity and may retard reaction (2).
Increase of rain acidity (presence of ions of carbonates, sulphates or nitrates) increases the wash-out process for all matrix components.
Concluding, in the presence of acid rain there appears so called acid corrosion, well described in cement chemistry and concrete technology.
The mechanism of acid corrosion is complex. It is not only a simple washing out process depending on solubility. As the result of chemical reaction of acid (or acids), with calcium hydroxide, calcium aluminate and calcium silicate hydrates, new compounds are formed, much more soluble and accelerating corrosion.
Some new compounds like, e.g., CaSO4 and CaSO3 formed in reaction with sulphuric and sulphurous acids present in acid rain, are responsible for expansion. The product (sulphate) has larger volume than the substrates. It produces tension, and may accelerate corrosion.
3. 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.
3.1. Unpainted flat roof sheets
Samples of unpainted flat sheets were taken in Southwest Poland, from the roof of a large, four-storied building in an industrial agglomeration at a busy street junction. This roof was covered by flat, pressed plates 40 × 40 × 0.6 cm of low porosity, lower than corrugated sheets. Plates were over 20 years old. Roof and plates were in good technical conditions.
SEM observation (Photo 1) shows very occasionally asbestos fibres, covered by calcium silicate hydrates, partly carbonised. Fibres are well connected with matrix. Risk of asbestos fibre liberation is rather small.
Photo 2 shows needle like secondary gypsum crystals. These result from reaction of calcium present in the matrix with sulphate anions present in acid rain. Nevertheless, on the surface appear a number of calcium carbonate crystals. Corrosion generally is rather slight. It is due to matrix low porosity and additional porosity reduction due to pores filling with carbonisation products.
3.2. Unpainted corrugated roof sheets
These samples were collected from the roof of an office block (two-stories) in a small town in south Poland. It is an agricultural area where the air is unpolluted and acid rain appears only occasionally. Only traffic pollution should be taken into consideration.
The roof and roof sheets were over 30 years old. Roof and plates were in good technical condition.
Asbestos fibres on the sheet surface are rarely visible in the SEM. All observed fibres are covered by cement reaction products and stick well to the sheet surface. Examples of observed asbestos, as rather long fibres, are seen on Photo 4. Risk of asbestos dust release is estimated as medium high.
Other unpainted corrugated roof sheets were investigated on the older (40 years old) roof of a building in an industrialised area in Southwest Poland, where acid rain is rather frequent.
Acid rain wears down the matrix and asbestos fibres are exposed - Photo 5. Fibres are clean, uncovered by calcium carbonates or calcium silicate hydrates and specifically not connected to the matrix. Here, calcium compounds reacted with acid to produce more soluble chemical compounds which were dissolved. As a result of this, on the sheet surface are found asbestos fibres, which can rather easily break away. Risk of asbestos dust release is high.
3.3. Autoclaved facade sheets, unpainted but covered by inorganic coloured layer
Facades were occasionally finished in Poland with autoclaved sheets. These sheets were formed by dry technology, then coated with admixtures of powdered coloured quartz grains, pressed again and autoclaved (typical grain dimension 0 - 0.2 mm). Our observations suggest that the layer rich in quartz grains and well connected with the A plate provides good protection from corrosion.
Asbestos was not visible on the surface of any of the samples observed at lower magnification (photo 6.). Nevertheless, corrosion is significant. On the surface a number of craters are clearly visible.
At a higher magnification it is possible to have a look in these craters (holes). Inside these craters corrosion washed out the cement matrix and asbestos fibres are visible.
On the asbestos fibres, cement-water reaction products form small, differently shaped grains. These are mostly calcium silicate hydrates partly carbonised.
Further observation showed that corrosion may lead to scaling effects and the coloured layer may partly drop out. The drop out effect causes high risk of asbestos dust release.
3.4. Corrugated sheets from cooling tower
In traditional cooling towers an increase of the surface of heat exchange between water and air may be obtained by inserting into the tower a number of AC sheets. The sheets are continuously washed by water. The average temperature in cooling towers is over 20°C
The samples examined were from a cooling tower 15 years old, which means that for15 years they were washed with water. The sheets, in relatively good condition, had been exchanged for thinner PVC sheets to achieve an increased total surface area. (Neglecting other technical details, as a result, the final water temperature is lower, so cooling efficiency is improved.)
The presented SEM observations of these sheets - Photos 8 and 9 - occasionally show asbestos fibres: rather short and partly covered by cement-water reaction products.
As long as plates are wet, and are not submitted to any mechanical action like, e.g., stripping, risk of asbestos pollution is small. After drying the risk becomes serious.
3.5. Element of ventilation chimney collecting also exhaust gases from gas water heaters
The next photos, 10 and 11, present typical results of SEM observation of the inside surface of elements of a ventilation chimney. These chimneys also vented exhaust gases from gas water heaters. Exhaust gases were wet; temperature of these fumes only occasionally exceeded 100°C.
The observed samples were taken from 23 years old AC elements of this ventilation system.
Seemingly, the elements of the ventilation chimney looked well, but the inside surface was corroded and carbonised.
Observed exposed asbestos fibres are partly covered with aggregates of calcium carbonate small crystals (Photo 11) and are poorly connected to the sheet surface (Photo 10). Risk of asbestos dust release is significant.
3.6. The risk of asbestos dust release due to mechanical damage
For a number of reasons AC products may be mechanically broken down or cracked. Presence of cracks, split-off or broken AC sheets are particularly dangerous. See Photo 12.
From the surface formed by breaking (more or less perpendicular to the sheet surface) a number of asbestos fibres are sticking out. This is due to the pull out effect. The pull out effect accompanies the cracking process in composites reinforced by short fibres. When, under bending or shear, stress cracks gradually open, some fibres are broken and some are pulled out. Fibres are pulled out because the mechanical strength of these fibres is higher than the strength of their attachment to the matrix.
Any mechanical influence on these protruding fibres may produce a large number of asbestos dust particles.
It should be emphasised, that the fibres in Photo 12 and in earlier photos, independently of magnification, differ in length and in dimensions. In fact, we observe not single asbestos crystals but fibres formed by hundreds and often thousands of asbestos long mono-crystals. Indeed, we observe bunches of a number of asbestos mono-crystals.
Forces keeping single fibres-mono-crystals together in the bunch are rather weak
From one asbestos fibre, protruding as we see on Photo 12, we may get hundreds or thousands of respirable asbestos fibres; it is only a question of even feeble mechanical action or other tension. For example, movement caused by thermal expansion could be sufficient.
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 fibres (asbestos fibre exposure). Exposed asbestos fibres may cause asbestos dust release.
The risk of fibre liberation increases proportionally with corrosion. But independently of the degree of corrosion, any cracks in or other mechanical damage to AC sheets (and other AC products) lead to a high risk of asbestos dust release.