Durability enhancement for cementitious materials through waterproofing by organosilicon chemistry

Cementitious materials like concrete are mainly composed of aggregates of various sizes such as sand and rock, which are bonded together by fresh cement paste and become a rigid mass over time. Although mostly hard and dense, cement-based systems are by their very nature still porous and thus susceptible to damage and deterioration caused by deleterious substances. Typical examples of such deterioration processes are alkali-silica reaction, sulphate attack, carbonation, frost damage etc. A common feature generally behind all these processes is the presence of water, in its liquid, solid or gas state. Water can either act as a transport medium for the deleterious substances or be directly involved in the chemical or physical processes. As a result, most construction projects require a certain type of proofing system to make cementitious products impervious to external water, as well as other substances.

Schematic illustration of silane molecules forming a polysiloxane network and exhibiting hydrophobic effects on mineral substrate © WACKER Chemie AG

For decades, organosilicon-based chemicals have been used in the construction industry for the protection of various building materials including cementitious types against moisture and consequential deteriorations. For instance, silanes carrying reactive and inert groups have the capability to create a pronounced hydrophobic effect on finished mineral-based substrates after treatment. When they are applied to the substrate material, the reactive groups such as alkoxy silyl groups can react with hydroxyl groups existing on the surface to form stable covalent bonds.

The silanes themselves can also react with each other through hydrolysis and condensation processes between the remaining alkoxy silyl groups and can form a very thin but permanent layer of polymer network covering the substrate surface. In contrast, the non-polar groups as the inert part of the silane molecules, e.g. alkyl groups, render the substrate surface hydrophobic. As a result, water or moisture is repelled and cannot penetrate into the substrate structure. With this, water-associated processes such as blistering, fungi growth, salt attack etc., which may easily cause health issues and safety damage to the materials and structures in use, can be significantly suppressed.

Schematic illustration of the inner surface of a concrete capillary pore after treatment by a) an oil-based hydrophobic admixture (left) and b) an organosilicon-based hydrophobic admixture (right), © WACKER Chemie AG

Organosilicon-based products have very effectively been applied to treat and protect a wide range of buildings and public works. By means of surface impregnation, coating, etc., these products help achieve excellent hydrophobic effects on treated substrates against water-induced damage. Besides hydrophobicity, silane and siloxane chemistry also offers additional benefits such as anti-stain effects or brilliant wall colors, contributing to the overall aesthetics of construction works.

The application of organosiloxane agents can generate pronounced hydrophobicity within the top layer of the surfaces of walls, floors or other building products. This provides exterior protection against water-associated damage to a very high level. However, there are several occasions where the surface protection alone is insufficient or inappropriate. For instance, if the protective surface layer gets damaged during service, the unprotected inner part will be subject to possible deleterious effects from the surrounding environment. Even with a tiny failed spot on the surface, the subsequent capillary effect can draw moisture from the surrounding environment into the dry mass, leading to water-induced internal deteriorations. Besides, the surfaces of the treated objects need to be dry so that the penetrating and hydrophobizing processes can take place. This means that materials with wet surfaces are not suitable for direct surface treatment. In the case of freshly mixed concrete, a sufficient drying time of several days or weeks needs to be ensured before hydrophobic agents can be applied. In order to provide longstanding comprehensive protection, it is also necessary to hydrophobized the interior of the target object and convert the whole of it into a water repellent mass. This is especially critical for cement-based materials, as water is always present inside the cementitious matrix – either as a chemical constituent of the hydration process or from external sources. Moreover, the porous and hydrophilic nature of the matrix renders it susceptible to the deteriorations resulting from transport processes of water, thus impairing the durability of the material significantly.

Find the whole article in which the waterproof treatment of a cementitious mass by using organosilicon chemicals will be elaborated here: www.wacker.com