What is verzanden (sanding out and silting up)?
The Dutch term verzanden has two meanings in construction. In mortar and concrete it is the loss of the correct ratio between binder and filler, usually through segregation of the mix. In hydraulic engineering it is the silting up of a harbour, river or canal with sand.
Sanding out of mortar and concrete
A mix consists of a binder (cement, lime), sand or gravel as filler, and water. If these constituents separate from each other, for example through vibrating for too long, too much water or drying out too quickly, the ratio is locally no longer right. Too little binder is then left at the surface. The result is a weak, sandy top layer that starts dusting: the sand grains come loose when you sweep or rub over it.
A common cause is drying out too quickly. Cement needs water to harden (hydration). If the surface dries out too fast because of sun, wind or an absorbent substrate, the top layer does not cure properly.
Prevention in practice
- Curing: keep fresh concrete and freshly laid screeds moist for a few days or cover them with plastic sheeting, or use a curing compound.
- Do not add extra water to make the mix easier to work. More water means a greater risk of segregation and a weaker top layer.
- Pre-wet or prime absorbent substrates before applying mortar or a screed.
- Do not work in bright sun or strong wind without protection, and not in frost either.
- Check after hardening: scratch the surface with a nail or a key. If a lot of sand comes loose, the top layer is too weak for tiles, coatings or paint and must first be sanded or consolidated.
Silting up in hydraulic engineering
Rivers and the sea carry sand and silt that settle in calm water. Without dredging, harbours and navigation channels therefore slowly silt up. A well-known historical example is Bruges: because the Zwin silted up, the city lost its direct connection with the sea.
Related terms
- Dusting
- Mix (mortar or concrete)
- Mortar
- Concrete
- Curing of poured concrete
- Screed
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