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Characteristics of carbonate building stones that influence the dry deposition of acidic gases
Construction and Building Materials 13 Ž1999. 101]108
Characteristics of carbonate building stones that influence the dry deposition of acidic gases C.M. Grossi a,U , M. Murray b a
STATS Consultancy, Porters Wood, St. Albans, Herts AL3 6PQ, UK Building Research Establishment, Garston, Watford WD2 7JR, UK
b
Received 8 April 1998; accepted 6 April 1999
Abstract The behaviour of several porous carbonate building stones used in Spanish and English monuments was compared. Stones were exposed to urban and suburban environments and subjected to simulated SO 2 atmospheres in the laboratory. Physical properties that affect the transfer of moisture were determined before and after exposure, and related to the degree of reaction of the stones. The degree of reaction was determined by analysing the reaction products. Results showed that physical properties influenced the reactivity of the stones. These characteristics also changed as the stones weathered due to the different concentrations of impurities in the material. Moreover, stones with a high specific surface area andror a deliquescent salt content may promote more NO x dry deposition. Q 1999 Elsevier Science Ltd. All rights reserved. Keywords: Porous carbonate building stones; Physical properties; Acidic deposition
1. Introduction Chemical degradation of carbonate building stones is considered to be due to atmospheric conditions such as dry deposition of acidic gases and aerosols, dissolution by acidic species in rainwater and dissolution by unpolluted water w1]3x. However, the degradation also depends on stone properties. 1.1. En¨ ironmental conditions The dry deposition of gaseous SO 2 and subsequent oxidation to sulphate is considered to be the main weathering agent for carbonate building stones. Additional sulphate can be deposited by particle deposition. The rate of uptake of SO 2 by carbonate stones depends on the relative humidity w4]8x, especially in
the case of porous stones w9x, suggesting the importance of condensed water in the pore structure for sulphation. NO x can deposit on carbonate stones by dry deposition although at a low rate w2,6x. According to Wittemburg and Dannecker w10x a source of nitrates in the stones might be dry deposition of gaseous nitric acid and nitrogen oxides followed by oxidation. NO 2 can also influence the uptake of SO 2 . In the presence of humidity NO 2 can promote the oxidation of SO 2 w6x although there is no strong evidence for this outside laboratory studies. Hutchinson et al. w11x studied the dry deposition of HCl on carbonate stones and determined that in moist conditions HCl is quickly absorbed and reacts with the calcium carbonate to produce CaCl 2 which is easily removed by water. 1.2. Properties of stone
U
Corresponding author. Tel.: q44-1727-83361, ext. 323; fax: q441727-811528. E-mail address: [email protected] ŽC.M. Grossi.
The uptake of acidic gases depends on the amount of
0950-0618r99r$ - see front matter Q 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 9 5 0 - 0 6 1 8 Ž 9 9 . 0 0 0 1 9 - 7
C.M. Grossi, M. Murray r Construction and Building Materials 13 (1999) 101]108
102
moisture in the stone, which is determined to a great extent by properties that affect the transfer of humidity. These properties are: absorption and permeability of liquid water and hygroscopic moisture content, evaporation and permeability of water in the vapour phase. However these properties can all be related to porosity and specific surface area Žpore size distribution.. Porosity can be regarded as the principal characteristic of the stone as the rest of the properties are determined by open porosity and pore size distribution. Hygroscopic moisture content refers to the uptake of moisture from the ambient air w12x. It is considered to be very important in the dry deposition of acidic gases. Hygroscopic adsorption depends on the relative humidity of the air, the nature of the material, the diameter of the pores w13x and the specific surface area. Several authors have indicated that SO 2 deposition is proportional to the surface area of the stone wetted by adsorbed water w8x. Hygroscopic salts Žincluding corrosion products such as nitrates, chlorides and previous deposits of SO 2 . can influence the moisture content of porous materials. These salts attract moisture from the air and could be important in maintaining the moisture film on the surface of carbonate stones. Non or slightly hygroscopic materials can become strongly hygroscopic when such salts are entrapped in their pores. This can lead to increase absorption and oxidation rates of atmospheric SO 2 to form calcium sulphate w12,15,16x. The presence of soluble impurities can also function as a catalyst in sulphation w11x. The role of the specific surface area is very important with respect to the adsorption of water to material surface at low relative humidity w14x. At higher relative humidity capillary condensation takes place. For condensation to take place, pore radii must be less than the radius given by Kelvin’s equation. This paper investigates the characteristics of carbonate building stones that influence the dry deposition of acidic gases. In order to do this, different stones were subjected to polluted atmosphere tests in the laboratory as well as being exposed on-site. Some physical properties of the stones were measured and related to the degree of reaction of these stones. Finally re-
lationships between stone properties and corrosion products were established.
2. Materials and initially physical properties tests The selected materials were three carbonate stones used as building materials in Spanish and English monuments: Laspra dolomite, Hontoria limestone and Portland limestone. Laspra is a micrite composed mainly of dolomite Žapprox. 90%.. The material used in these experiments corresponds to blocks from the Cathedral of Oviedo ŽAsturies, Spain.. Hontoria and Portland are a bioclastic and an oolitic limestone, respectively. The material from this research was extracted from the quarries of Hontoria de la Cantera ŽBurgos, Spain. and Bowyers ŽPortland, UK.. The following properties related to transfer of moisture have been measured: v
v
v
Open porosity, pore size distribution and specific surface area by mercury porosimetry ŽCarlo Erba Serie 200. ŽTable 1.. Porosity is very high in Laspra and high in Hontoria and Portland. Free water absorption by total immersion Ž Cw . ŽNormal 7r81. w17x ŽTable 2.. Hygroscopic water content Ž CH . Žat T s 238C; RH s 60%. ŽTable 2..
The amount of absorbed water seems to be related to open porosity}amount of accessible voids} whereas hygroscopic moisture content}water adsorption and capillary condensation}depends on the pore size distribution and therefore on the specific surface area.
3. Exposure experiments The three carbonate stones have been subjected to: v
v
An exposure to simulate SO 2 polluted atmospheres in a climatic chamber; and a 1-year exposure on-site.
Table 1 Porosity Žmercury porosimetry.
Laspra Hontoria Portland
Open porosity Ž%.
Specific surface area Žm2 rg.
Specific pore volume Žmm3 rg.
Specific pore volume radius - 1 mm Žmm3 rg.
Specific pore volume radius - 0.1 mm Žmm3 rg.
31 20 20
4.03 0.26 1.21
164 94 94
155 27 48
34 2 17
C.M. Grossi, M. Murray r Construction and Building Materials 13 (1999) 101]108 Table 2 Physical properties a
Laspra Hontoria Portland
surface composition by Fourier Transform Infra-Red ŽFT-IR. spectroscopy and weight change. Cw Ž%.
CH Ž%. ŽRHs 60%.
12 6 7.5
1.25 0.005 0.04
a
Cw s free water absorption by total immersion; CH s hygroscopic moisture content ŽT s 238C, RHs 60%..
3.1. Laboratory exposure Laspra and Hontoria were exposed to dry deposition of SO 2 in an atmospheric flow chamber following the methodology used by Johnson et al. w18x and Lewry et al. w19x. The results were compared with those obtained in Portland limestone under the same conditions of exposure w7x. The concentration of SO 2 was equivalent to an atmospheric concentration of 20 mgrm3 w7x. During the exposure period, half the samples were subjected to simulated rain; the other half remained dry. In this way, dry deposition on both wet and dry surfaces has been simulated. The simulated rain consisted of cycles of 8-h wetting followed by 16-h drying ŽT s 19 " 38C; RHs 84%.. The aqueous ‘run-off’ from the surface of each sample was collected weekly for analysis. The exposure period was 30 days, simulating a year’s exposure in an area with annual average rainfall of 800 mm. After exposure a chromatographic analysis of cations and anions was carried out both of run-off solutions and extracted reaction products Žsoluble salts.. 3.2. On-site exposure The selected materials have been exposed for 1 year in two different environments: v
v
103
Central London Žexterior of Westminster Abbey.; and Garston, Hertfordshire Žexposure site at the Building Research Establishment, BRE., suburban environment.
At each of these sites, pairs of freely rotating carousels of stone tablets Ž5 = 5 = 1 cm. were installed following the methodology used by researchers at the BRE. One of the carousels was sheltered from rainfall and exposed only to dry deposition. The other was exposed to wet and dry deposition. The concentration of ions was determined by ionchromatography in successive drillings of 0.5 mm from the surface to a 2-mm depth. Further analysis was carried out on samples exposed both in the laboratory and on-site. These included
4. Results and discussion Sulphates were the main products of reaction. Sulphur dioxide is the main acidic species involved in dry acid deposition of these stones w1,20x. The most deteriorated stone was Laspra dolomite both in laboratory and on-site experiments. The amount of sulphate deposited on Laspra surface was higher than on Hontoria and Portland stones. Surface recession was also higher in laspra dolomite. Both characteristics have been related to the physical properties of these materials in previous works w21,22x. 4.1. Reaction products Sulphite ŽSO 32y . }the intermediate product in the oxidation of SO 2 to SO42y}was found in laboratory tests. However, it is never found in real exposure due to the presence of NO x , O 3 , particulate matter and other compounds that facilitate the oxidation to SO42y. Most of the sulphates newly formed in sheltered samples were calcium sulphate even in Laspra dolomite. In Laspra samples from on-site exposure, SO42y balances perfectly well with Ca2q. This was corroborated by determining the ion balance SO4 with Ca or Ca and Mg according the methodology followed by Butlin et al. w1x. Moreover, the weight ratio SrCa varies between 0.7 and 1. The weight ratio SrCas 0.8 in pure gypsum was used by Nord w23x to determine the calcium bound as gypsum. Fig. 1 shows the CarSO4 ratio in all the samples and the Ca and MgrSO4 for Laspra dolomite. MgSO4 ŽFig. 1. has been found as a reaction product in the Laspra run-off samples from climatic chamber experiments, where the experimental conditions are different: the only polluting gas is SO 2 and the water is deionised. Interestingly the proportion of Ca and Mg sulphate varies over the duration of the test ŽFig. 2.. Sodium, potassium, chlorides and nitrates were detected in Laspra run-off Žclimatic chamber.. These ions were present in the stone before the test. Laspra is an old stone with a high specific surface area that facilitates the uptake of moisture. This fact can favour gas and aerosol deposition. Deposited soluble compounds might migrate and accumulate inside the stones. Another possible source of these salts is ground water coming to the stone by capillary suction. Laboratory results agree with those obtained on-site. In the reference and sheltered samples there is a high concentration of chlorides and especially of nitrates, whereas samples exposed to rainfall show a lack of
104
C.M. Grossi, M. Murray r Construction and Building Materials 13 (1999) 101]108
Fig. 1. CarSO4 and ŽCa and Mg.rSO4 ratio. Ža. Sheltered Hontoria and Portland on-site; Žb. sheltered Laspra on-site; Žc. run-off Laspra climatic chamber.
soluble salts which have dissolved and removed ŽTable 3.. By subtracting the data for rain-exposed samples from that for reference samples and by subtracting all chamber run-off sulphate and its Ca and Mg equivalents it was shown that some calcium and magnesium
were in the form of chlorides and nitrates in the reference sample ŽTable 3.. The best correlation was found between sodium and nitrates and chlorides. As a result in Laspra dolomite there is a mixture of deliquescent salts that can increase dry deposition by increasing the water content of the stones.
C.M. Grossi, M. Murray r Construction and Building Materials 13 (1999) 101]108
105
Fig. 2. Variation of the proportion of Ca and Mg sulphate over the test. Run-off water, climatic chamber experiment.
4.2. On-site results In sheltered Laspra, Hontoria and Portland samples of on-site experiments an increase in the amount of nitrates and chlorides was also found. Sulphates tended to concentrate in the first layers of the stone, whereas nitrates and chlorides which generally are more soluble and hygroscopic were evenly distributed across the sample ŽFig. 3. w24]26x. In Laspra dolomite the concentration of new formed nitrates was high. Moreover, as stated above, Laspra shows a high nitrate concentration in the reference samples ŽTable 3. which was higher than that for chlorides. NO x can transform to NOy 3 in the water adsorbed on the surface of the stone. However, the process is slow, the large Laspra surface area could allow more nitrate to be absorbed into the surface
moisture layers w22,27x. Gaseous HNO3 in the air might also be the source of nitrates. Wittemburg and Dannecker w10x from experiments carried out on-site found from regression analysis that the nitrate input was strongly correlated with the gaseous HNO3 in the air. Because the reaction of gaseous NO x with carbonate stone is much slower than that of SO 2 ŽSO 2 concentration in these experiments are 12]21 m g my3 of SO 2 and 42]61 m g my3 of NO x ., the characteristics of the stone pore network are very important in the deposition of nitrates. Stones with high surface area will have a high moisture content which will favour deposition of nitrates both from gaseous species and particulate. 4.3. Changes in hygroscopic water content Hygroscopic moisture content was measured at 60%
Table 3 Concentration of impurities in Laspra dolomite
Ca2q Mg2q Naq Kq Sum of cations SO42y NOy 3 Cly Sum of anions a
Concentration reference sample on-site Žmeq.rg.
Concentration exposed to rainfall Žmeq.rg. on-sitea
Concentration impurities on-site Žmeq.rg. Ž A.
Amount of ions in run-off chamber Žmeq.. Ž B .
BrA Ž=10y3 .
65.2 45.8 33.0 16.6
28.7 25.3 1.0 3.6
36.5 20.5 32.0 13.0
0.09 0.07 0.11 0.04
2.5 3.4 3.4 3.1
3.8 72.9 40.2
?
? 72.3 39.7
102 0.6 0.5
0.31 subtracted 0.22 0.11
112
3.0 2.8 0.33
It is considered to be the original concentration of the stone. In chamber run-off samples the amounts of sulphates were subtracted together with the equivalents of calcium and magnesium.
106
C.M. Grossi, M. Murray r Construction and Building Materials 13 (1999) 101]108
Fig. 3. Distribution of anions across sheltered Portland samples Žon-site exposures.. Ža. Sulphates; Žb. nitrates; Žc. chlorides.
RH and 238C in both reference samples and on-site exposed samples ŽGarston exposure site.. This property was measured in tablets with an exposure period of 5 and 11 months in positions both sheltered and exposed to rain-fall. This property was found to vary when stones have been exposed on-site. In Laspra dolomite the hygroscopic moisture content
decreased dramatically when samples were exposed to rainfall. In the case of Hontoria and Portland stones the highest variation was detected in sheltered samples. The variation of this property after on-site exposure has been related specially to change in concentration of hygroscopic salts inside the stone. Table 4 shows the differences in concentration of
C.M. Grossi, M. Murray r Construction and Building Materials 13 (1999) 101]108
107
Table 4 Laspra dolomite: concentration of chloride q nitrate and hygroscopic moisture content Žon-site. Clyq NOy 3 Žmeq.rg.
CH Ž%. ŽRHs 60% T s 238C.
Sheltered Reference Garston 4 months West. 4 months Garston 12 months West. 12 months
Exposed
Sheltered
113.1 84.1 129.1 115.0 117.3
Exposed 1.25
1.3 16.4 1.0 1.3
chloride q nitrates of sheltered and unsheltered Laspra samples and the hygroscopic moisture content at 60% RH and 238C. Laspra dolomite had a high concentration of hygroscopic salts in the reference stone. When exposed to rainfall, the stone lost soluble salts ŽTable 3.. The hygroscopic moisture content after exposure to rainfall was reduced by approximately 50% Žtest conditions: RHs 60%, T s 238C.. In sheltered stones, this property does not show any variation with respect to the original values. In Hontoria and Portland limestones the hygroscopic moisture content seems to be higher in sheltered stones which show a increase in concentration of chlorides and nitrates. The results are not as convincing as in Laspra dolomite. The difference in salt concentration is not as dramatic as in Laspra, and there are not enough data to ensure that there is real variation in the property.
Garston 5 months
1.42
0.62
Garston 11 months
1.17
0.60
which allows more acidic gases to absorb. Ž5. Unless rain washed away, these salts can be retained indefinitely.
Acknowledgements
The authors wish to acknowledge Comision ´ InŽCICYT ., terministerial de Ciencia y Tecnologıa-Spain ´ project: SEC95-0501 and Fundacion ´ para el Fomento en Asturias de la Investigacion ´ Cientıfica ´ Aplicada y la ŽFICYT ., project: PB-REC96-08. Tecnologıa-Spain ´ Also to Dr Rosa M Esbert and Mr Francisco Dıaz´ Pache from the University of Oviedo ŽSpain..
References 5. Conclusions As expected, the Spanish and English porous carbonate stones are very reactive to rainwater and acidic gaseous deposition Žmainly to SO 2 .. The main product of reaction found in external atmospheres was gypsum ŽCaSO4 .2H 2 O.. Sulphite was also formed as an intermediate product in laboratory studies. It was allowed to crystallise due to the pure and ozone free environments. The nature of the reactivity, however, could be related to specific physical properties of the stones: Ž1. porosity is important as the open structure allows more water into the sample and so more carbonate to dissolve. Ž2. Pore distribution and specific surface area influence the uptake of moisture from the air. High surface areas will have a high moisture content which will favour dry deposition. That could be important in the case of the deposition of NO 2 Žwhich is less soluble than SO 2 or HCl.. Ž3. Dry deposition causes an accumulation of deliquescent salts as well as gypsum. Ž4. Deliquescent salts can, at the same time, increase dry deposition by increasing the water content of the stones
w1x Butlin RN, Coote AT, Devenish M et al. A four-year study decay in different pollution climates in the United Kingdom. Lisbon: 7 th International Congress on Deterioration and Conservation of Stone, 1992:345]349. w2x Livingstone RA. Graphical methods for examining the effects of acid rain and sulphur dioxide on carbonate stones. Lisbon: 7 th International Congress on Deterioration and Conservation of Stone, 1992:375]386. w3x Cooke RU, Gibbs GB. Crumbling heritage: studying stone weathering in polluted atmospheres. Swindon: National Power Plc, 1993:68. w4x Spedding DJ. Sulphur dioxide uptake by limestone. Atmos Environ 1969;3:683]685. w5x Goturk ¨ ¨ HM, Volkan M, Kahveci S. Sulfation mechanism of travertines: effect of SO 2 concentration, relative humidity and temperature. Conservation of stone and other materials. Paris: RILEM, 1:83]90. w6x Johansson LG, Linqvist O, Mangio RE. Corrosion of calcareous stones in humid air containing SO 2 and NO 2 . Dur Build Mater 1988;5:439]449. w7x Lewry AJ, Asiedu-Dompreh J, Bigland DJ, Butlin RN. The effect of humidity on the dry deposition of sulphur dioxide onto calcareous stones. Constr Build Mater 1994;8Ž2.:97]100. w8x Spiker EC, Comer VJ, Hosker RP, Sherwood SI. Dry deposition of SO 2 on limestone and marble: role of humidity.
108
w9x
w10x
w11x
w12x w13x w14x w15x w16x w17x w18x
C.M. Grossi, M. Murray r Construction and Building Materials 13 (1999) 101]108 Lisbon: the 7th International Congress on Deterioration and Conservation of Stone, 1992:397]406. Weber J. Natural and artificial weathering of Austrian building stones due to air pollution. Laussanne: the 5th International Congress on Deterioration and Conservation of Stone, 1985:527]533. Wittemberg C, Dannecker W. Salt enrichment in building stones by deposition of sulphur and nitrogen containing species from urban atmospheres. Venice: The Conservation of the Stone in the Mediterranean Basin, 1994:179]183. Hutchinson AJ, Johnson JB, Thompson GE, Wood GC, Sage PW, Cooke MJ. Stone degradation due to dry deposition of HCl and SO 2 in a laboratory-based exposure chamber. Atmos Environ 1992;26AŽ15.:2785]2793. Vos BH. Fundamentals of heat and moisture transfer. The deterioration and conservation of stone. Studies and documents on the cultural heritage. UNESCO, 1988:67]88. Torraca G. Porous building materials. ICCROM, 1988:149. Quenard D, Sallee ´ ´ H. Le transfert isotherme de la vapeur d’eau condensable dans les materiaux microporeux du batiment. ´ ˆ Cahiers du CSTIB, livraison 323, cahier 2525, 1991:19. Lipfert FW. Atmospheric damage to calcareous stones: comparison and reconciliation of recent experimental findings. Atmos Environ 1989;23Ž2.:415]429. Webb AH, Bawden RJ, Busby AK, Hopkins JN. Studies on the effects of air pollution on limestone degradation on Great Britain. Atmos Environ 1992;26BŽ2.:165]181. Normal 7r81, Assorbimento d’acqua per immersione totale. Capacita ` d’imbibizione. Rome: CNR-ICR, 1981. Johnson JB, Haneef SJ, Hepburn BJ, Hutchinson AJ, Thomp-
w19x
w20x w21x
w22x w23x w24x w25x w26x w27x
son GE, Wood GC. Laboratory exposure system to simulate atmospheric degradation of building stone under dry and wet deposition conditions. Atmos Environ 1990;24AŽ10.:2585]2592. Lewry AJ, Butlin DJ, Butlin RN. A chamber study of the effects of sulphur dioxide on calcareous stones. Lisbon: the 7th International Congress on Deterioration and Conservation of Stone, 1992:641]650. Jaynes SM, Cooke RU. Stone weathering in south-east England. Atmos Environ 1987;21:2425]2433. Grossi CM, Esbert RM, Lewry AJ, Butlin RN. Weathering of building carbonate rocks under SO 2 polluted atmospheres. Balkema, Rotterdam: 7th International IAEG Congress, 1994:3573]3582. Grossi CM, Murray M, Butlin, RN. Response of porous building stones to acid deposition. Water Air Soil Pollut 1995;85Ž4.:2707]2712. Nord AG. Analysis of deteriorated sandstone at the Royal Palace, Stockholm, vol 117. GFF, 1995:43]48. Arnold A, Zehnder K. Salt weathering on monuments. Bari: The Conservation of Monuments in the Mediterranean Basin, 1989:31]58. Auger F. World limestone decay under marine spray conditions. Bari: The Conservation of Monuments in the Mediterranean Basin, 1989:65]69. Smith BJ, McGreevy JP. Contour scaling of a sandstone by salts weathering under simulated hot desert conditions. Earth Surf Process Landforms 1988;13:697]705. Haneef SJ, Johnson JB, Dickinson C, Thompson GE, Wood GC. Effect on dry deposition of NOx and SO 2 gaseous pollutants on the degradation of calcareous building stones. Atmos Environ 1992;26A:2963]2974.
Characteristics of carbonate building stones that influence the dry deposition of acidic gases C.M. Grossi a,U , M. Murray b a
STATS Consultancy, Porters Wood, St. Albans, Herts AL3 6PQ, UK Building Research Establishment, Garston, Watford WD2 7JR, UK
b
Received 8 April 1998; accepted 6 April 1999
Abstract The behaviour of several porous carbonate building stones used in Spanish and English monuments was compared. Stones were exposed to urban and suburban environments and subjected to simulated SO 2 atmospheres in the laboratory. Physical properties that affect the transfer of moisture were determined before and after exposure, and related to the degree of reaction of the stones. The degree of reaction was determined by analysing the reaction products. Results showed that physical properties influenced the reactivity of the stones. These characteristics also changed as the stones weathered due to the different concentrations of impurities in the material. Moreover, stones with a high specific surface area andror a deliquescent salt content may promote more NO x dry deposition. Q 1999 Elsevier Science Ltd. All rights reserved. Keywords: Porous carbonate building stones; Physical properties; Acidic deposition
1. Introduction Chemical degradation of carbonate building stones is considered to be due to atmospheric conditions such as dry deposition of acidic gases and aerosols, dissolution by acidic species in rainwater and dissolution by unpolluted water w1]3x. However, the degradation also depends on stone properties. 1.1. En¨ ironmental conditions The dry deposition of gaseous SO 2 and subsequent oxidation to sulphate is considered to be the main weathering agent for carbonate building stones. Additional sulphate can be deposited by particle deposition. The rate of uptake of SO 2 by carbonate stones depends on the relative humidity w4]8x, especially in
the case of porous stones w9x, suggesting the importance of condensed water in the pore structure for sulphation. NO x can deposit on carbonate stones by dry deposition although at a low rate w2,6x. According to Wittemburg and Dannecker w10x a source of nitrates in the stones might be dry deposition of gaseous nitric acid and nitrogen oxides followed by oxidation. NO 2 can also influence the uptake of SO 2 . In the presence of humidity NO 2 can promote the oxidation of SO 2 w6x although there is no strong evidence for this outside laboratory studies. Hutchinson et al. w11x studied the dry deposition of HCl on carbonate stones and determined that in moist conditions HCl is quickly absorbed and reacts with the calcium carbonate to produce CaCl 2 which is easily removed by water. 1.2. Properties of stone
U
Corresponding author. Tel.: q44-1727-83361, ext. 323; fax: q441727-811528. E-mail address: [email protected] ŽC.M. Grossi.
The uptake of acidic gases depends on the amount of
0950-0618r99r$ - see front matter Q 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 9 5 0 - 0 6 1 8 Ž 9 9 . 0 0 0 1 9 - 7
C.M. Grossi, M. Murray r Construction and Building Materials 13 (1999) 101]108
102
moisture in the stone, which is determined to a great extent by properties that affect the transfer of humidity. These properties are: absorption and permeability of liquid water and hygroscopic moisture content, evaporation and permeability of water in the vapour phase. However these properties can all be related to porosity and specific surface area Žpore size distribution.. Porosity can be regarded as the principal characteristic of the stone as the rest of the properties are determined by open porosity and pore size distribution. Hygroscopic moisture content refers to the uptake of moisture from the ambient air w12x. It is considered to be very important in the dry deposition of acidic gases. Hygroscopic adsorption depends on the relative humidity of the air, the nature of the material, the diameter of the pores w13x and the specific surface area. Several authors have indicated that SO 2 deposition is proportional to the surface area of the stone wetted by adsorbed water w8x. Hygroscopic salts Žincluding corrosion products such as nitrates, chlorides and previous deposits of SO 2 . can influence the moisture content of porous materials. These salts attract moisture from the air and could be important in maintaining the moisture film on the surface of carbonate stones. Non or slightly hygroscopic materials can become strongly hygroscopic when such salts are entrapped in their pores. This can lead to increase absorption and oxidation rates of atmospheric SO 2 to form calcium sulphate w12,15,16x. The presence of soluble impurities can also function as a catalyst in sulphation w11x. The role of the specific surface area is very important with respect to the adsorption of water to material surface at low relative humidity w14x. At higher relative humidity capillary condensation takes place. For condensation to take place, pore radii must be less than the radius given by Kelvin’s equation. This paper investigates the characteristics of carbonate building stones that influence the dry deposition of acidic gases. In order to do this, different stones were subjected to polluted atmosphere tests in the laboratory as well as being exposed on-site. Some physical properties of the stones were measured and related to the degree of reaction of these stones. Finally re-
lationships between stone properties and corrosion products were established.
2. Materials and initially physical properties tests The selected materials were three carbonate stones used as building materials in Spanish and English monuments: Laspra dolomite, Hontoria limestone and Portland limestone. Laspra is a micrite composed mainly of dolomite Žapprox. 90%.. The material used in these experiments corresponds to blocks from the Cathedral of Oviedo ŽAsturies, Spain.. Hontoria and Portland are a bioclastic and an oolitic limestone, respectively. The material from this research was extracted from the quarries of Hontoria de la Cantera ŽBurgos, Spain. and Bowyers ŽPortland, UK.. The following properties related to transfer of moisture have been measured: v
v
v
Open porosity, pore size distribution and specific surface area by mercury porosimetry ŽCarlo Erba Serie 200. ŽTable 1.. Porosity is very high in Laspra and high in Hontoria and Portland. Free water absorption by total immersion Ž Cw . ŽNormal 7r81. w17x ŽTable 2.. Hygroscopic water content Ž CH . Žat T s 238C; RH s 60%. ŽTable 2..
The amount of absorbed water seems to be related to open porosity}amount of accessible voids} whereas hygroscopic moisture content}water adsorption and capillary condensation}depends on the pore size distribution and therefore on the specific surface area.
3. Exposure experiments The three carbonate stones have been subjected to: v
v
An exposure to simulate SO 2 polluted atmospheres in a climatic chamber; and a 1-year exposure on-site.
Table 1 Porosity Žmercury porosimetry.
Laspra Hontoria Portland
Open porosity Ž%.
Specific surface area Žm2 rg.
Specific pore volume Žmm3 rg.
Specific pore volume radius - 1 mm Žmm3 rg.
Specific pore volume radius - 0.1 mm Žmm3 rg.
31 20 20
4.03 0.26 1.21
164 94 94
155 27 48
34 2 17
C.M. Grossi, M. Murray r Construction and Building Materials 13 (1999) 101]108 Table 2 Physical properties a
Laspra Hontoria Portland
surface composition by Fourier Transform Infra-Red ŽFT-IR. spectroscopy and weight change. Cw Ž%.
CH Ž%. ŽRHs 60%.
12 6 7.5
1.25 0.005 0.04
a
Cw s free water absorption by total immersion; CH s hygroscopic moisture content ŽT s 238C, RHs 60%..
3.1. Laboratory exposure Laspra and Hontoria were exposed to dry deposition of SO 2 in an atmospheric flow chamber following the methodology used by Johnson et al. w18x and Lewry et al. w19x. The results were compared with those obtained in Portland limestone under the same conditions of exposure w7x. The concentration of SO 2 was equivalent to an atmospheric concentration of 20 mgrm3 w7x. During the exposure period, half the samples were subjected to simulated rain; the other half remained dry. In this way, dry deposition on both wet and dry surfaces has been simulated. The simulated rain consisted of cycles of 8-h wetting followed by 16-h drying ŽT s 19 " 38C; RHs 84%.. The aqueous ‘run-off’ from the surface of each sample was collected weekly for analysis. The exposure period was 30 days, simulating a year’s exposure in an area with annual average rainfall of 800 mm. After exposure a chromatographic analysis of cations and anions was carried out both of run-off solutions and extracted reaction products Žsoluble salts.. 3.2. On-site exposure The selected materials have been exposed for 1 year in two different environments: v
v
103
Central London Žexterior of Westminster Abbey.; and Garston, Hertfordshire Žexposure site at the Building Research Establishment, BRE., suburban environment.
At each of these sites, pairs of freely rotating carousels of stone tablets Ž5 = 5 = 1 cm. were installed following the methodology used by researchers at the BRE. One of the carousels was sheltered from rainfall and exposed only to dry deposition. The other was exposed to wet and dry deposition. The concentration of ions was determined by ionchromatography in successive drillings of 0.5 mm from the surface to a 2-mm depth. Further analysis was carried out on samples exposed both in the laboratory and on-site. These included
4. Results and discussion Sulphates were the main products of reaction. Sulphur dioxide is the main acidic species involved in dry acid deposition of these stones w1,20x. The most deteriorated stone was Laspra dolomite both in laboratory and on-site experiments. The amount of sulphate deposited on Laspra surface was higher than on Hontoria and Portland stones. Surface recession was also higher in laspra dolomite. Both characteristics have been related to the physical properties of these materials in previous works w21,22x. 4.1. Reaction products Sulphite ŽSO 32y . }the intermediate product in the oxidation of SO 2 to SO42y}was found in laboratory tests. However, it is never found in real exposure due to the presence of NO x , O 3 , particulate matter and other compounds that facilitate the oxidation to SO42y. Most of the sulphates newly formed in sheltered samples were calcium sulphate even in Laspra dolomite. In Laspra samples from on-site exposure, SO42y balances perfectly well with Ca2q. This was corroborated by determining the ion balance SO4 with Ca or Ca and Mg according the methodology followed by Butlin et al. w1x. Moreover, the weight ratio SrCa varies between 0.7 and 1. The weight ratio SrCas 0.8 in pure gypsum was used by Nord w23x to determine the calcium bound as gypsum. Fig. 1 shows the CarSO4 ratio in all the samples and the Ca and MgrSO4 for Laspra dolomite. MgSO4 ŽFig. 1. has been found as a reaction product in the Laspra run-off samples from climatic chamber experiments, where the experimental conditions are different: the only polluting gas is SO 2 and the water is deionised. Interestingly the proportion of Ca and Mg sulphate varies over the duration of the test ŽFig. 2.. Sodium, potassium, chlorides and nitrates were detected in Laspra run-off Žclimatic chamber.. These ions were present in the stone before the test. Laspra is an old stone with a high specific surface area that facilitates the uptake of moisture. This fact can favour gas and aerosol deposition. Deposited soluble compounds might migrate and accumulate inside the stones. Another possible source of these salts is ground water coming to the stone by capillary suction. Laboratory results agree with those obtained on-site. In the reference and sheltered samples there is a high concentration of chlorides and especially of nitrates, whereas samples exposed to rainfall show a lack of
104
C.M. Grossi, M. Murray r Construction and Building Materials 13 (1999) 101]108
Fig. 1. CarSO4 and ŽCa and Mg.rSO4 ratio. Ža. Sheltered Hontoria and Portland on-site; Žb. sheltered Laspra on-site; Žc. run-off Laspra climatic chamber.
soluble salts which have dissolved and removed ŽTable 3.. By subtracting the data for rain-exposed samples from that for reference samples and by subtracting all chamber run-off sulphate and its Ca and Mg equivalents it was shown that some calcium and magnesium
were in the form of chlorides and nitrates in the reference sample ŽTable 3.. The best correlation was found between sodium and nitrates and chlorides. As a result in Laspra dolomite there is a mixture of deliquescent salts that can increase dry deposition by increasing the water content of the stones.
C.M. Grossi, M. Murray r Construction and Building Materials 13 (1999) 101]108
105
Fig. 2. Variation of the proportion of Ca and Mg sulphate over the test. Run-off water, climatic chamber experiment.
4.2. On-site results In sheltered Laspra, Hontoria and Portland samples of on-site experiments an increase in the amount of nitrates and chlorides was also found. Sulphates tended to concentrate in the first layers of the stone, whereas nitrates and chlorides which generally are more soluble and hygroscopic were evenly distributed across the sample ŽFig. 3. w24]26x. In Laspra dolomite the concentration of new formed nitrates was high. Moreover, as stated above, Laspra shows a high nitrate concentration in the reference samples ŽTable 3. which was higher than that for chlorides. NO x can transform to NOy 3 in the water adsorbed on the surface of the stone. However, the process is slow, the large Laspra surface area could allow more nitrate to be absorbed into the surface
moisture layers w22,27x. Gaseous HNO3 in the air might also be the source of nitrates. Wittemburg and Dannecker w10x from experiments carried out on-site found from regression analysis that the nitrate input was strongly correlated with the gaseous HNO3 in the air. Because the reaction of gaseous NO x with carbonate stone is much slower than that of SO 2 ŽSO 2 concentration in these experiments are 12]21 m g my3 of SO 2 and 42]61 m g my3 of NO x ., the characteristics of the stone pore network are very important in the deposition of nitrates. Stones with high surface area will have a high moisture content which will favour deposition of nitrates both from gaseous species and particulate. 4.3. Changes in hygroscopic water content Hygroscopic moisture content was measured at 60%
Table 3 Concentration of impurities in Laspra dolomite
Ca2q Mg2q Naq Kq Sum of cations SO42y NOy 3 Cly Sum of anions a
Concentration reference sample on-site Žmeq.rg.
Concentration exposed to rainfall Žmeq.rg. on-sitea
Concentration impurities on-site Žmeq.rg. Ž A.
Amount of ions in run-off chamber Žmeq.. Ž B .
BrA Ž=10y3 .
65.2 45.8 33.0 16.6
28.7 25.3 1.0 3.6
36.5 20.5 32.0 13.0
0.09 0.07 0.11 0.04
2.5 3.4 3.4 3.1
3.8 72.9 40.2
?
? 72.3 39.7
102 0.6 0.5
0.31 subtracted 0.22 0.11
112
3.0 2.8 0.33
It is considered to be the original concentration of the stone. In chamber run-off samples the amounts of sulphates were subtracted together with the equivalents of calcium and magnesium.
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C.M. Grossi, M. Murray r Construction and Building Materials 13 (1999) 101]108
Fig. 3. Distribution of anions across sheltered Portland samples Žon-site exposures.. Ža. Sulphates; Žb. nitrates; Žc. chlorides.
RH and 238C in both reference samples and on-site exposed samples ŽGarston exposure site.. This property was measured in tablets with an exposure period of 5 and 11 months in positions both sheltered and exposed to rain-fall. This property was found to vary when stones have been exposed on-site. In Laspra dolomite the hygroscopic moisture content
decreased dramatically when samples were exposed to rainfall. In the case of Hontoria and Portland stones the highest variation was detected in sheltered samples. The variation of this property after on-site exposure has been related specially to change in concentration of hygroscopic salts inside the stone. Table 4 shows the differences in concentration of
C.M. Grossi, M. Murray r Construction and Building Materials 13 (1999) 101]108
107
Table 4 Laspra dolomite: concentration of chloride q nitrate and hygroscopic moisture content Žon-site. Clyq NOy 3 Žmeq.rg.
CH Ž%. ŽRHs 60% T s 238C.
Sheltered Reference Garston 4 months West. 4 months Garston 12 months West. 12 months
Exposed
Sheltered
113.1 84.1 129.1 115.0 117.3
Exposed 1.25
1.3 16.4 1.0 1.3
chloride q nitrates of sheltered and unsheltered Laspra samples and the hygroscopic moisture content at 60% RH and 238C. Laspra dolomite had a high concentration of hygroscopic salts in the reference stone. When exposed to rainfall, the stone lost soluble salts ŽTable 3.. The hygroscopic moisture content after exposure to rainfall was reduced by approximately 50% Žtest conditions: RHs 60%, T s 238C.. In sheltered stones, this property does not show any variation with respect to the original values. In Hontoria and Portland limestones the hygroscopic moisture content seems to be higher in sheltered stones which show a increase in concentration of chlorides and nitrates. The results are not as convincing as in Laspra dolomite. The difference in salt concentration is not as dramatic as in Laspra, and there are not enough data to ensure that there is real variation in the property.
Garston 5 months
1.42
0.62
Garston 11 months
1.17
0.60
which allows more acidic gases to absorb. Ž5. Unless rain washed away, these salts can be retained indefinitely.
Acknowledgements
The authors wish to acknowledge Comision ´ InŽCICYT ., terministerial de Ciencia y Tecnologıa-Spain ´ project: SEC95-0501 and Fundacion ´ para el Fomento en Asturias de la Investigacion ´ Cientıfica ´ Aplicada y la ŽFICYT ., project: PB-REC96-08. Tecnologıa-Spain ´ Also to Dr Rosa M Esbert and Mr Francisco Dıaz´ Pache from the University of Oviedo ŽSpain..
References 5. Conclusions As expected, the Spanish and English porous carbonate stones are very reactive to rainwater and acidic gaseous deposition Žmainly to SO 2 .. The main product of reaction found in external atmospheres was gypsum ŽCaSO4 .2H 2 O.. Sulphite was also formed as an intermediate product in laboratory studies. It was allowed to crystallise due to the pure and ozone free environments. The nature of the reactivity, however, could be related to specific physical properties of the stones: Ž1. porosity is important as the open structure allows more water into the sample and so more carbonate to dissolve. Ž2. Pore distribution and specific surface area influence the uptake of moisture from the air. High surface areas will have a high moisture content which will favour dry deposition. That could be important in the case of the deposition of NO 2 Žwhich is less soluble than SO 2 or HCl.. Ž3. Dry deposition causes an accumulation of deliquescent salts as well as gypsum. Ž4. Deliquescent salts can, at the same time, increase dry deposition by increasing the water content of the stones
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