Hydrophobic treatment

Fibre Reinforced Facade Encyclopedia: Finishes

A hydrophobic treatment is a low viscosity silane or siloxane solution that soaks into a porous cementitious surface and chemically bonds to the pore walls, leaving a water repellent lining a few millimetres deep. It does not build a film. Liquid water is repelled because the pore walls will no longer wet, while water vapour still moves freely through the open pores, so the panel keeps drying outwards. That combination, repellent to liquid and open to vapour, is what separates an impregnation from a sealer.

At a glance

How it works

Cementitious materials take up water by capillary action: the pores are fine and their walls readily wetted, so water is drawn in against gravity carrying dissolved salts and airborne dirt with it. Silanes and siloxanes are silicon based molecules with a reactive alkoxy group at one end and a water repelling alkyl group at the other. The reactive end hydrolyses in the alkaline, slightly damp pore environment and bonds to the silicate pore walls, leaving the alkyl end pointing into the pore.

The result is a pore that is still open but no longer wettable. Capillary suction reverses, so it takes substantial applied pressure to force liquid water in, while water vapour, which moves by diffusion rather than capillary flow, passes through as before. That is why a treated panel repels driving rain and still dries from the inside. Silanes penetrate deeper and suit dense substrates; siloxanes stay nearer the surface, lose less to evaporation and suit open absorbent ones. Most commercial products are blends, and many are supplied as a cream or gel so they stay in contact with a vertical surface long enough to soak in.

What it does and does not do

Reduced water absorption means less water carried into the panel, and therefore less salt movement and less efflorescence, less staining from dirt washed in with the water, better freeze-thaw resistance where that applies, and less chloride ingress in a coastal environment. On a facade the visible benefit is cleaner weathering, because surfaces that shed water rather than absorb it collect less of the dirt that run-off carries.

• It is not a waterproofing membrane: it resists capillary uptake, not hydrostatic pressure, and does nothing for a crack or a failed joint
• It does not bridge cracks; anything wider than a hairline is a water path regardless
• It does not stop dirt deposition, only the staining carried in by absorbed water
• It does not change the fire performance of the panel, and no fire claim should be made for it without a named test and testing body
• It does not remove existing stains; the surface must be cleaned first or the treatment locks them in

How it differs from a film-forming sealer

• Location: an impregnation is inside the surface, a sealer is a film on top of it
• Vapour: an impregnation retains most vapour permeability, an acrylic, epoxy or polyurethane sealer substantially reduces it
• Appearance: an impregnation is essentially invisible, a sealer darkens the surface and usually adds sheen
• Failure mode: an impregnation wears away gradually and invisibly, a sealer fails visibly by blistering, peeling, yellowing or crazing
• Recoat: an impregnation is reapplied over itself, a sealer generally has to be stripped first, which on a porous textured face is difficult and damaging
• Risk: a sealer over a cementitious panel can trap moisture and salts behind the film, which is what causes the blistering

Service life, recoating and substrates

Impregnations do not last the life of the building. The alkyl groups near the face are degraded slowly by ultraviolet and the treated zone erodes as the surface weathers, so performance declines gradually. Published service lives are typically five to ten years for conventional products on vertical facades, and fifteen to twenty for deeply penetrating cream silanes on dense substrates, but these are manufacturer figures that vary with exposure, orientation and porosity. Horizontal surfaces are always the shortest lived.

Because the treatment fails invisibly, it is managed by schedule and test rather than inspection. A water bead check or a Karsten tube absorption test on a representative area every few years tells the owner whether it is still working, and reapplication is straightforward: clean, dry, apply over the existing treatment. That obligation belongs in the maintenance schedule at completion.

Substrate matters. Impregnations work on porous mineral substrates: glass fibre reinforced concrete, precast, ultra high performance concrete, fibre cement and masonry. They do nothing useful on a fibre reinforced polymer panel, where the face is a gel coat with no open pore structure. On very dense substrates absorption is already low and the benefit smaller, which is why a polished panel gains less than an acid etched or sandblasted one.

Standards and references

• EN 1504-2 Products and systems for the protection and repair of concrete structures, surface protection, class H hydrophobic impregnation
• EN 13580 water absorption and resistance to alkali, and EN 14630 determination of carbonation depth
• EN 1062-3 Paints and varnishes, determination of liquid water permeability
• ASTM C642 water absorption of concrete, and ASTM E514 water penetration of masonry
• AS/NZS 4456 Masonry units, methods of test, for absorption reference on mineral substrates
• Manufacturer’s published penetration depth, absorption reduction and vapour permeability data for the specific substrate

Related terms

• Water absorption
• Weathering
• Maintenance
• Durability and service life
• Efflorescence
• Anti-graffiti coating
• Surface finish

Questions specifiers ask

Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.