Silicic acid ester

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Tetraethylorthosilicate , a silicic acid ester

Silicic acid esters (KSE) are esters of silicic acids . The esters of orthosilicic acid with the general formula (Si (OR) 4 ) are of practical importance . These compounds are formed by the reaction of silicon tetrahalides (e.g. silicon tetrachloride ) with alcohols , such as. B. methanol and ethanol . The silicic acid ester with ethanol is called tetraethylorthosilicate . Silicic acid esters tend to hydrolyze , orthosilicic acid is formed, which converts into polysilicic acid in a condensation reaction (dehydration), from which amorphous silicon dioxide is finally formed:

Si (OC 2 H 5 ) 4 + 4 H 2 O → SiO 2 • n H 2 O + 4 C 2 H 5 OH

Conservation of natural stone

Main article: Stone preservation

Silicic acid esters are used in building protection to consolidate and preserve natural stone and plaster, as they are produced during hydrolysis, e.g. B. by humidity, silicon dioxide (SiO 2 ) form, the finest cracks and crevices in the stone with a silica gel film ( silicification ).


Patented in Austria around 1900, silicic acid esters were first used in England around 1920 for stone consolidation. However, no success in terms of consolidation could be determined with these applications. The silicic acid esters were picked up again in Czechoslovakia in the early 1960s, and their development in Germany was mainly driven by Wacker-Chemie . The first stone object treated in Germany is the sandstone bay of the castle in Burgsteinfurth (Westphalia).

Conservation method

In addition to tetramethyl orthosilicates, which are no longer used today because of their toxicity, and which split off methanol , only tetraethyl orthosilicates, which split off the less dangerous ethanol , are used. The strengthening effect of the silicic acid ester is based on the formation of bridging, water-containing, amorphous SiO 2 gels in the pore space of the rock. The simultaneously formed ethanol evaporates into the atmosphere. Released from Propylsilikaten propanol evaporates worse than ethanol.

The setting of a silicic acid ester takes place in a sol-gel process in two steps: In the first step hydrolysis takes place, the second step leads to the final gel formation via condensation and polymerization processes. During hydrolysis in the presence of water, ethanol is split off and unstable orthosilicic acid is formed, from which amorphous, water-containing SiO 2 gel is formed by splitting off water .

In practice, the hydrolysis must be accelerated by adding catalysts . In addition to the acidic and alkaline catalysis that was used in the past , in which acid or base had to be added to the CFE immediately before use , nowadays it is usually catalyzed with organometallic compounds ( dibutyltin dilaurate ). Organometallic compounds catalyze the hydrolysis more slowly than acidic or alkaline catalysts, so that application errors can be avoided better. The alkali-catalyzed hydrolysis continues to play a role in restoration when bonding with rapidly hydrolyzed KSE. During the further gel formation, condensation processes in the HO-Si structures lead to the formation of disordered SiO 2 tetrahedral networks with H 2 O splitting off.

Continuous condensation of the free OH groups in the silica gel leads to volume contractions and thus to tensions . If the cohesive forces in the gel are exceeded, a polygonal, characteristic crack system develops. The development of this crack system leads to the formation of secondary porosity in the pore system of the treated stones, which can affect the hygric behavior of the stone . Due to the hydrophobic ethyl groups that have not fully reacted during gel formation, the treated stone surfaces show an initial hydrophobicity that can last for several years. The hydrolysis of the CFE is initiated by the adhesive water on the mineral surfaces. A certain amount of sorption moisture, but without capillary condensation, is therefore essential for successful consolidation. In practice, it is generally recommended that stones to be strengthened be conditioned at a relative humidity of around 65 percent.

The silicic acid esters available on the market from different manufacturers differ in their solids content or gel deposition rate, the content of solvents and possible additions of hydrophobic substances and are selected individually depending on the application.

The aim is to further develop the silicic acid ester in order to reduce the embrittlement of the solidified stone material and prevent clods from forming on the treated surfaces.


Some monument preservationists warn against the excessive silicification of natural stone, as the strengthening of the upper layers in connection with the hydrophobizing effect when the facade is heated by solar radiation and frost-thaw cycles can lead to the formation of peelings and crusts and possibly an accumulation of salt. As a result, due to the stresses caused by weathering, a separating joint can form between the solidified layers and the softer material underneath.
In order to prevent this, the amount of strengthening agent added should be precisely dosed and suitable measures should be taken to penetrate as deeply as possible into the material.

Individual evidence

  1. ^ Polemics on the conservation of natural stone by Konrad Fischer, accessed in February 2016