Stone preservation

The term stone conservation is understood to mean measures that serve the purpose of preserving, maintaining or conserving the condition of a weathered monument rock . Since weathering is almost always the result of the chemical and / or physical interaction of water with the rock-forming minerals or their structure, one approach to stone conservation is to keep water away from the stone and, if necessary, to replace dissolved or lost binding agents.

Since it is predominantly the weathering surfaces made of soft sandstones as well as lime-bound or gypsum-containing natural stones that have to be strengthened and protected from further erosion, the work area is often summarized under the term natural stone conservation.

Stone conservation has a long tradition. The means and application techniques used have changed over the course of time based on the current state of research and development, but the goal of preserving and protecting cultural assets made of natural stone has remained the same. It must always be borne in mind that any preservation measure on porous materials must be classified as irreversible.

Silicic acid ester consolidation

Currently, the consolidation of weathered rock surfaces with silicic acid ester is practiced. The silicic acid ester is applied to the rock surface with an injection syringe, a brush or a compress, penetrates the near-surface pore structure of the rock and hydrolyzes there by reacting with the moisture in the rock to form amorphous silica . As a thin film in the pore space, this silica gel replaces the bond that has been lost due to weathering and thereby strengthens sanding and easily flaking rock surfaces of sedimentary rocks. Deeper damage such as cracks or peeling peelings from the surface cannot be repaired with this procedure. This requires more complex measures such as backfilling the cracks or backfilling the shells with mortar adapted to the material . The binding agent of these mortars is often also a silica ester or a silica sol .

Problems with the silicic acid ester consolidation are the penetration depth of the strengthening agent and the achievement of a balanced strength profile in the stone cross-section. The depth of penetration in the application techniques that can be used on site at the structure depends on how readily the material soaks up the silica solution due to its capillarity . In the case of very fine-pored rocks with an average pore radius <1 µm, the penetration depth is only a few millimeters, while in large-pored rocks (> 5 µm) it can be a few centimeters. If the weathered zone extends deeper into the stone than the penetration front of the silicic acid ester, then a hard, outer shell is created on a weakly consolidated zone, which then becomes the predetermined breaking point.

Acrylic resin impregnation

For more than 30 years the process of acrylic resin impregnation has been used for degradable objects , in which the problematic peeling does not occur. In large autoclaves , with the support of vacuum and pressure, monomeric methyl methacrylate (MMA) is introduced into the previously completely dried out pore system. After complete penetration, the MMA is polymerized by supplying heat , so that PMMA (acrylic glass, plexiglass) is created in the pore system . Adhesion promoters achieve good adhesion to the pore wall, so that a high level of strength, which is uniform over the entire cross-section of the rock, is achieved, and water absorption is completely prevented. After problems that initially occurred, which were due, among other things, to insufficient previous drying, the method has now proven itself for numerous, but by no means all, types of rock that absorb moisture and are therefore susceptible to weathering. Due to the strong interference in the rock substance in the initial phase, the procedure was used exclusively as a last resort for objects that could no longer be rescued in any other way. This restriction has now largely been lifted. The procedure is of particular importance in the case of statically endangered objects (e.g. the staircase of the Wendelstein in Torgau). Low-strength natural stones with a high pore volume, such as Weiberner tuff , are not suitable for acrylic resin impregnation due to the high probability of consequential damage in the form of cracks. A prophylactic application of this procedure should be avoided.

Minimally invasive acrylic resin impregnation AVT X

In a recent further development, the acrylic resin is now fully impregnated with a variable proportion of polymerizable binder. The suffix "+ X%" is added to the old designation AVT (AcrylharzVollTränkung). An AVT50 therefore has a 50% proportion of polymerizable MMAs. AVT90 to AVT20 are currently used for sandstone and marble, but primarily AVT50. The decisive difference to the classic acrylic resin impregnation is that the stone pore is no longer completely filled and closed, but only lined with a thin PMMA film. The film thickness adhering to the pore wall varies with the proportion that can be polymerized. The thermal expansion of the consolidated rock largely corresponds to that of the non-consolidated rock. The pore volume is reduced depending on the type of rock and film thickness and the water absorption is reduced accordingly. It can be completely re-treated with conventional restoration means and measures.

Full preservation with functional silanes

In addition to the previously described full acrylic resin impregnation, there is a newer process of full preservation of structurally damaged objects with a mineral strengthening agent. The "full preservation with functional silanes " can only be carried out on degradable and mobile sandstone objects, since large pressure and vacuum capable autoclaves are also required here. The concept of full conservation counteracts the formation of interfaces between consolidated and non-consolidated areas. Since the strengthening agent consists of a mixture of functional organoalkoxysilanes that can chemically bond to silicate surfaces, it is suitable for strengthening sandstones , but not for strengthening calcitic rocks such as e.g. B. marble . The strengthening agent is introduced into the stone as a monomer and polymerized to a stable polysiloxane film in the pore space by hydrolysis and polycondensation . Only the inner surface of the rock pores is coated. The pore space itself remains open.

The sequence of full preservation with functional silanes:

• Conditioning the stone: The stone will be conditioned to prepare the pore space to receive the solidifying agent. However, it does not have to be completely dried because the strengthening agent requires moisture to react.
• Impregnation: The conditioned stone is stored in a soaking tub. The soaking tub is flooded with the soaking medium in a pressure / vacuum autoclave and placed under vacuum. Air escapes from the stone. This is followed by a pressure phase that transports the impregnation medium into the stone. After several pressure / vacuum phases, the stone is removed from the impregnation medium.
• Complete reaction: The complete reaction takes place under a controlled, humid and warm climate. In addition, the ambient pressure is adapted to the course of the reaction. The complete reaction takes 14 days to 3 weeks, depending on the type and format of the substrate.
• Restoration: All conventional restoration measures can be carried out on the consolidated pieces without restrictions.
• Reversibility and re-treatment: In principle, no preservation measure is completely reversible. "Retreatability" is of greater importance today. The "re-treatability" is completely given after the full preservation with functional silanes. Since the pore space is not filled, but only encased, repeated consolidation can be carried out with the same system, later with other, new consolidation agents, but also with the aforementioned full acrylic resin impregnation.

Water repellency

For a long time, hydrophobization has been viewed as a gentle - because "only" water-repellent - preservation measure . The indication that the stone could still “breathe” closed the view of the problems that can arise behind the hydrophobized layer. The water repellency works not only from the outside in, but also from the inside out. This means that the water in the stone can only leave the stone through this layer via the vapor phase. Behind this layer, water can build up and, in the case of salt-laden stones, a salt concentration can occur. The hydrophobization therefore requires a very extensive preliminary examination of the suitability of the rock and the individual place of use in order to exclude damage from frost and salt exposure. Different types of stone, in particular the Rhenish tuff , but also bricks or bricks very often show consequential damage to be attributed to the hydrophobization in the form of shell formation depending on the thickness of the rock surface covered by the hydrophobization. A prerequisite for a successful preservation measure is the ability of the stone to be treated to absorb the preservative offered. Very fine-pored rocks u. U. only the solvent, while a deposit of the active ingredient can be observed on the surface. The suitability of the preservative for the respective rock (or, conversely, of the rock for an otherwise proven preservative) must therefore be checked on a case-by-case basis, not least with regard to the prevailing weathering status and any measures taken earlier. The readiness of the capillary system to accept must also be checked. A pore filled with water cannot absorb a preservative. In-situ treatment comes up against natural, physical limits. The damage patterns and restrictions formulated here, however, reflect experiences that can only be transferred to the current state of the art to a limited extent, as modern raw materials and further developed waterproofing agents also often have an additional impregnating effect and are effective in the long term due to the associated depth effect. Certain natural stones and especially concretes are protected in this way and a supplier and processing industry specialized in this area has developed and established accordingly. Due to the large number of raw materials and / or water repellants formulated from them and the resulting confusion of the range, it is in any case necessary that both users and providers of water repellants carefully check the suitability of the substrate or the water repellent intended for this individually in preliminary tests. This prerequisite is not always given, which, however, should not, conversely, lead to a general association of hydrophobic products with damage patterns. On the other hand, there are a large number of building projects that have been successfully hydrophobized in the long term at home and abroad.

Biological process

An unusual new method was recently tried out at the Duomo in Milan. A research group led by the microbiologist Francesca Cappitelli had two processes compete against each other at two different weathered spots, which can be summarized under the heading “Chemistry versus Biology”. The use of microbes of the Desulfovibrio vulgaris species turned out to be the gentler method. They cleaned the dark, gypsum-based crust of the marble evenly and without leaving any residue.

Other procedures

Attempts have been made with both lime water and barite water to consolidate crumbly natural stones or plastering mortar by introducing the solution into the capillary system so that the precipitation products stabilize the grain structure.
However, it has been scientifically proven that, if there is any increase in strength, this is mainly due to a redistribution of calcium hydroxide still present in the mortar, so that the effect can also be achieved by a simple soaking with water.

literature

• Rolf Snethlage: Guidelines for stone conservation. Planning of investigations and measures for the preservation of monuments made of natural stone , 3rd revised edition, Fraunhofer IRB Verlag, Stuttgart 2008, ISBN 978-3-8167-7554-6
• DBU research project AZ 25200-45 "Preservation of valuable, environmentally damaged cultural assets made of natural stone through innovative preservation with a mixture of functional silanes"

See also

• Concrete repair
• Natural stone surfaces

Individual evidence

1. ^ Polemics on the conservation of natural stone by Konrad Fischer, accessed in February 2016
2. ↑ Literature overview on acrylic resin impregnation at the IRB  ( page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. , Die Acrylharzvolltränkung Ibach, H.; Source: Naturstein, 1987 ISSN 0028-1026 ;   
3. ↑ (Odermatt, W .; in: Sustainability and Monument Preservation. Contributions to a culture of circumspection, 2003, pp. 127–137)
4. ↑ (Der Spiegel 38/2007, p. 169)
5. ↑ F. Cappitelli, L. Toniolo, A. Sansonetti, D. Gulotta, G. Ranalli, E. Zanardini, C. Sorlini: Advantages of using microbial technology over traditional chemical technology in removal of black crusts from stone surfaces of historical monuments. In: Applied and environmental microbiology. Volume 73, Number 17, September 2007, pp. 5671-5675, ISSN  0099-2240 . doi : 10.1128 / AEM.00394-07 . PMID 17601804 . PMC 2042061 (free full text).