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INFLUENCE OF DIFFERENT CONSOLIDATING AGENTS ON THE WATER VAPOUR DIFFUSION PROPERTIES OF SELECTED STONES. HILBERT, G.

Remmers Chemie GmbH & Co., 49624 Loningen

WENDLER,E.

Fachlabor fiir Konservierungsfragen in der Denkmalpflege, 81245 Miinchen

SUMMARY

Depending on the type of agent used for consolidation, morphologically different types of strengthening solids are formed in the pore space of natural stones. Considering the different distributions of pore radii of different stones, it is evident, that the influence of a treatment on the water vapour transport may vary within a wide range. The present paper deals with laboratory investigations on vapour transport of different types of natural stone (with different types of pore radii distribution). treatet with different types of strengtheners. These results are illustrated by SEM images.

1. INTRODUCTION

Stone conservation includes not only steps to diminish the velocity of decay like hydrophobing agents reducing immisions and freeze damages

substances inhibiting hydroswelling.

In most cases the use of consolidating agents is necessary. In Germany beneath acrylates (esp. in Thuringia) two systems of consolidating materals are generally used:

Ethyl silicates TEOS Polyurethanes PU

TEOS consolidation mechanisms and effiency have been broadly discussed by SA TILER 1992, a similar work on PU was published by HONSINGER 1990. Figure 1 (a-e) shows HONSINGERs (1990) sketch of principal deposition mechanisms of the different conservation agents in natural stones porous space. Consolidating agents discussed in this paper show deposition features postulated in Figure 1 b and1 c.

a) hydrophobation b) classic (TEOS) consolidation

c) (PU) film forming

ti on

e) complete impregnation

Fig.1: Deposition of different stone consolidating agents in pore space of natural stone, sketch in HONSINGER 1990.

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While classic TEOS stone consolidants tend to form plates of Si02-gel in small extents between

grains, PU consolidants develope a fabric stabilizing film. Whether a stone consolidating measure was successful or not may be considered by the strength

profile, as SATTLER 1992 and SNETHLAGE & WENDLER 1995 worked out. Besides strength

parameters, the influence of consolidating agents on the humidity economy of stone is an important

factor in valuation. The present paper deals with the influence of the conservation measures on the

water vapour diffusion of natural stone. According to figure 1, TEOS and PU should perfonn

differently regarding non-capillar humidity transport paths: • Plates of Si02-gel formed by TEOS consolidants agent tend to decrease single capillary pores'

space without closing them completely. Humidity transport paths should remain more or less

constant, since smaller pores are formed due to fissures in the rigid gel. • PU-consolidation of stone establishes a water vapour resistant film. Coating the stone components

and attaching them, this film closes the smallest pores thus preventing that water is able to induce

swelling to the clay minerals \'Aachen model", HONSINGER 1990). Considering "breathe activity",

the Aachen model postulates enough permissivity of vast pore channels to be remaining after the consolidation. Furthermore the Aachen model needs a netlike connection of coated big pores to

establish humidity transport by gas diffusion. Regarding these aspects this paper tries to answer the following two questions: 1. How changes humidity transport, characterised by µ-values, in a pore fabric of stone lacking big

interconnected pores after consolidation? 2. Are big pores really interconnected and thus able to establish humidity transport after closure of

small pores due to a PU consolidation?

2. EXPERIMENTAL PROCEDURE

To answer the questions above, three varieties of natural stone with basicly different pore structures

were used.

• Bucher sandstone: maximum of pore radii > 100 µm, weak grain contacts between quartz

• Sander Schilfsandstone:

• Baumberger sandstone:

bimodal maximum of pore radii distribution, 1 O µm, 0, 1 µm, clay-rich matrix.

bimodal pore radii distribution, maxima at 2 and 0, 1 µm, carbonatic sandstone.

As shown in tab.1, slices of these natural stones were treated for half an hour with different

consolidating agents.

VarietY of stone TEOS Remmers, 'LOnlngen · P:UERabe; Kl~irfosthei rn ·. ···: ? :: Bucher sandstone lBUCl FUNCOSIL 510 RABOSEAL S 2 Sander Schilfsandstone (SAN) FUNCOSIL 300 RABOSEAL N 2 Baumberg_er sandstone (BMB) FUNCOSIL 100 RABOSEAL N 1

Tab1 : Varieties of stone and consolidants used

The solutions used have been selected specifically by the manufacturers of the products themselves,

regarding accurate concentration of consolidating agent for the different types of stone necessary for

an efficient consolidation. Therefore, an efficiency test was not carried out in this work assuming the accuracy of the producers statements.

The amount of liquid agent absorbed was 4 - 5 m-% in case of Baumberger and Sander sandstone

and 7.5 - 8.5 % in case of the Bucher sandstone, regardless of the type of agent used. After storing at

20 °C and 75% r.h. during four weeks, water vapour diffusion properties of treated and untreated

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reference samples (REF) were measured by the dry- and wet-cup method. Additionally, small samples were examinated by SEM to detect structural alterations.

3. RESULTS

Figure 2 shows a graphic summary of the water vapour diffusion resistance measurements.

µ-Value BMB ,SAN, BUC

80

RAB RAB

~ RAB REM REF

REF REF REM REM

60

:>. 4 0 I-

20 I-

I l I l ll I I I I I I I I BMB SA N BUC

I c:JDrycup •Wetcup I Fig. 2: Graphic illustration of µ-value (water vapour diffusion resistance number) measurements. Sander Schilfsandstone - SAN, Baumberger sandstone - BMB, Bucher sandstone - BUC

The following facts can be drawn from the results: • Samples treated with PU-consolidatants show the highest µ-values in both dry and wet-cup. • Changes of water vapour diffusion of TEOS treated samples compared to the reference samples are minimal. In case of the Bucher sandstones even a µ-value decrease is observed. • The extent of all changes, including µ-value-increase in case of PU-consolidating is relevant to the physics of constructions only under extreme conditions, i.e. extreme humidity behind the strengthened zone. • In spite of a totally different structure, all three stones show a similar water vapour diffusibility. Considering the two questions mentioned above, it may be stated that even the fine pored Baumberger sandstone remains water vapour permissive after treatment with film forming consolidating agents. Fig. 3 und 4 show deposition of the different types of consolidating agents in Bucher Sandstone pore space. The difference between formation of gel plates including the formation of secondary pores (TEOS) and the closure of a part of the smaller pores by formation of films (PU) is clearly recognizable. In this case, the Aachen model is verified.

Fig. 3: Plates of Si02-gel in Bucher sandstone, following TEOS treatment

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Fig. 4: Film formation after PU treatment in Bucher Sandstone

Fig. 5: Formation of film fragments in Baumberger sandstone after PU treatment

Fig. 5 shows the pore space of a PU-treated sample of Baumberger sandstone. No connecting film, as in case of the high concentrated product used for Bucher sandstone was to be found. Only fragments of consolidating agent can be detected, which are sometimes difficult to be identified. The formation of a complete film could not be verified in this experiments obviously. If the Aachen model would be valid in general, a surrounding film should connect all structural components closing nearly all the pore space available in the narrow structure of the Baumberger sandstone. As a consequence, the water vapour diffusion would drop dramatically , which was not the case. A similar result is found in case of Sander Schilfsandstone. The formation of PU-film fragments leading to a still existing permissive pore space is verified by the experimental results.

Regarding the PU consolidating agent applied in laboratory experiments, the low concentration of consolidating agent used for fine porous materials has inhibited formation of a connected film. Netlike

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connected big pores postulated by the Aachen model are not needed to establish water vapour permissivity after a PU consolidation.

4. CONCLUSIONS It is clearly shown that water vapour ransport resistance is not enhanced by a TEOS treatment, regardless of the stone type. The slight increase of the µ-values in case of PU-treatments is only relevant in the presence of high moisture loads from the interior/ground.

The Aachen model is valid only for stones having a portion of larger capillary pores sufficient to ensure the transport of large amounts of agent into the pore structure. In these cases, the smaller pores are closed. On the other hand, dense materials lacking of larger capillary pores (but absorbing

due to pores in the range of 1 - 1 O µm) cannot develope a connecting film which would close the smaller pores. An enhancement of the amount of agent deposited by application of higher­concentrated solutions, which possibly would allow the formation of a connecting film, is not possible

due to the restricted capillarity of these materials.

REFERENCES

HONSINGER, D (1990) : Strukturmerkmale polymerimpragnierter Sandsteine. - Diss. Fak. Bauing.- u. Vermessungswesen, RWTH Aachen.

SA TILER, L (1992): Sandsteinfestigung mit Kieselsaureester. - Diss. Universitat Munchen, Bayr. Landesamt fi.ir Denkmalpflege, Zentrallabor, Forschungsbericht 9/92. SNETHLAGE, R. & WENDLER, E. (1995): Methoden der Steinkonservierung - Anforderungen und Bewertungskriterien. in: Denkmalpflege und Naturwissenschaft, Steinkonservierung I

(Verbundprojekt Steinzerfall und Steinkonservierung); Ernst & Sohn.