Water repellents: how to make cement-based products hydrophobic
How powdered silicone, zinc stearate and calcium stearate cut water uptake in wet-area products and insulation renders.
Why water repellency?
Hardened cement mortar is porous and draws water in through its capillaries. That water breaks the mortar up as it freezes and thaws, carries salts to the surface as efflorescence, and gives mould and staining somewhere to start. Water-repellent additives make the pore surfaces hydrophobic: liquid water struggles to get in, while water vapour still largely passes through, so the mortar keeps breathing.
Additives used
- Powdered silicone. A silicone-based hydrophobic additive in powder form. In insulation renders, tile grouts and wet-area products it gives a clearly noticeable gain in water repellency.
- Zinc stearate. A metallic soap. It makes the finished cement-based product water repellent.
- Calcium stearate. Works much like zinc stearate. The choice between the two usually comes down to the balance of cost and performance.
Where is it used?
- ETICS (EIFS) renders and adhesives
- Tile grouts
- Renders and screeds in wet areas such as bathrooms and kitchens
- Waterproofing mortars
Points to watch
Set the dosage so it does not hurt adhesion or strength. Too much additive can weaken the mortar's bond to the substrate. The MSA technical team can help you pick the right additive and dosage for your product.
Let's pick the right product together.
Tell us what you make and we will suggest the product that fits.