What is a filler material?
A filler (Dutch: vulstof) is a fine, usually mineral powder added to a product to add bulk, fill voids or improve properties such as density and workability. A filler is neither a binder nor a pigment: it does not bind by itself and adds (almost) no colour, but it strongly affects the result.
Fillers by application
- Concrete and mortar: fines such as limestone powder fill the space between cement and sand. This gives a denser, more workable mix and a smoother surface, for example in fair-faced and self-compacting concrete. The concrete standard EN 206 calls these additions: type I is nearly inert (such as stone powder), type II is reactive (such as fly ash).
- Asphalt: the filler is the finest fraction of the aggregate (smaller than 0.063 mm). Together with bitumen it forms the mastic that binds the stones.
- Paint, filler pastes and sealants: chalk (calcium carbonate), talc, kaolin and barytes are common fillers. They add opacity, filling power and a flatter finish, and lower the price.
- Plastics: minerals or glass fibres as filler make plastics stiffer or cheaper.
Tips
- More filler is not always better. Cheap wall paint often contains a lot of filler and relatively little binder; it chalks sooner and is less scrub-resistant. Compare the scrub resistance class on the label, not just the price per litre.
- Do not add filler yourself to ready-made mortar, sealant or paint. The formulation is balanced; extra filler can reduce adhesion and strength.
- Use a suitable filler paste for repairs: fine for thin coats, coarser for deeper holes in one go.
- For concrete, mind the mix design. The ready-mix plant sets the amount and type of addition; for exposed surfaces ask for a mix designed for fair-faced concrete.
Related terms
- Mineral filler
- Joint filler
- Binder
- Pigment
- Asphalt
- Fair-faced concrete
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