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The mechanisms of water uptake by building materials are as varied as the possible forms of damage to the building. This chapter deals with the mechanisms of capillary water uptake, condensation, and hygroscopic water uptake, as well as the consequences for t he building fabric.

Water Repellent Treatment: Realiable Protection Against Moisture

The term “water-repellent treatment” refers to the treatment of mineral substrates, especially facades of fair-faced masonry and concrete, with hydrophobic impregnating agents. The term “impregnating agent” is frequently used on its own in this context, since it is considered self- evident that these agents are hydrophobic, that is, water-repellent. By definition, however, “to impregnate” just means to saturate an absorbent material with a low-viscosity, capillary-active liquid.

MOISTURE: ABSORPTION BY MASONRY AND RESULTING DAMAGE

It follows that impregnations ‑ also of masonry ‑ do not necessarily imply water‑repellent treatment.

They may serve, for example, to strengthen and consolidate the masonry, or to imbue it with biocidal properties.

In this article, however, impregnating a g e n t s a r e u n d e r s t o o d t o b e hydrophobic agents, that is, compounds which confer water repellency on mineral substrates.

The purpose of water‑repellent treatment is to protect exposed facades from moisture and attendant damage by applying a colourless, non‑film‑

forming agent which prevents capillary uptake of water and the aggressive substances dissolved therein. Because

the impregnating agent does not block the capillaries, the substrate retains its vapour permeability.

When mineral building materials come into contact with water, they absorb an amount which

depends on their porosity. The result is various forms of damage, including:

● Penetration of moisture through the wall ● Cracks caused by swelling and shrinkage

● Damage caused by frost and de icing salt ● Destruction of concrete caused by

corrosion of the reinforcing steel ● Efflorescence and salt damage

c a u s e d b y h y d r a t i o n a n d crystallization

● Lime leaching ● Rust stains and curtaining ● Dirt pick up and curtaining Attack

By Wacker Chemie AG

Editor’s note: The paper was presented at the seminar on “Deterioration and Damaging of Mineral-Based Structures and Building - The Contemporary Protection Solution” held on November 16, 2006 and is produced courtesy of IEFI Sdn Bhd.

by fungi, moss, lichens and algae ● Chemical corrosion, e. g. binder

transformation caused by acidic gases (SO

2, NO

2)

● Impaired thermal insulation

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Many of these forms of damage can be prevented, or at least reduced or kept at bay for longer, by means of impregnation. Creation of a water‑repellent zone considerably reduces the uptake of water and aggressive substances; the masonry remains dryer, and is consequently less prone to the kinds of damage referred to above.

However, this is only true of capillary water uptake, which is the "natural" water uptake by building materials when they come into contact with water ‑ when a facade is exposed to rain, for example. There are various mechanisms of water absorption other than capillary water uptake, and

Stone fabric damaged by the effects of salt and moisture.

Algal growth on exposed construction elements with a high moisture content.

these include condensation, capillary condensation, and hygroscopic water uptake.

Capillary Water Uptake

Capillary water uptake is responsible for the penetration of large volumes of water into the building material within a short time. The amount of water absorbed depends primarily on the radius of the capillary pores in the building material. There are three pore categories:

Micropores have a radius of less than 10 ‑7 m, gel pores less than 10‑8 m. These small pores do not permit capillary water transport. Water can only penetrate these pore spaces in the form of vapou r. Consequently, building materials in which micropores dominate are practically impervious to water penetration by capillary action. They are likewise difficult to impregnate, since the water repellents cannot penetrate into these pores, either.

Pores with a radius of between 10‑7 m and 10‑4 m are referred to as macropores or capillary pores. These pores support capillary action, and are able to transport

water and other liquids in the building material to a degree dependent on their capillarity. Building materials with a high proportion of such pores are generally well suited for impregnation with water repellents.

Air pores, as the third pore category, have a radius exceeding 10 ‑ 4 m. These large pores, like micropores, are unsuitable for capillary water transport.

Capillary water absorption by mineral building materials is usually according to the “square‑root‑of‑time law.” If water absorption W is plotted against the square root of the time t, a straight line is obtained, at least for the initial phase of capillary absorption. The gradient is referred to as the water‑absorption coefficient, or often just as the w value: Strictly speaking, the water absorption coefficient w describes the rate of capil lar y absorption characteristic of a particular building material, but it is frequently used as a measure of the capillary absorbency of building materials in general. The water absorption coefficient is determined as per ISO 52617.

Fig. 1 shows characteristic water‑absorption curves for different building materials. The w values range from 0.15 kg/m2h0.5 for the very dense bridge concrete to 11.5 kg/m2h0.5 for the highly absorbent brick.

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concentration in the masonry, and by the moisture content of the ambient air. Hygroscopic water absorption is especially serious when nitrates are present in the masonry.

Hygroscopic Water Absorption

The presence of soluble salts increases the equilibrium moisture content of building materials due to the salts’

hygroscopicity, i.e. their ability to attract water vapour. The extent of this hygroscopic moisture absorption is determined pr imari ly by the chemical nature of the salts, by their

A mineral surface with (a) a hydrophobic impregnation, (b) filled pores, and (c) a sealant film.

be determined quantitatively as the contact angle θ.

Untreated surfaces of mineral building materials are wetted immediately by water, i. e. the drops of water spread out and are rapidly absorbed by the building material (Fig. 3a). If the same building material is treated with an impregnating agent, the drops of water are repelled in the form of beads and do not penetrate into the substrate (Fig. 3b).

Water Vapour Can Diffuse

Since the pores in hydrophobically treated masonry remain open, the building material retains its vapour permeabil it y, or “breathabil it y.” Accordingly, the passage of water vapour is impaired only slightly, if at all. This is of great importance, since moisture contained in the building material can diffuse to the outside in the form of water vapour without causing any damage, e.g. blistering and subsequent spalling, which frequently occur with thick surface coatings.

WATER REPELLENTS: A SURE GUARANTEE OF A COMFORTABLE AMBIENCE

It may be concluded from the above comments about the various water‑uptake mechanisms that provided

the building material has pores which do permit capillary action, or the salt content ‑ at least in the surface zone ‑ is not excessively high, capillary moisture absorption without doubt poses the most serious problem. In this case, water‑repellent treatment is certainly one of the best ways to protect the masonry from moisture damage.

Water repellents need to fulfill the following requirements: ● Drastic reduction in water uptake

‑Retention of high water‑vapour permeability

● Extensive penetration ‑Adequate resistance to alkalis

● Resistance to UV light ● Surfaces not rendered shiny or tacky,

or caused to yellow ● Environmental compatibility

Unlike film‑forming coatings, such as those based on acrylic, polyurethane or epoxy resins, organosilicon water repellents do not seal the pores at the surface of mineral masonry, but simply form a very thin layer on the pore walls (Fig. 2).

Water can no longer penetrate in liquid form into capillaries that have been

Unlike the various film-forming coatings, building materials that have been treated with a water repellent retain their vapour permeability. This is because organosilicon water repellents do not seal the pores at the surface of the mineral masonry substrate, but form a very thin layer on the pore walls. Siliconized pores are no longer wetted by water, and capillary water uptake is prevented.

rendered water‑repellent, since, as a polar liquid, it is unable to interact with a non‑polar, hydrophobic surface. In other words, pores which have been siliconized and are therefore hydrophobic can no longer be wetted by water. The degree of wetting can

Wetting of a hydrophilic porous surface Wetting of a hydrophobic porous surface

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Organosilicon compounds have been recognized for over 40 years as the ideal active agents

for the hydrophobic impregnation of absorbent mineral building materials. However, it is not the ubiquitous silicone rubbers (joint sealants) and silicone fluids (release agents and lubricants) that are used in masonry protection, but the third important category of silicones ‑ silicone resins.

Silicone resins are three‑dimensionally cross‑linked polymers with a silicon and oxygen backbone. The silicon atoms carry organic groups R and functional groups OR’ (usually, R is a methyl group and R’an ethyl group) (Fig. 4). The resins range from liquid to solid depending on the degree of

curing, the molecular weight and the kind of functional group (Fig. 5). They are soluble in organic solvents such as white spirit or alcohols.

When applied to the building material, these silicone resins react with water, the remaining alkoxy groups being split off, to form a three‑dimensional, densely cross‑linked polysiloxane which is firmly attached to the building material by way of covalent Si‑O‑Si bonds (Fig. 6).

A comparison of the molecular structure of a fully cured silicone resin (Fig. 7a) with that of natural quar tz ( Fig. 7b ) makes the close resemblance clear. The fully cured silicone resin can be regarded as

superficial but extends to a good depth.

The water repellent must accordingly be formulated such as to al low ma ximum p enetrat ion into the building material. Only then can the effectiveness of the hydrophobic impregnation be guaranteed for years or even decades.

rain. However, properly applied water repellents are perfectly suff icient to render many standard building materials, such as sand‑lime brick, clinker brick and plasters, resistant even to driving rain with velocities of 100 km/h and more. “Properly applied” in this context means, for example, that the water repellent produces a hydrophobic zone which is not merely

Water under Pressure is a Problem

Ground water can be a serious problem in cellars, as can driving rain for highly exposed facades. The larger the pores in the building material are, the greater the problem. Since the pores are open, a water‑repellent treatment obviously cannot always protect a building material from ground water or driving

SILICONE RESINS: RELIABLE VEHICLES OF WATER REPELLENCY

The close structural similarity between fully cured silicone resins and natural quartz is the reason for the high affinity of silicone resins for silicate building materials, and for the exceptional durability of surfaces treated with these resins. Silicone resins boast excellent water repellency, and are completely resistant to many chemical, physical and biological influences.

Consistency of solid silicone resins.Molecular structure of silicone resins. Molecular structure of quartz.

Crosslinked silicone resin bonded to a substrate.

Molecular structure of fully cured silicone resin.

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Wh e n s i l i c o n e m a s o n r y protection was in its infancy, there were two main sorts of

water repellents, namely solutions of silicone resins in organic solvents, and methyl siliconates in water.

Silicone Resin Solutions and Siliconates

Solutions of silicone resins are only of minor importance today as water repellents. On account of their relatively high molecular weight, they do not penetrate deeply enough into dense building materials. Under alkaline conditions, standard silicone resins are broken open and methyl siliconates are formed. These are water‑soluble and are leached out of the facade by rain (Fig. 8). The silicone resin, along with the water repellency it confers on the substrate, thus dwindles with time. Aqueous solutions of potassium methyl siliconate (CH3‑Si(OH)2O‑K+) are frequently used for in‑plant impregnation of building materials of f ired clay and fibrous gypsum. Alkali carbonates are formed as by‑products.

However, siliconates are no longer used for impregnating facades because

they are apt to form a white deposit. The poor degree of penetration and insufficient resistance of silicone resin solutions to alkalis prompted the search for alternative active agents back in the mid 1970s.

are oligomeric alkylalkoxy siloxanes derived therefrom.

Resistance to Alkalis - a Criterion of Durability

The second most important discovery on the way to developing today’s highly effective water repellents was how to influence the resistance of the active agent to alkalis through use of the appropriate organic group R. If R is always a methyl group, the reaction of Fig. 8, in the presence of alkali, will eventually lead to a complete loss of hydrophobicity. But if longer‑chain organic groups are used in place of methyl groups, the reaction of Fig. 8 can still proceed, even though this is not easy for steric reasons. However, the resulting siliconate becomes increasingly water‑insoluble as the length of the R group lengthens, thus preventing the siliconate from being leached out by rain. In consequence, the degradation reaction is suppressed, and the active agent remains in the form of a silicone resin network on the masonry.

The iso‑octyl group has proved to be particularly suitable. Silanes and siloxanes with iso‑octyl groups are readily available, can be incorporated in a large number of products and exhibit outstanding properties. Water repellents frequently contain a mixture of silicone components with short‑chain methyl and long‑chain iso‑octyl groups.

a quartz structure that has been modified with organic groups. This close structural resemblance is the reason for the high affinity of silicone resins for silicate building materials, and for the exceptional durability of the water‑repellent treatment.

T h e o r g a n i c g r o u p R c o n f e r s e x c e l l e n t w a t e r r e p e l l e n c y o n t h e s i l i co n e r e s i ns . S i n ce t h e y are also ful ly resistant to many chemical,physical and biological i n f l u e n c e s , t h e r e s i n s w a t e r ‑rep e l l ent p ro p er t y is gener al ly

maintained for decades,provided that the water‑repellent treatment is carried out properly,i.e. provided that the right amount of the right product for the substrate is applied, and that the concentration of active agent is high enough.

SILANES AND SILOXANES: STABLE COMPOUNDS

The ultimate aim of all hydrophobic impregnations is to guarantee effective masonry protection for as long as possible. This goal was not always achieved with the organic-solvent based silicone resins frequently used in the past, or with water-based methyl siliconates. Today, silanes and siloxanes form the basis of modern water repellents. They can be incorporated easily in a large variety of products, and have outstanding properties.

Silanes and Siloxanes

Better penetration is achieved by using low‑molecular rather than high‑molecular compounds. This discovery marked the birth of water‑repellent agents based on silanes and siloxanes. As is evident from the chemical formulae (Fig. 9), the silanes, on closer examination, are monomeric alkyltrialkoxy silanes, and the siloxanes

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The depth to which the active ingredient of an impregnating agent penetrates is directly

dependent on the concentration of the active ingredient. The more a product is diluted, the less able it is to penetrate into the building material and fully occupy the walls of the pores and capillaries so as to prevent them from absorbing water. The capillary absorbency of building materials, for their part, varies enormously. If, for example, a vertical bridge‑concrete surface is saturated with water repellent by flooding, the high‑grade concrete will absorb 80‑100 g/m2 at the most. An absorbent brick or sandstone facade, by contrast, will easily take up ten times this amount under the same conditions. To ensure that the active agent penetrates to an acceptable depth into the low‑absorbency concrete, it is thus necessary to use a much more concentrated product than for the more absorbent sandstone or brick.

In practice, use is made of undiluted silanes or silane/ siloxane mixtures to render concrete water‑repellent, while for natural‑stone facades it is common to use products containing only 5‑10 % active agent.

Silanes and Siloxanes in Solution

In keeping with their hydrophobic nature, silanes and siloxanes are soluble to an almost unlimited extent in organic solvents. Since, however, contrary to widespread opinion, silicone active agents penetrate into the pores of building materials better when dissolved in petroleum ether than in alcohols such as ethanol and 2‑propanol, the latter

are only used if there is any risk of the water repellentís coming into contact during application with plastics or paints that are attacked by petroleum ether. Otherwise, low‑odour, dearomatized white spirit or synthetic iso‑paraffins are the most commonly used solvents. Care must be taken that whenever solvents are used, these are, if possible, anhydrous. The current trend is definitely toward solvent‑free, that is, water‑based products.

SILRES ® BS SMK – A Triumph over Difficulties

The f irst break through was the development of silicone microemulsion concentrates (sold under the name SILRES ® BS SMK).

Silicone microemulsion concentrates are solvent‑free mixtures of silanes,siloxanes and silicone surfactants, which, on dilution with water,yield ready‑to‑use silicone microemulsions.

The advantage offered by these products results from the nature of the surfactants (emulsifiers and co‑emulsifiers), which only function temporarily as such; once the water repellent has been applied to the building material,they transform into hydrophobic active‑

Silane and Siloxane Blends

In this way, it is possible to formulate water repellents that are customized in terms of properties such as penetration power, resistance to alkalis, water‑beading, etc. A water repellent for concrete will require, for example, more silane and more long‑chain organic groups than a product used to impregnate sandstone or brick. Whereas concrete is usually very dense

and highly alkaline, sandstone and brick are absorbent and more or less neutral.

Specialty Products for Maximum Effectiveness

There is a whole range of building materials with properties between those of concrete and sandstone or brick. These materials include l imestones and sand‑lime brick ,

clinker brick, mineral‑based plasters and paints, aerated concrete, fiber‑re inforce d concrete, and many othe r s . I t i s thus imp o r t ant to have balanced, general‑purpose products, i. e. impregnating agents that are acceptably effective on all standard substrates. Where maximum effectiveness for a particular substrate is desired, silanes and siloxanes may be suitably blended to formulate a specialty product.

CUSTOMIZED RATHER THAN STANDARDIZED ACTIVE-AGENT CONCENTRATIONS

A standard solution will not be able to confer maximum water repellency on every type of building material, as each material has different properties. Whatever the substrate, whether high-grade concrete or absorbent brickwork, the right impregnating agent in the right concentration is always needed. Impregnating agents may be used in concentrations ranging from highly diluted to undiluted. Among the diluted products, aqueous water repellents are becoming increasingly important.

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agent components. If the particle sizes of emulsions and microemulsions (Figs.10a and b) are compared, it is clear that microemulsions, on account of the vastly greater particle surface area,will require considerably more emulsifier than emulsions.The use of conventional surfactants would impair the hydrophobicity.

The great advantage of SILRES ® BS SMK products,apart from their being water‑dilutable and easy to handle,is the fact that they come in concentrated form. This makes for smaller container sizes, greatly reduces transport,storage and waste‑disposal costs and lessens environmental impact.

A disadvantage of SMK technology, however, is that the microemulsions have to be used within 24 hours of diluting them. Their appearance does not change in days or even weeks,but the alkoxysilanes and alkoxysiloxanes in the micropar ticles hydroly ze and polymerize to an increasing extent,making it harder and harder for the active agents to penetrate into the building materials.

Impregnating Agents in the Form of Emulsions

Aside from the product classes already mentioned, there are silane/siloxane

emulsions which boast both dilution and storage stability. These emulsions are diluted with water in a ratio of 1:4 to 1:9, the exact ratio depending on the type of substrate and the extent of water repellency required.

The problem of using emulsions for impregnations, however, as was explained before, is that, if the products are to have properties comparable to those of conventional solvent‑thinned impregnating agents, only low‑molecular alkylalkoxy silanes and siloxanes are suited as active agents. But these silanes and siloxanes react with moisture, and therefore special tricks are needed to stabilize them.

The key to success lies primarily in the correct choice of emulsifiers, in

adjusting the pH to the optimum value, and in the chemical nature and thus the reactivity of the active agents. It is advisable, for example, to employ ethoxy silanes and siloxanes rather than their methoxy counterparts, as the former hydrolyze far less quickly. The reactivity is, moreover, largely dependent on the nature of the organic group attached to the silicone: long‑chain groups, such as the iso‑octyl group, drastically reduce the tendency of silanes and siloxanes to hydrolyze.

Impregnating Cream

Unlike all other water repellents, impregnating agents in the form of cream may be applied without wastage even during overhead work.

SILRES® BS creams are best applied using airless equipment, with which up to 400 g/m≤ may be applied in a single operation.

The impregnating cream penetrates completely into the substrate within a period of some minutes to some hours; the exact time required will depend on the amount applied. Ver y high depths of penetration are achieved due to the long time the cream is in contact with the construction material.

Comparison of emulsion (a) and silicone microemulsion (b)particle size.

GENERAL-PURPOSE AND SPECIALTY PRODUCTS: SOLUTIONS FOR EVERY APPLICATION

Wacker Chemie AG offers both long-established water repellents and new, innovative masonry protection agents. Our product spectrum includes both general-purpose water repellents and specialty products for concrete.

Wacker Chemie AG has been intensively researching and developing silicone masonry

protection agents for some 50 years. A large number of products have been developed during this time, many of which are still in use.

However, the constant development of new water repellents necessitated streamlining the product portfolio and adapting it to today’s requirements. The outcome of these measures is a clearly organised product spectrum comprising the t wo categories :

“general‑purpose water repellents” and “specialty products for concrete.”

General‑purpose products are defined as those impregnating agents which, in principle, confer good water repellency on a large variety of very different mineral substrates, ranging from highly alkaline, dense concrete to neutral, absorbent brick and natural stone.

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Specialty products for concrete, by contrast, are those products that confer long‑term protection on high‑grade reinforced concrete of the type used in bridge building. These products must be able to penetrate extensively into the building material, even into very dense concrete, and they must show excellent resistance to attack by alkalis.

Table 1 contains a survey of the various products, their properties, and their fields of application.

General-Purpose Water Repellents

The best‑known and most frequently used water repellent is SILRES® BS 290. When used in the recommended concentrations of 1:11 to 1:15 (parts by weight) , it produces excellent results on almost al l absorbent mineral substrates. Particularly worth mentioning are its resistance to alkalis, its good penetration power and its outstanding beading effect. For many types of natural stone, in particular,

there is to date still no alternative to SILRES® BS 290. The hydrophobic silicone resin network forms within a few hours, even on non‑alkaline substrates. SILRES® BS SMK 1311 is a well‑established, aqueous alternative to solvent‑thinned products, and is suitable for a wide variety of building materials, e. g. sand‑lime brick, bricks and mineral plaster. It should be

diluted in a ratio of 1:10 to 1:14 to suit the substrate in question. By virtue of its autocatalytic behavior, SILRES® BS SMK 1311 reacts to form the hydrophobic silicone resin just as quickly as SILRES® BS 290.

However, on many types of natural stone, including sandstone, limestone and stone containing clay mineral, the

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effectiveness of the aqueous product may dif fer markedly from that of SILRES® BS 290. Before treating a large area of masonry, therefore, it is a good idea to check the effectiveness of the water repellent on a test area. Like all silicone microemulsion concentrates, SILRES® BS 1311 should only be diluted with tap water on the day it is to be processed.

Another of the general‑purpose water repellents, SILRES® BS 1001, is a 50% silane/siloxane emulsion that is diluted with water to suit the consistency of the substrate in question. Before the emulsion is diluted and applied, it is advisable to stir it briefly so as to ensure that it is homogeneous. SILRES® BS 1001 rapidly confers discernible water repellency on the building material, although the active agent takes a few days to some weeks to form completely. The exact length of time will depend on the alkalinity of the substrate.

Since the emulsion breaks once it has been applied, and there is a pronounced beading effect as soon as the water has evaporated, the amount of active agent leached out by rain during the relatively long curing period is negligible. Like all aqueous products based on alkylalkoxy silanes and siloxanes, SILRES® BS 1001 is markedly less effective on some types of natural stone, especially limestone, than, for instance, solvent‑thinned SILRES® BS 290. It is therefore always essential to check its effectiveness on a test surface.

Specialty Products For Concrete

SILRES® BS 1701 is a 100% silane‑based impregnating agent that is used to confer water repellency on concrete and reinforced concrete. It is usually applied to the concrete by flooding (wet on wet) at least twice in undiluted form. Of course, this water repellent may be diluted with organic solvents or alcohols in any mixing ratio desired. It should be remembered, however,

that the more a given quantity of water repellent is diluted, the less deep the active agent will penetrate. Because of its iso‑octyl silane groups, SILRES® BS 1701 is much less volatile than other commercial silanes. However, under certain conditions, evaporation may cause some loss of active agent. This happens most frequently when the concrete is old, of low alkalinity, and is hot and dry at the surface due to exposure to the sun. In such cases it is advisable to resort to an aqueous product, in particular to the impregnating cream described below.

SILRES® SMK 2101 is s imi lar in composition to SILRES® SMK 1311, but contains substantially more iso‑octyl silane. This enhances its penetration power and resistance to alkalis. When SILRES® SMK 2101 is used to treat reinforced concrete, it should be diluted with water in a ratio of 1:4. Like all silicone microemulsions, the diluted product ought to be used on the same day it is mixed. SILRES® SMK 2101 is also especially suitable as a primer for concrete coatings.

Unlike all the other products listed in Table 1, SILRES® BS Creme C is not a liquid. Its thixotropic consistency makes it possible to apply the product

without wastage even when working overhead. SILRES® BS Creme C is best applied using airless equipment, with which up to 400 g/m2 may be applied in a single operation. Such quantities often cannot be applied even in three operations when conventional low‑viscosity products are used on high‑grade concrete. The impregnating agent penetrates completely into the concrete within a period of some minutes to some hours, the exact time depending on the quality of impregnating agent applied. No visible traces of the cream are left on the surface, and very great depths of penetration can be achieved.

Thank s to the ver y pronounced penetration depth of several mm, even concrete with a damaged surface (cracks, spalling) is adequately protected. Tests have confirmed that a hydrophobic impregnation not only protects the concrete from moisture, but also minimizes pollutant ingress into the concrete. In test samples of treated masonry, chloride uptake is superficial only.

In untreated building material, by contrast, considerable concentrations of chloride are detectable even at a depth of 4 mm (Fig. 11).

Chloride migration in concrete (strength class C35/45) Untreated specimens and those treated with SILRES® BS Creme C Specimens conditioned for 10 days in 10% NaCl solution

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Capillary Water Uptake

The most important requirement which a water repellent must fulfill is a significant reduction

in capillary water uptake. A common specification is that the amount of water absorbed by a building material during 24 hours’ immersion in water be reduced by at least 80 %.

Depth of Penetration

In order that the reduced tendency to absorb water may be long‑lasting, it is obviously not sufficient if the water‑repellent effect is limited to the surface of the building material. It is essential that the hydrophobic zone extend deep below the surface, but it is difficult to give a general answer to the frequently‑asked question as to exactly how deep. Of course, from a technical point of view, there is

nothing against striving for maximum possible penetration, but it must be remembered that this is only achieved by applying a large quantity of active agent, which incurs high costs. For many substrates, a penetration depth in the order of a few millimeters is technically adequate and economically acceptable.

Beading Effect

The third important criterion which water repellents must satisfy is the production of a beading effect. This effect is measured objectively by means of the contact angle Θ. The following classification is often used:

The Effectiveness of WACKER Products

Tables 4a and 4b show test results for water uptake, beading and penetration depth of various products applied to

different substrates. White spirit with a low aromatics content was used as diluent for the solvent‑thinnable products, and normal tap water for the aqueous products. All substrates were impregnated by means of immersion (immersion times: 1 minute for mortar and concrete, 5 minutes for all other substrates).

The quantities of impregnating agent absorbed correspond approximately to the amounts absorbed when the products are applied to vertical surfaces by flooding in two wet‑on‑wet operations.

Water absorption was determined 14 days after impregnation, again by way of immersion (specimens were covered with 5 cm of water, in accordance with EN 12859). This method was chosen because the hydrostatic pressure exerted by the covering of water simulates, to a certain extent, the effect of driving rain, making the procedure more realistic than the sponge immersion test described in ISO 15148, in which only capillary absorption is measured.

To determine the penetration depth, a specimen of each product was broken 14 days after impregnation and dyed water was dripped onto the fracture surface. The hydrophobic zone is not wetted by the dyed water.

Gas Permeability

The term “gas permeability”, when used in the context of water repellents, refers primarily to the impregnated structure’s permeability to water vapour and carbon dioxide.

Water‑vapour permeability is essential for allowing any moisture beneath

WACKER WATER REPELLENTS FOR MAXIMUM EFFECTIVENESS

The most important criteria which water repellents must satisfy are reduction in water uptake, penetration by the hydrophobic active agent, permeability to gas and water vapor and a beading effect. In the following section, these requirements will be described in more detail, and the effectiveness of the WACKER products compared on the basis thereof.

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the surface to dry. It is determined as follows, in accordance with ISO 7783‑2: a sample of the substrate is cemented ab ove a saturated salt solution contained in a cup. The salt solution provides for a constant and very specific relative humidity within the cup. In line with the recommendation given in the standard, frequent use is made of ammonium dihydrogen phosphate, with which a relative humidity of precisely 93% is obtained. The cup is kept under standard conditions at 23°C and 50% humidity. By weighing it repeatedly over a period of several days, it is possible to determine how much water vapor diffuses through the substrate. Since the surface area of the substrate is known, the water‑vapour permeability, WVP, can be calculated in [g/m2d]. Table 3 shows the water‑vapour permeability of sand‑lime brick specimens (diameter: 90 mm, thickness: 5 mm).

The liquid products SILRES® BS 290, SILRES® BS SMK 1311 and SILRES® BS 1001 were applied by immersion (immersion time: 5 minutes) , and SILRES® BS Creme was applied by

brush. Once the WVP measurements had been completed, the test discs were broken and the penetration depths determined. In all of the specimens, the hydrophobic zone extended through the entire thickness of the disc.

As Table 3 shows, the impregnating agents reduced the water vapour permeability by less than 20% , in spite of the fact that the specimens had been rendered hydrophobic throughout their entire thickness, and that sand‑lime brick is a relatively dense substrate. In the case of coarse‑pored building materials, e.g. many kinds of brick s, mineral plasters and aerated concrete, there is no measurable impairment of water‑vapour permeability whatsoever. Permeability to carbon dioxide is necessary in order that carbonated substrates may achieve their ultimate s trength. Determining whether carbonation is af fected by water repellents is most easily done by measuring the depth of carbonation i n t r e a te d a n d u nt r e a te d l i m e mor tar specimens . To this end, phenolphthalein solution is trickled

onto the specimen. On account of their high pH, zones which have yet to undergo carbonation show up red, while carbonated zones are colorless. It is generally correct to say that silane/siloxane‑based water repellents neither hinder nor promote the carbonation reaction.

Coatability

The question of ten ar ises as to whether building material surfaces that have been rendered water repellent can be overpainted. This question is answered automatically when one recalls that the same products as the impregnating agents discussed here, or similar ones, are also recommended specifically as water‑repellent primers for protective coats on facades. Facades that have been rendered water‑repellent can easily be overpainted with all paints that contain wetting agents, e.g. silicone resin emulsion paints , emulsion paints and silicate emulsion paints. Wetting and adhesion dif f iculties are only encountered, as would be expected, with purely mineral‑based coatings such as whitewash.

STONE STRENGTHENING WITH ETHYL SILICATES

Untreated sand Fully cured Stone Strengthener SILRES® BS OH 100

Sandstone consolidated with Stone Strengthener SILRES® BS OH 100

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As long ago as the 19th century, it was suggested that ethyl silicates might be useful for consolidating, i.e. strengthening, stone. However, it is only in the last 30 years that this idea has been taken up. Several thousand objects have since been restored with ethyl silicates, bearing witness to their enduring effectiveness.

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Principles of Stone Conservation

In s t o n e c o n s e r v a t i o n , t h e microstructure of the deteriorated s t o n e i s s t a b i l i z e d t h r o u g h

the introduc tion of binder. This strengthening measure is generally co m b i n e d w i t h a hyd r o p h o b i c impregnation to protect against further deterioration. The products used to conser ve the stone are impregnating agents which are applied liberally so as to saturate the building material. Once applied, the impregnating agent reacts with the water in the capillary pores to form a silica‑gel‑based mineral binder (SiO2 aq).

How stone strengthener reacts

The binder stabilizes the building material by way of covalent Si‑O‑Si bonds.

On the basis of today’s knowledge, a s t o n e c o n s e r v a t i o n a g e n t s h o u l d m e e t t h e f o l l o w i n g requirements:

● Deposition of fresh, weather‑resistant binder

● Good depth of penetration into the stone, at least down to the

undeterio rated core ● No crust formation but, rather,

a build up of a uniform strength profile through the cross‑section of the stone

● No formation of harmful, salt‑like by‑products

● No discolouration of the stone surface● No modification or impairment of

important physical properties of the stone. This applies in particular to water‑vapour permeability and to thermal and hygric behaviour.

● O p t i m i z a t i o n o f p h y s i c a l properties such as tensile adhesive strength.

Stone strengtheners based on ethyl silicates meet these requirements best. This is why modern stone conservation uses this class of products almost to the exclusion of others.

WACKER Stone Strengthener

WACKER currently offers the stone strengthener SILRES® BS OH 100. It contains a blend of different pre‑

condensed ethyl si l icates and is solvent‑free. A neutral , organo ‑m e t a l l ic co n d e ns at i o n c at a l ys t controls the reaction between the ethyl silicate and the moisture in the stone such that the reaction proceeds at the optimal rate. Gel precipitation is 32 % , but may be adjusted to a lower value if desired, for example by addition of anhydrous solvent (Fig. 13).

Scanning electron micrographs after consolidation

Gel precipitation by SILRES® BS OH 1300 stone strengthener.

Scanning electron micrographs before consolidation

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Processing

Sto n e s t r e n g t h e n e r s b a s e d o n ethyl silcates are generally applied by spraying or f looding. Movable p a r t s m a y a l s o b e t r e a t e d b y immersing them in a bath. In special cases , compresses can b e use d to ensure a maximum period of contact between the stone and the strengthener.

Important notes

● Freshly treated surfaces must be covered for two to three days to protect them against rain.

● Considerable loss of the active ingredient by evaporation may occur at temperatures exceeding 25°C. At such temperatures, the freshly consolidated sur faces have to be protected against direct sunlight.

● Processing temperatures below 5°C cause the stone strengthener to react very slowly. This may result in discolouration or glaze on the surface.

● The time needed for deposition of the si l ica gel depends on the relative humidity and the temp erature. T herefore, i t is advisable to wait for a week before carrying out any further r e s t o r a t i o n w o r k o n f r e s h l y strengthened sections. This will allow 90‑95% of the ethyl silicate to be precipitated. It takes from one to at most three weeks for deposition to be completed.

● On no account should water be added to the ethyl silicate preparation in an attempt to speed up the reaction. This can cause extensive surface glazing that is extremely difficult, if not impossible, to remove.

STONE STRENGTHENERS AT WORK

Cross-section through deteriorated sandstone and hardness prof i les demonstrating successful and unsuccessful consolidation.

Moais on Easter Island

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The Significance of Penetration Depth

The success of stone conservation hinges largely on the depth to which the conservation agent penetrates into the stone. In any event, the stone strengthener silicate sandstone must penetrate through the weathered zone and replenish the depleted binder levels with fresh binder. After treatment, the consolidated zone

should not be stronger than the undeteriorated, intact core of the stone. Otherwise crusts may form that can cause the surface zone to undergo slate‑like cleavage.

Fig. 14 illustrates crust formation a n d th e ha rd n e ss p ro f i l e s f o r successful and unsuccessful stone consolidation work. The likelihood of a poor hardness prof i le can practically be eliminated by perform ing preliminary tests to ascertain the amount of material needed, the penetration depth and the required gel deposition rate.

Applications Sto n e s t r e n g t h e n e r s b as e d o n ethyl silicates are highly suitable for consolidating absorbent silicate sandstone, calcareous and porous t u f f . S u i t a b i l i t y te s t s m u s t b e per formed in advance when the stone consists of pure limestone or is a swellable sandstone containing clay minerals. Outstanding results are obtained with all ceramic, absorbent construction materials, such as bricks, roofing tiles and terracotta.

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Re f e r e n c e o b j e c t s v i v i d l y demonstrate their successful performance in the f ield of

building maintenance. Provided that use is made of the correct products, that these products are prepared and applied properly, and that their water‑repellent effectiveness is understood and assessed in the correct sense, i.e. as a reduction in capillary water uptake and not just as a superficial beading effect which is lost relatively quickly, long‑term protection can be guaranteed.

There are numerous structures which today provide the best proof that impregnations, given the correct choice of product and its professional application ‑ are able to fulfill their function as a means of protecting buildings and building materials against the uptake of moisture and aggressive substances for at least 20 years. Examples to be found in Munich are the Olympic Village, whose white‑concrete curtain walls were treated in 1972 with a product similar to todayís SILRES® BS 290, and the Alte Pinakothek, on which, in 1975, the first test surfaces were treated with stone strengthener and subsequently with water repellent. When these surfaces were re‑examined

LONG-TERM STUDIES:WELL-KNOWN REFERENCE OBJECTS

Select examples whose importance makes them familiar beyond Germany’s borders serve to illustrate how silicone resins, the active agents on which all established organosilicon water repellents are based, are able to protect buildings against water and attendant damage for decades by virtue of their chemical, physical and biological stability.

in 1995, neither the strengthener nor the water repellent showed any reduction in effectiveness.

Another well‑known example of both natural‑stone and concrete repair and preservation is the Kaiser‑Wilhelm Memorial Church in Berlin. With the help of WACKER masonry protection agents, the ruins of the old church and the concrete structure which is the new church were repaired at the beginning of the 1980s and preserved with water repellents. Successfully, as a recent follow‑up examination showed!

One of the oldest examples of a building that underwent water‑repellent treatment with organosilicon compounds, although less well‑known than those mentioned so far, is certainly just as informative:

On WACKER’s production premises in Burghausen, there is a small building with a lime‑finish facade fully exposed to the weather. In 1954, a test area was treated with methyl siliconate, today’s SILRES® BS 16. Even today, more than 50 years later, the water repellency is clearly recognizable when the surface is sprayed with water.

In the Karstens tube test, the untreated plaster absorbs 5 ml of water in 10 minutes, while the water‑repellent surface absorbs practically nothing ‑ an amazing finding after more than 50 years!

These examples go to prove that silicone resins, the active agents on which all established organosilicon water repellents are based, are able to protect buildings and structures against water and attendant damage for decades thanks to their excellent chemical and physical stability.

Munich’s Alte Pinakothek - restoration and preservation of natural stone.

The Olympic Village in Munich - in-plant concrete impregnation.

Lime-finish surface impregnated with methyl siliconate, in 1954.

Testing the same lime-finish surface for water repellency by the Karstens tube method, in 1997.

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