The effects of sulfur dioxide on calcareous stone: a chamber study

The effects of sulfur dioxide on calcareous stone: a chamber study A. J. Lewry*, D. J. Bigland and R. N. Butlin BuUding Research Establishment, Garston, Watford WD2 7JR, UK A sulfur dioxide (S02) polluted atmosphere has been simulated with an atmospheric flow chamber (AFC). Portland limestone and White Mansfield dolomitic sandstone were subjected to S02 concentrations in air ranging from 0.5 to 6.0 ppm for periods of 30 days. The relative humidity of the gas flow was maintained at 84%. The rate of presentation of the gas to the surface of the stones was controlled so that 30 days exposure in the AFC was equivalent to a year's external exposure. During exposure in the AFC, half the samples were subjected to simulated rain whilst the other half remained dry. Analysis of the 'run-off' produced each week was combined with analysis of the samples to determine the total weight of sulfate formed. Damage functions were formulated for both stone types and it was found that: (i) deposition onto a wet stone surface produced squareroot relationships with a reasonable statistical fit; (ii) deposition onto a wet surface is an order of magnitude greater than that onto a dry surface. Keywords: a1mospheric flow chamber; sulfur dioxide; calcareous stone

The degradation of building stone due to natural processes such as wet/dry and freeze/thaw cycles is in general relatively slow'. As a result serious damage does not often occur within the design life of the building. In the majority of cases this can be taken as a lifetime or approximately 70 years. In recent years, the focus has been on increased deterioration rates of historical structures and monuments, which have been mainly attributed to the effects of S02 and other pollutants--'. Most research work has therefore concentrated on the materials most sensitive to these pollutants, i.e. calcareous building stones and metals', Processes leading to the transfer of pollutants from the atmosphere to surfaces are divided into two groups; dry deposition and wet depositions". Dry deposition is the direct collection of gaseous, aerosol and particulate species on a dry or wet surface; wet deposition includes the incorporation of pollutants in cloud droplets ('rainout'), their removal by falling precipitation ('washout'), and the deposition of the resultant liquid onto a surface. Damage to materials in polluted atmospheres can be attributed to dry or wet deposition of pollutants, or, in the case of stone, direct dissolution by rainfall". In order to predict the economic cost of damage from acid deposition it is necessary to know how decay rates are related quantitatively to pollutant concentration and meteorological parameters. The derived mathematical expressions are called 'damage functions".

"Correspondence to Dr A. J. Lcwry 0950-0618/94/04/0261-05
There is, therefore, much importance attached to national and international exposure programmes that have been established to assess the responses of a range of materials to different environments. However, the number of variables in field exposure programmes is usually high and, although nearly all can be measured, almost none can be controlled. As a result, complementary laboratory studies under controlled conditions are needed to measure the effect of one variable while keeping the others constant. The data from field and laboratory can then be combined to improve mathematical models and to predict rates of decay more accurately'", .

Experimental conditions Design philosophy There are many factors contributing to the interaction of atmospheric reactants that must be taken into account when considering the design of an experiment to simulate and accelerate the degradation of building materials. For example, type and concentration of pollutant, wetting/drying cycles, relative humidity and temperature. Dry deposition. Two of the main factors controlling dry deposition in real conditions are the gas concentration and the wind velocity. Together these determine the presentation rate of a pollutant to the material surface. The air flow (wind velocity) in outdoor environments is typically greater than that practically possible in laboratory exposure chambers. Therefore, to match the pollutant presentation rates of natural environments in laboratory chambers the product of air volume flowing onto a unit area of sample in unit time and pollutant

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The effects of S02 on calcareous stone: A. J. Lewry et al.

concentration should be the same for both situations: that is, lower wind velocities are compensated for by increasing pollutant concentration. Assuming an average wind velocity of 2 m s -1 and an S02 gas concentration of 60 j.Lg rrr' (21 ppb), the presentation rate to a sample area of 1 ern? is 1.2 X 10-5 mg em? S-I.11 Therefore if the air flow in the chamber is at 8.3 X 10-5 m3 S-1 the gas concentration must be 7.5 ppm S02 to compensate. Relative humidity and rainfall. Dry deposition can occur either on a completely dry surface or on one that is damp due to rainfall. The average annual temperature and relative humidity (RH) for the UK are 12°C and 85% respectively's, The rainfall for the Greater Manchester area is 800 mm per annum which equates to 2.2 mm per day. On a sample which has a surface area of 1500 mm? this means a daily rainfall volume of 3.25 ml. Exposure chamber The atmospheric flow chamber (AFC) consists of four exposure chamber units (Figure 1). Each unit has a primary Perspex box (470 x 150 x 150 mm) into which a pollutant atmosphere, at controlled humidity and a temperature of 292 ± 3 K, is passed at 8.3 X 10-5 m3 s'. A secondary container is located along one side of the box, through which water can be passed to control the temperature. Wetting/drying cycles. The four exposure chambers are normally divided into two sets of two, where each set has a 'wet' and 'dry' chamber. The wetting/drying cycle for the wet chambers consists of 8 hours of dripping de-ionized water saturated with CO 2 at ambient

temperatures followed by 16 hours when the samples are allowed to dry. De-ionized water saturated with CO2 was used to simulate natural rainwater. Ten weighed and characterized material samples (50 X 30 X up to 25 mm thick but usually 5 mm) are placed within the chamber. Drip feed valves supply water, which has been equilibrated with CO 2, at a controlled rate to each individual sample to simulate the dampening effect due to rain. The 'run-orr solution from each sample is collected in a separate vessel and removed weekly for analysis. An investigation into changes in the composition and volume of the 'run-orr is possible by rotating the chamber about the horizontal axis to vary the angle of the samples. The exposure period is 30 days during which time samples are wetted on 22 days. Humidity control. The AFC is supplied with air from a compressed-air line (-60 psi, 4 bars) which is split into two lines: a 'dry' line at 44% RH and a 'wet' line at 100% RH. By mixing these air lines the RH of 84% required for these experiments was obtained. Pollutant gas supply. It was shown above that an external concentration of 60 j.Lg m-3 produced a presentation rate of 1.2 X 10-5 mg em? S-I. As each box contains 10 samples the total area of exposed material is ISO cm-. The AFC air stream is supplied at a flow rate of 5 I mirr'; hence the concentration of gas required to keep the presentation rate constant is 7.5 ppm. The AFC has two gas blenders (Figure 2), each of OW

DW

Water inlet

/

Inlet for polluted air

front panel removeable for sped men access

•

_

Drip feed nozzle and adjustable qutter for run-off to specimens

40 p.s.i, Air

8412%

RH Run-off guliV -

Run-off collection vessel 25 ml - -

®

Gas blender

02

Ozonizer

F

Flowmeter Exposure chamber To exh aust Run·oft coUection

EC TE

RC DW

----- Run-off lap

Figure 1 Schematic cross-section of the exposure chamber

262

Inlet controls

GB

S02

N02

NO

1000 vppm in N2

De-ionized water C02 equi libraled

Figure 2 Block diagram of the atmospheric flow chamber (AFC)

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The effects of 8°2 on calcareous stone: A. J. Lewry et al.

which supplies a set of chambers with three streams. By the use of a 1000 vppm gas cylinder a pollutant gas supply to the exposure chamber of 0-10 vppm of pollutant can be produced in a controlled humidity air flow. Sample preparation and analysis Sample preparation. Portland limestone and White Mansfield dolomitic sandstone were chosen as the exposure materials for this work owing to their extensive use in exposure programmes'r'-". Before exposure, the stone samples (50 x 30 x 5 mm) were washed and then dried for 20 days at 55-60°C to achieve constant weight (± 0.05%). Sample analysis. At the end of the exposure period the stone samples were again dried for 20 days at 5560 °C and reweighed. Each chamber contained five Portland and five White Mansfield samples and the following techniques were used to examine each set of five samples:

Samples I, 2 and 3 were ground down to a powder using a micronizing mill and then the soluble salts were extracted with water. The solutions obtained were then analysed using an ion chromatograph for sulfate and nitrate contents. 2 A petrographical thin section (-20 urn) of the crosssection of one sample was prepared and examined using optical microscopy. 3 One tablet was kept for further analysis.

White Mansfield sandstone samples. White Mansfield sandstone consists of silica grains bound together with dolomite (CaMg(C03)z)' When this stone is exposed to SOz in air the dolomite will react to form calcium magnesium sulfate (CaMg(S04)z), The sulfate extracted from the samples was therefore assumed to be in the form of CaMg(S04h and a mass of extracted sulfate (in milligrams) was calculated. For the 'wet' samples the calcium and magnesium in the 'run-off' were also assumed to be from sulfate washed off the stone surface. The amount of sulfate was then calculated and added to that obtained by extraction. Experimental results and discussion Portland stone. Portland limestone was exposed to seven different S02 atmospheres at 84% RH, which spanned the equivalent range of annual mean S02 concentrations found in the UK (see section on humidity control above) and clean air. The amount of sulfate formed during the 30 day exposure periods is represented graphically in Figure 3 with the data shown in tabulated form in Table 1. Gaseous deposition onto a dry surface quickly reached a limit of approximately 10-11 mg of sulfate formed during the exposure period. This implies that there are a limited number of reactive sites and the greater the rate of presentation the more quickly these sites are saturated. 200r------~--------~____,

'Run-off solutions. 'Run-off' solution from each wetted stone sample was collected weekly and its volume determined by weight difference. The solution from each sample was then analysed quantitatively for calcium (Ca2+), magnesium (Mg2+), potassium (K+), sodium (Na"), sulfate (SO~ ) and nitrate (NO:!). Flame emission spectroscopy was used to detect K+ and Na" whilst atomic absorption spectroscopy was used to analyse Ca2+

o

WET

x

DRY

150

a>

o

E

'il 13 lOO

o

s: a. :i

o

If>

o o

and Mg2+. Ion chromatography was used to analyse the 'run-off' solutions for SOl- and NO]" contents.

o+---...---~--~--~--~-­ 3 .. o

Results and discussion

502 (ppm)

Analysis of results For each set of samples the total weight of sulfate formed during the exposure period was calculated for each individual stone sample (50 X 30 X 5 nun) from the analytical data collected. Portland limestone samples. Portland limestone consists mainly of calcium carbonate which reacts with sulfur dioxide (S02) in air to form calcium sulfate dehydrate, gypsum 13- 15• The sulfate extracted from the stone samples was assumed to be in the form of calcium sulfate (CaS04) and the mass of sulfate (in milligrams) extracted was calculated. For the 'wet' samples the calcium found in the 'run-off' was also assumed to be from CaS04 washed off the stone surface. The amount of CaS0 4 was then calculated and added to the mass obtained by extraction.

Figure 3 The weight of sulfate formed as a function of the S02 concentration for Portland stone

Table 1 Tabulated data from the exposure of Portland stone to various S02 atmospheres at 84% relative humidity S02 (ppm)

0 0.5 1.5 2.5 3 4 5 6

Equivalent Sulfate deposition onto a Sulfate deposition wet surface (mg) onto a dry surface outside (mg) concentration (ug m-3) Run-off Extraction Total 0 4 12 20 24 32 40 48

6.0 25.8 18.4 35.9 20.8 27.2 32.3 59.7

2.7 32.5 47.6 62.1 83.5 86.5 108.3 J25.7

8.7 58.3 66.0 98.0 104.3 113.7 140.6 185.4

2.2 8.7 11.2 12.0 10.6 10.3 10.2 9.3

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The amount of sulfate formed during the exposure of a damp surface increases as a function of the gaseous concentration. A log(S02) versus log(sulfate) plot gives a straight-line least-squares fit with a gradient of approximately 0.5. This indicates that the following relationship exists16: Total sulfate (mg) cc ,)S02 (ppm) A least-squares fit of the experimental data using the above relationship gives a satisfactory fit (r 2 =0.93) and the following damage function for Portland stone at 84% RH: Total sulfate formed (mg)

= 63.24 ";S02 (ppm)

+ 2.57

Using the equivalent outside concentration of sulfur dioxide, by keeping the presentation rate constant (see section on dry deposition above), this damage function now becomes Total sulfate formed (mg) = 22.36 ..JS02 (~~ rrr-') + 2.57 White Mansfield dolomitic sandstone. The results obtained from White Mansfield sandstone are very similar to those of Portland lUnestone (see Figure 4 and Table 2). The main difference is that White Mansfield sandstone appears to be more reactive and less homogeneous than Portland stone in the same exposure 250

o

WET

x

DRY

o

200

~

12

' 50

regime. For example, with deposition onto a dry surface the White Mansfield sandstone reaches a limit of approximately 30 mg compared with that of Portland stone which is 10-11 mg. This could be due to the greater solubility and mobility of combined calcium/magnesium salts when compared with their calcium counterparts. For example, the molar solubility for dolomite (CaMg(C0 3)0 is 10 times that of calcite (CaC0 3) I7. This is reflected in the data from the run-off analysed, where the amount of sulfate in solution is far greater for the White Mansfield stone. However, the sulfate that remains in the stone is slightly less for the White Mansfield sandstone than the Portland limestone. The more heterogeneous nature of White Mansfield stone is reflected in the fit obtained (r2 =0.79), from the experimental data, which give the following damage function at 84% RH: Total sulfate formed (mg) = 78.69 ..JS02 (ug rrr") - 11.05 The uncertainty in the fit due to the spread of the data has led to a negative intercept which is unrealistic because this implies that the stone increases its mass in clean air. However, more data points or a fitting technique that forces the fit through the measured intercept may solve this problem. The measured intercept at zero concentrations of S02 is a realistic approach because there is dissolution of the stone's surface due to the action of carbonic acid in the water. In this case, for convenience, it is represented as sulfate formed in the results. However, this type of data manipulation does add another constraint to the data. When comparing the AFC experiment undertaken with the outside environment, a good measure is the 'deposition velocity' 18, which can be defined by the following equation:

"

s:

[L

""5100 tI1

o 0

0

50

3

where Vd = deposition velocity (m S-I) m = mass of pollutant deposited (ug m-2) C = atmospheric concentration of pollutant (ug rrr ')

5

4

S02 (ppm)

== time (s)

Figure 4 The weight of sulfate formed as a function of the 502 concentration for White Mansfield stone Table 2 Tabulated data from the exposure of White Mansfield stone to various S02 atmospheres at 84% relative humidity S02 (ppm)

Equivalent Sulfate deposition onto a Sulfate deposition wet surface (mg) onto a dry surface outside (mg) concentration Run-off Extraction Total (flog mt)

o

0

0.5 1.5 2.5

4 12 20 24 32 40 48

3 4 5 6

264

3.4 35.7 26.9 45.5 92.8 33.2 150.2 172.5

2.1 26.8 28.9 58.1 22.4 55.0 46.0 51.1

5.5 62.5 55.8 103.6 115.2 88.2 196.2 223.6

1.8 13.4 24.5 33.3 20.6 30.7 20.0 29.1

A typical example from the AFC is that an average 115 mg of sulfate have been formed on a single tablet during a period of 30 days in an atmosphere of 3 ppm S02' This equates to a deposition velocity of 1.6 X 10-3 m S-I. This is similar to deposition velocities found outside, i.e. 1.7 X 10-2 m

S-I.19

Another approach is to compare the AFC results with the sulfate contents from the sheltered tablets of the United Kingdom National Materials Exposure Programme (NMEP). Taking an outside S02 concentration of 24 ug m-3 the damage functions derived from the NMEP data would predict a sulfate content of 6700 ILg g' per year". The AFC produces 115 mg of sulfate in a 20 g sample in 30 days. This 30 day period is an accel-

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The effects of 802 on calcareous stone: A. J. Lewry et al.

erated test to simulate a year's natural exposure and th~refore the sulfate content would be 5800 J.Lg s' per year. The majority of damage functions obtained from site work have been linear correlations-'. However, the expressions obtained from the AFC have followed squareroot relationships. The apparent conflict between the site and the AFC devised functions might arise if the range of values measured on sites was too narrow. If this was the case the measured values could be on a small part of the curve which would appear to be a straight line.

and Technology, who were responsible for the design of the atmospheric flow chamber.

References

2 3 4 5

Conclusions Atmospheric flow chambers (AFCS) can be used to generate damage functions for stone samples under a range of pollutant exposure regimes. Thirty days of exposure in the AFC appears to be approximately equivalent to a year outside. The experiments show that a square-root relationship between the amount of sulfate from the stone samples and SOz concentration, at 84% relative humidity, can be deduced for deposition onto a wet surface. A reasonable statistical fit can be obtained from the experimental data to provide a reliable damage function. Deposition on a wet surface is an order of magnitude greater than that onto a dry surface. This technique may be used to investigate the effects of humidity and pollutants on stone and other materials which then could be integrated into a damage function. The data will also provide a validation for the mathematical modelling of the information collected from the United Kingdom National Materials Exposure Programme (NMEP). An experimental programme is currently in progress to investigate the effects of relative humidity on the dry deposition of SOz onto dry and wet surfaces.

6 7 8 9 10 11

12 13 14 15 16

17

18

Acknowledgements

19

This work was funded by the Air Quality Division of the UK Department of the Environment. This paper has been published by permission of the Chief Executive of the Building Research Establishment. This project was part of a collaborative programme with the University of Manchester Institute of Science

20 21

Coote, A.T., Lewry, AJ., Yates, T.J.S. and Budin, R.N. Acid deposition and building materials. In Acid Deposition: Sources, effects and controls, ed. J.W.S. Longhurst, British Library, London, 1989 House of Commons Environment Committee. Acid Rain. 4th Report, HMSO, London, 1984 Amoroso, G.G. and Fassina, V. Stone decay and conservation, Materials Science Monographs II, Elsevier, Amsterdam, 1983 Baboian, R. Materials Degradation Caused by Acid Rain, ACS Symposium Series 318, ACS, Washington, DC, 1986 Yates, TJ.S., Butlin, R.N. and Coote, A.J. Predicting the degradation of building materials in the UK, Proc. 6th Int. Con! on Durability of Building Materials and Components, Omiga, Japan, 1993, to be published Garland, J.A. Dry and wet removal of sulphur from the atmosphere, Atmos. Environ. 1978, 12,349-366 Jaynes, M.S. and Cooke, R.D. Stone weathering in S E England. Atmos. Environ. 1987,21, 1601-1622 Budin, R.N. Effects of air pollutants on buildings and materials. Proc. R. Soc. Edinburgh 1991, 97B, 255-272 Topol, L.E. and Vijayakumar, R. Material damage from acid deposition. Proc. 6th World Congr. on Air Quality, Vol. 3, SEPIC, Paris, 1983, pp. 51-57 Building Effects Review Group Report, HMSO, London, 1989 Johnson, J.B., Haneef, SJ., Hepburn, B.J. Hutchinson, A.J., Thompson, G.E. and Wood, G.c. Laboratory exposure systems to simulate atmospheric degradation of building stone under dry and wet deposition conditions. Atmos. Environ. 1990, 24A, 2585-2592 Lacy, R.E. Climate and Bui/ding in Britain, BRE, HMSO, London, 1977 Johansson, L.G., Lindqvist, 0. and Mangio, R.E. Corrosion of calcareous stones in humid air containing S02 and N0 2• Durability Bui/d. Mater. 1988,5,439-449 Schaffer, R.J. The weathering of natural building stones. Building Research special report No. 18, BRE (UK), 1989, pp. 24-39 Winkler, E.M. Scone: Properties, Durability in Man's Environment, Springer-Verlag, New York, 1973, pp. 87-101 Lowry, AJ., Bigland, OJ. and Butlin, R.N. A chamber study of the effects of sulphur dioxide on calcareous stone. Proc. 7th Int, Congr. on Deterioration and Conservation of Scone, Lisbon 1992, pp. 641-650 Handbook of Chemistry and Physics, 48th edn, Chemical Rubber Company, Boca Raton, FL, 1967-68 Fowler, O. Transfer to terrestrial surfaces. Phi/os. Trans. R. Soc. 1984, B305, 281-297 Martin, A. Sulphur in air and deposition from air and rain over Great Britain and Ireland. Environ. Pollut. Ser. B, 1980, 1, 177-193 Yates, TJ.S., Coote, AJ. and Budin, R.N. The effect of acid deposition on buildings and building materials. Constr. Build. Mater. 1988.2,20-26 Lipfert, W.P. Atmospheric damage to calcareous stones: comparison and reconciliation of recent experiment findings. Atmos. Environ. 1989,23,415-429

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