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ENVIRONMENTAL STUDIES

ARTIFICIAL CORROSION OF UNTREATED AND TREATED SAREMA DOLOMITE

KRAGE,L.

Riga, Latvia

I. INTRODUCTION

According to D.B. Honeybome (J. Ashurst and F.G. Dimes) there are three main causes of deterioration of natural stone and masonry involving loss of substance. These are:

1. salt crystallisation, 2. attack by acidic gases in the air, 3. frost action.

All those negative factors have been observed also in the processes of the corrosion of stone objects in Riga Cathedral (Latvia).

The increased amount of air pollutants S02, NOx, soot and dust, presence of soluble salts in ground water, humidity, rain and frost actions during winter and drying during summer causes the decay of dolomite carvings exposed to the environmental conditioins in the church yard. Among all those factors the air pollution and salt crystallisation is the most important because it is potentially the most damaging. It attacks porous materials irrespective of their chemical composition and often enhances the effects off the other primary causes of decay (J. Ashurst and F.G. Dimes).

However the sources of solubie salts in the cross vaulted cloister passages of the church have not been definitely detected. They possibly can arrive:

from action of polluted air on stone-, from fired - clay bricks (the walls of church are built from bricks); sea water (as the city Riga is located not far from the sea side); road and pavement de-icing salt (as the salt was used in order to avoid the ice on the surrounding

streets during many years); salt used for preserving meat etc. (some years ago there was the salt storage at the building close

to monument).

Practical stone conservation at the Riga Cathedral was begun in 1984. The cleaning and desalination of the mechanically strong surfaces was carried out beginning from 1986. In order to consolidate the stone, saturated limewater was applied repeatedly 40 times (Vitina, et al, 1990). The lime poultice method was also used in order to clean and consolidate the stone. Thus as conservation methods for this study the

lime method was chosen for dolomite samples exposed to different environmental conditions.

The following samples of Sarema dolomite were used for experiments:

I) untreated Sarema dolomite; 2) Sarema dolomite treated with lime poultice;

3) Sarema dolomite treated with lime water.

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1.1 . Short description of lime methods

There is much to be said for the view that the composition of a surface treated should be close to that of the stone to be conserved (J.Ashurst and F.G. Dimes). The use of lime water and lime poultice in Riga

Cathedral was based on literature studies (mainly the works of Ashurst 1984). Usually the stone surface was cleaned with lime poultice and then the consolidation process was continued by lime water

application.

1.1.1 Consolidation with lime water

Lime water is a solution of slaked lime (calcium hydroxide, Ca (OH)i) in water. A litre of this solution

contains about 1.7 g of solid. Carbon dioxide gas in the air will react with the calcium hydroxide to form calcium carbonate (limestone). The chemical change is represented by the equation:

Repeated applications are necessary to produce sufficient calcium carbonate to consolidate stone (Clifton, 1982).

According to Ashurst (1984) limewater is siphoned from the slaking tank after lime has been slaked in an excess of water and after all slaking has ceased and the water is clear. But usually, lime putty is stirred into a container of water and left to sediment until the water is clear. It is important that the limewater is protected from the air, since it will otherwise carbonate and become ineffective. Only clear limewater must be used. Application can continue as long as the surface will absorb, but excess limewater should not be allowed to stay on the surface of the stone and should be removed.

1.1 .2. Cleaning with lime poultice

The principle use of lime for cleaning is to achieve a progressive softening of dirt deposits, sufficient to enable them to be removed by brushing. The traditional method of cleaning associated with it is by using a hot lime poultice (Ashurst, 1984). The lime putty has to be pressed into the surface of pre-wetted stone and have to remain damp and soft during a period of two to three weeks. When the packing is finally removed, the lime is carefully lifted off in small areas at a time with spatulas or small trowels, bringing away some of the dirt from the contact surface.

II. EXPERIMENTAL 11.1 . Preparation of samples

Samples, sawed from blocks of Sarema dolomite were firrst polished in distilled water with 220 and later with 400 mesh to measure 40 46 120 mm. The samples were placed in distilled water and submerged in an ultrasonic bath for 10 minutes three times with an exchange of water after each cleaning. After cleaning the samples were dried at 60+5° C until constant mass was attained. Samples prepared in this manner were later used untreated or treated with lime methods.

Three types of samples were used for experiments: 1) untreated Sarema dolomite; 2) Sarema dolomite treated with lime poultice; 3) Sarema dolomite treated with lime water.

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11.2. Conservation treatments

11.2.1. Consolidation with lime water

In order to prepare the lime water, ca 50 g Cao (reagent grade) was mixed with distilled water and slaked

for 48 hours. The treatment vessels with lime water was closed in order to protect the lime water from

carbonatization. For treatment, only clear lime water was used. All samples were treated with lime water 40 times during 12 days by immersion. This involved immersion

of the samples for 30 minutes each time, 3-4 times per day. After each immersion, excess solution was

removed by blotting with damp absorbent paper and the samples were let to dry at rooms temperature.

After every treatment the weight gain was detected in order to calculate the eventual deposition of Cao or

theoretical quantity of CaC03 (look tabie 1.).

11.2.2.Treatment with lime poultice

Ca 200g Cao (reagent grade) was mixed with ca 550 ml distilled water and slaked 24 hours. The pre­

wetted stone samples, which were immersed in distilled water for 24 hours, were covered with a 1 cm thick

lime putty on all sides. The samples were packed at first with wet wool-cotton then with parafilm and at

last with Aluminium folly. The samples were remained then for two weeks. The poultice was kept damp

and soft during the whole period. After two weeks the packing was removed, the lime lifted off and the samples washed with the help of a

brush and distilied water.

11.3. Surface roughness

Stone samples, dried at 600 C for 24 hours and weight (until they reached constant surface roughness

parameters, such as Ra. Rt and Rz were calculated from surface profiles, recorded with a "Hommel Tester

T500" (Hommelwerke GmbH, Germany). In this report, Ra (µm) - arithmetic mean roughness value - is

the arithmetic mean of the profile deviation of the measuring length Lm (4.8). Rt - maximum individual

peak to valley high - is the vertical distance between the maximum highest points of the filtered roughness

profile within the reference length. Rz - ten point high - is the main value of the absolute values of the 5

highest peaks and the 5 lowest valleys within the reference length Lm (determined from the filtered

roughness profile). 10 measurements were performed on each of two parallel untreated and treated stone sampies. The

same measurements were done also after the exposure to artificial climatic conditions.

11.4. Artificial weathering

Prior to artificial weathering test in climatic chamber the untreated and treated weight were immersed in a

1 m NaCl (Amoroso and Fassina, 1983) solution for 24 hours (Teutonico. 1988). After that samples were

exposed to the following cycle:

12 hours + 40° C and RH= 50 %

12 hours - 5° C and RH= 90%

12 hours + 40° C and RH= 50%.

After each cycle the optical observation, weight changes and surface roughness measurements of

samples were carried out, following by new treatment with 1 M NaCl solution and new weathering cycle.

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Ill. RESULTS

111.1 . Conservation treatments

After the treatment with lime water and lime poultice, the formation of theoretical CaC03, was calculated according to the weight gain of samples. Table 1. shows the formation of CaC03, after treatment with both

methods, calculated as:

m2-m1 M(%) = -------------- 100

m1

where m1 - initial mass of dry sample (g);

m2 - mass of dry treated samples (g);

M - formation of CaC03 (%).

Tabie 1. Formation of CaC03 in dolomite after the treatment with lime methods

Formation of CaCO, after the treatment with

Sample lime poultice(%)

LP 1 0.108

LP 2 0.114

LP 3 -3.161

LP 4 0.0-97

Sample Formation of CaC03 after the treatment with lime water(%)

LW 1 0.243

LW2 0.215

LW3 0.245 LW4 0.241

111.2. Surface roughness measurements

The surface roughness parameters measured after each exposure cycle is showed in table 2.-4. and depicted graphically in figure 1.

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Table 2. Changes of surface roughness of untreated dolomite

N° of Ra Rz Rt exp. C2YCles

0 3.87 28.71 37.00 1 4.47 34.07 47.89 2 4.31 31 .34 42.59 3 5-01 35.30 53.-62 4 6.24 44.27 55.18 5 6.88 38.38 51 .10 6 6.77 37.47 52.13 7 6.77 36.84 49.25 8 7.88 43.17 61 .25 20 8.23 43.38 61.51

Tabie 3. Changes of surface roughness of dolomite treated with lime poultice

N° of Ra Rz Rt exp. cycles

0 5.04 43.15 61.38 1 4.53 34.85 48.78 2 4.46 35.23 51 .66 3 5.07 38.57 57.11 4 4.63 37.21 60.41 5 5.23 36.58 45.20 6 5.49 33.37 48.94 7 6.40 35.79 52.89 8 5.93 34.70 50.81

20 7.97 43.29 56.63

Tabie 4. Changes of surface roughness of dolomite treated with Linie water

N° of Ra Rz Rt exp. ~cle

0 4.07 34.02 41 .99 1 5.25 40.33 58.14 2 3.59 29.98 40.61 3 4.24 33.07 50.19 4 3.91 30.48 43.86 5 4.58 28.07 38.64 6 4.28 26.53 53.42 7 4.09 26.32 37.06 8 1 .. 64 27.55 39.83 20 5.68 32.50 50.42

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9 s 7 6

5 Ra

4 3 2

0 0 1 2 3 4 5 6 7 8 20

Number of exposure cycles

Fig.1 . Changes of Ra values during exposure cycles

As we can see from Fig.1. and tab.2.-4. the values of Ra. Rz, and R, for untreated dolomite before the exposure is lower than for lime treated dolomite. Due to conservation treatments the crystals of Ca(OH), or eventually calcite CaC03 are deposited on the surface and in the pore system and the value of specific surface for treated dolomite is much higher than for untreated one. During the first steps of artificial corrosion the surface roughness parameters for untreated dolomite increase rapidly while in the case with both lime treated samples the decrease is obtainecl (look fig 1.). So for lime treated samples the Ra values decrease after 1st, 2nd and 3rd exposura cycies. then the increase is obtained. The similar phenomenon occur with Rz and Rt values, where decrease until 7th or 6th exposure cycle occur which is followed by rapidly increase. Thus the changes of surface roughness for lime treated dolomite can be divided in two steps: at first the NaCl salt solution dissolves the crystals of CaC03 deposited during conservation treatment and the suface roughness decrease. At the second step the soluble NaCl salts crystallises in the pores and on the surface - roughness increase. Ra. Rz and R1 values for untreated dolomite increase rapidly and after the 20th cycle are higher than for treated dolomite. Also the difference between both lime methods can be observed. Thus the surface roughness of dolomite treated with lime poultice is much higher than for the dolomite treated with lime water and in the case of Ra and R, it is equal with the values of untreated dolomite. It can be explained with the different deposition of Ca(OH)2 during both treatments. Thus the Ca(OH)2 from lime poultice deposits mainly on the surface and fills the large pores while the Ca(OH)2 from lime water have possibly the deeper deposition depth and better boundary with the stone. As the values of Rz and R, are dispersed in a large scale, they are not depicted as a curie. It could be explaired with the pore structure of dolomite, as the pore sizes differs a lot between themselves. The Rz and R, values are more sensitive to changes in surface and to distinguish the results influenced by artificial corrosion process and those due to unhomogenous structure of stone is very difficult. In the case of Ra the roughness average is measured and the results does not depend so much from individual peaks ard valleys.

Some more test methods of the changes in structure and other evaluation methods are neccessary in order to give some more objective conclusions. However this work was limited by time and the further research is required.

IV. CONCLUSIONS

The presence of NaCl in rising ground water or in air leads to dramatic changes of the surface of stone. Surface with increased roughness will better absorb air pollutants and moisture and this will contribute to more rapid stone decay.

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Stone treated with lime water is more resistant to the influence of soluble salt - NaCl as untreated or lime poultice treated stone samples. It means that as the both methods are used in Riga Cathedral in order to consolidate the stone, the treatment with lime water is more effective.

ACKNOWLEDGEMENTS

The financial support from Swedish Institute and Helgo Zetervall Foundation is gratefully acknowledged.

I am also thankful to Prof. S.Massa and P. De Carli from CNR (Italy) for advice and help.

REFERENCES Amoroso G.G., Fassina V. Stone Decay and Conservation. Elsevier, Amsterdam (1983).

Ashurst J. , Dimes F.G. Conservation of Building and Decorative Stone. Vol. 1. Butterworth Heinemann, UK (1990).

Aschurst j ., Dimes F.G. Conservation of Building and Decorative Stone. Vol.2. Butterworth Heinemann, UK (1990).

Ashurst, J. The Cleaninq -and Treatment of Limestone by the ,,Lime method" Part 1. Monumentum, Vol.

27. (1984) pp 233 - 252.

Ciiftofi J., Frohnsdorff G.l.C, Stone - consolidating materials: A Status report Conservation of Historic

Stone Buildings and Monuments. National Academy Press, 'Washington, D.C. (1982), pp 287 - 31 1.

Teutonico J.M. A Laboratoy Manual for Architectural Conservators. ICCROM, Rome (1988), pp 31 ,- 40, 50

- 55; 57.