1195

CONSOLIDATION EFFECT OF WACKER-SILICONES ON THE PROPERTIES OF SANDY LIMESTONE

HRISTOVA, JULIA

Central Laboratory of Physico-Chemical Mechanics, Bulgarian Academy of Sciences, Acad.G.Bontchev str., Bl.1 , 1113 Sofia,

Bulgaria

TODOROV, VALENTIN

National Academy of Fine Arts, Shipka str. 1, 1000 Sofia, Bulgaria

KEY WORDS: Consolidation, sandy limestone, Wacker-Silicone OH, Wacker-Silicone 290L, porosity, water sorption, ultrasonic modulus of elasticity, strength.

SUMMARY The results of the laboratory consolidation of the sandy limestone from Madara plateau in Bulgaria have been discussed. The consolidation was performed with Wacker-Silicones OH and 290L. In order to establish the influence of the consolidants parallel study were carried out on natural (non­consolidated) and consolidated samples. The test specimens were investigated by ultrasonic method, mercury porosimetry, scanning electron microscopy, sorption method, etc.The results showed that the applied silicones alter the basic physical and mechanical properties of the stone (pore sizes and volume, fluid permeability, modulus of elasticity, strength). Pore volume and fluid permeability decrease and the modulus of elasticity and strength increase. The extent of the properties alteration depends on the type of consolidant and on the physico-mechanical characteristics of the individual

sample.

1.INTRODUCTION

The famous relief of the 8th century "The Horseman of Madara" has been carved on the cliff face of a plateau situated in North Eastern Bulgaria near the village of Madara. This unique monument is included in the world cultural heritage list. Unfortunately, the monument is subjected to severe damage originating from the geological and sedimental characteristics of the Madara crags [1]. The present paper deals with some investigations carried out in the frame of a National Programme aiming at establishing the possibilities of different conservation techniques for a long-term

preservation of the Madara Horseman relief. Previous and recent geological studies have revealed that depending on their location the rocks forming the Madara plateau are sandy limestone or calcareous sandstone. The cliff face on which the relief and the inscriptions beneath have been carved, represents a set of continuous transitions in the

frame of a single qualitative state - sandy (gravel-sand) limestone [2]. The major components constituting the Madara limestone are biodetricious calcite fragments (preferably debris of echinoids and bryozoans and less of bivalves and crinoids), quartz and quartzite grains and calcite cement. Negligible quantities of other minerals are incorporated as well. The soft, porous and crumbling sandy limestone are simultaneously subjected to three types of weathering [3]: physical (preferably high and low temperature), chemical (leaching and weathering) and biological (mechanical and chemical destruction caused by plants and micro-organisms). These processes

strongly endanger the existence of the monument. Investigations on the possible reinforcement of the Madara rocks have been carried out via treatment (impregnation) with definite chemical substances, known as consolidants. The present communication reports the results obtained with two consolidants manufactured by Wacker Chemie (Germany),

namely, Wacker Silicone OH and Wacker Silicone 290L.

1196

Regardless of the fact that the rocks of the cliff face with the relief have similar sedimental

characteristics and appearance, they differ in their physico-mechanical properties. As it will be shown

below, in some cases the difference is considerable. The present study demonstrates the results of the parallel experiments with non-consolidated (natural) and consolidated rock samples. Five rock

fragments were taken from different spots on the cliff face on which the relief had been carved. They were further cut into test bodies of prismatic shape and dimensions of 4 x 4 x 16 cm.

2. CONSOLIDATION

Wacker-Silicone OH denoted further as OH is ready made for immediate application tetraethyl silicate. It has very low viscosity, excellent penetration ability and does not change the colour of the

treated object. The test bodies prepared from Madara sandy limestone were treated with OH by a single immersion until complete saturation was achieved. Wacker-Silicone 290L (polysiloxane resin)

denoted further as 290L is diluted prior to use. When applied with concentration of 4 - 6 % 290L

exerts hydrophobic effect, while with concentration of 15 % - reinforcing effect. We used in our experiments 290L diluted with turpentine (290L : turpentine=1 :3). The final concentration of the 290L

solution was 26 %, v/v. Test prisms were immersed in the diluted 290L solution until saturation was achieved.

Measurements of the consolidated test bodies were carried out at least 30 days after consolidation was performed. It was accepted that this period of time was enough to ensure the formation of new phases in the bulk of the rocks.

3. COMPARATIVE POROSITY STUDIES

3.1.Porosity of natural rock samples (non-consolidated)

This study aims at obtaining information about bonded or open pores which are accessible to fluid penetration and are of utmost importance for the consolidation technology and processing. In this study bonded pores are conventionally classified as macropores and micropores. The macropores are indirectly evaluated by measuring the sorption capacity of 5-10 test bodies in water by the gravitation method [4]. It is suggested that the macropores correspond to the initially sorbed liquid on exposure to water.

The results showed that the quantity of the sorbed water measured 1 O min after exposure amounted to more than 60 % of the total water sorbed when equilibrium was attained. Hence, it could be suggested that the initial sorption capacity is mainly provided by pores and cavities of greater dimensions which are interbonded and are filled with water immediately after immersion.

Micropores have diameters within 5 - 7500 nm determined by a mercury porosimetry. Micropores with diameters from 5000 to 7500 nm are dominating in the Madara sandy limestone. The second big group of microporese comprises micropores with diameters of 100 - 500 nm.

1197

Table 1

Porosity of the sandy limestone samples (non-consolidated)

Sandy Total Ope n p 0 res, % limestone porosity, Macroporosity Microporosity samples % (Sorbed water, %, after exposure (mercury

10 min 1 h Sh 100 h 400 h) porosimetry)

1 24.19 4.27 4.83 4.88 5.48 5.90 7.20 2 19.80 2.76 3.17 3.31 4.09 4.40 8.15 3 20.25 4.20 4.57 4.67 5.24 5.70 5.75 4 16.00 2.50 2.84 2.93 3.38 3.70 5.50 5 20.29 3.50 4.08 4.16 4.73 5.00 6.70

Table 1 lists data on the total relative porosity of non-consolidated (natural) rocks and the volume of bonded macro- and micropores in the studied samples, while Table 2 - the micropore size distribution.

Table 2

Micropore size distribution in natural (non-consolidated) rock samples

Sandy M i c r o p o r o s i t y, % limestone p 0 r e s i z e s, nm samples to 20 25-100 100-500 500-1000 1000-5000 5000-7500

1 25.5 7.5 20.0 6.0 10.5 25.6 2 8.0 14.0 28.0 12.0 12.0 6.0 3 12.5 18.0 22.5 6.5 13.5 27.0 4 6.5 0.0 5.5 20.0 19.0 28.5

It is seen that the total porosity varies in the range from 16.0 to 24.2 %. If one regards the bonded macro- and micropores as a portion of the total porosity it is established that the macropores amount to 23.1 - 28.1 %, while the micropores to 28.4 -41 .2 % That means that in natural rock samples bonded pores accessible to external fluids amount to 54.1- 63.4 % of the total porosity.

3.2. Effect of consolidants on open porosity

The effect of the consolidants on the macroporosity was indirectly evaluated from the change in sorption capacity of the rock samples in water. The results obtained show that the applied consolidants reduce the amount of the sorbed water and hamper the sorption process. An one hour exposure reduces the water amount by 70-90%. It can be suggested that part of the pores or cavities is filled (partially or completely) by the consolidant, the latter thus facilitating the transformation of bonded pores into closed ones. This process additionally prevents the penetration of water and causes a change in the sorption kinetics. The total macropore volume is reduced by 24.3 - 45.3 %

(Table 3).

1198

Table 3 Influence of the Wacker-Silicone OH on the open pores volume

Sandy limestone Pores volume, %, against natural samples

samples Sorbed water after exposure Mercury porosimetry

2

4 5

1 h 5 h 100 h

4.6 6.0

13.0 6.0

5.5 7.5

14.3

7.5

24.3 31 .3

45.3 28.7

82.2

90.5

A similar effect was observed with pores detected by a mercury porosimeter. Their total volume was

reduced by 9.5 to 17.8 %. As for the size distribution no general tendency could be established. E.g. application of the OH causes an increase in the relative share of the pores with diameters of 1000 -

5000 nm, while application of the 290L increases the relative share of the pores with diameters of 500 - 1000 nm.

3.3.Scanning electron microscopy of consolidated samples

These investigations aimed at establishing the location of the penetrated consolidants. As it was to be expected, the consolidants were deposited in the pores and in the less dense sections of the contact

zone. The latter were thoroughly examined. The consolidants were found to be present on the surface of the sand grains and on other structure units neighbouring the pores or located in pores, as well as in the contact zone (Fig .1).

Fig.1. Structural details of the sandy limestone treated with Wacker-Silicone

290L (a - 300x). Supermolecular structure of the consolidate observed on the details (b- 20000x).

1199

Electron microscopy observations confirmed the conclusions concerning consolidant deposition in the

pore area of the rocks derived from the sorption capacity and microporosity analyses. They provide additional information about the consolidants present in the contact zone as well.

4. COMPARATIVE ULTRASONIC TESTING

4.1. Ultrasonic characteristics of natural samples

Ultrasonic testing is a non-destructive technique which provides information about the structure density and mechanical properties of the studied materials (5] . It was applied in determining the ultrasonic characteristics (transit time, waves velocity, modulus of elasticity) of all test bodies prior to their consolidation and remaining tests. The results obtained reveal that the Madara sandy limestone samples differ in their ultrasonic characteristics (see Table 4, where are listed the mean values obtained by measuring of 20-25 test bodies). The values of ultrasonic modulus varies in the range of 9.89 GPa for sample 1 which has the worst mechanical parameters to 16.24 GPa for sample 5 with the best physico-mechanical characteristics, respectively. The transit time, which is an indirect characteristic of the structure density is also varied in the range from 59.03 µs to 75.42 µs .

Table 4

Ultrasonic test characteristics of the sandy limestone samples (non-consolidated)

Sandy limestone Transit time, Wave velocity, Modulus of

samples µs mis elasticity, GPa

1 75.42 2199 9.89

2 70.43 2308 12.15

3 65.24 2493 12.84

4 65.58 2471 14.03

5 59.03 2648 16.24

4.2. Effect of consolidants on ultrasonic characteristics

The effect of the consolidants was evaluated by comparing the ultrasonic characteristics of the test

bodies prior (natural state) and after consolidation. At least 10 test bodies of each rock sample were

tested. The results obtained confirmed the conclusion that the treatment with consolidants packed the limestone structure. As a result the transit time was reduced, while the ultrasonic wave velocity as

well as the modulus of elasticity were enhanced, i.e. the elastic properties were improved. As seen from Table 4 the reinforcing effect varies in a certain range depending on the sample. With OH the highest modulus was observed with the weakest sample 1 (123.3 %). The moduli of the remaining samples were also considerably altered - by 61 - 87 %. The ultrasonic pulse transient time was reduced by 29 - 31 %, respectively. The 290L consolidant also changed the dynamic characteristics

but the changes were 2 times less as compared to these with OH (see Table 5) .

'

1200

Table 5

Ultrasonic characteristics of the consolidated samples against the natural samples

Consolidant Sandy limestone Changes in the characteristics, %

samples Transit time Modulus of elasticity

Wacker-silicone OH 1 32.9 123.3

2 21 .2 61.1

4 23.2 68.0

5 25.8 86.7

Wacker-silicone 290L 4 12.3 30.7

5 11.9 29.0

5. COMPARATIVE STRENGTH TESTING

The flexible strength was determined by the three point flexion design with at least 5 test bodies. The

prismatic test bodies were loaded as a beam on two supports with a strength applied in the middle of the support spacing. The flexible strength was calculated from the destruction strength. The produced

halves of the prisms in the flexible strength test were further used in determining the compressive

strength. The compression loading was realised by two plates (disks) applied to the side planes of the prism halves. The mean strength values of non-consolidated (natural) rock samples are presented in

Table 6. It is seen that they vary in the range from 3 to 5 GPa with flexion and from 9 to 17 GPa with compression.

Table 6

Strength data about natural sandy limestone

Sandy limestone Flexible Compressive samples strength, GPa strength, GPa

1 2.90 9.02 2 3.89 10.15 3 4.56 12.40 4 4.86 16.72 5 4.78 15.83

The results obtained for OH consolidated samples show that the strength growth is more expressed

for the flexible strength (by 3 - 55 %) and for the sandy limestone of poorer physico-mechanical characteristics (by 27 - 55 %). The compressive strength was enhanced by 1 to 31%. The corresponding increase caused by 290L was 0 - 21 % for the compressive strength and 1 _ 24 % for the flexible strength. This increase is demonstrated in Table 7.

1201

Table 7

., Changes in the strength of the consolidated samples against natural samples

Consolidant

Wacker-Silicone OH

Wacker-Silicone 290L

Sandy limestone Changes in the strength values, % samples Flexible test Compressive test

1 2 4 5

4 5

55 27 3 7

1 24

31 2

12

0 21

The discussed results show that the consolidation should be applied with care and caution taking into account all factors and circumstances affecting the process.

6. CONCLUSIONS

The reinforcing effect of the Wacker Silicones OH and 290L on the porosity and mechanical properties of sandy limestone from Madara cliffs has been investigated. It is shown that the Wacker­Silicone OH exerts stronger reinforcing effect as compared to the effect of the 26-% solution of the Wacker-Silicone 290L.

REFERENCES

1. Frangov, G., P.lvanov, N.Dobrev, ll.lliev. Stability problems of the rock monument "Madara Horseman". - In: Proc. of the 7th Int. Congress on Deterioration and Conservation of Stone. Lisbon, 1992, pp. 1425-1435.

2. Hadzhiev, G., H.Katsov, Ts.Tsankov. Litholigical and structural studies of the Madara Horseman relief.- Museum and Cultural Monuments, 1977, No 4, pp.20-35 (in Bulgarian).

3. Venkov, V., N.Kosev. Study of the rock in the area of the Madara Horseman relief with a view to its conservation, in: Investigation and Conservation of the Cultural Monuments in Bulgaria, Nauka i lzkustvo, Sofia, 1974, pp.83-97 (in Bulgarian).

4. Todorov, V., J.Hristova. Effect of some chemical substances on the sorption ability of the stone samples from Madara Horseman rock. - Physico-Chemical Mechanics, Vol. 24, 1996 (in

Bulgarian). 5. Krautkramer, J., H.Krautkramer. Ultrasonic testing of materials. Springer verlag, Heidelberg,

1977.