What is a plasticizer?
A plasticizer is an admixture for concrete and mortar that makes the mix more workable without needing extra water. It works by dispersing the cement particles: left alone they flocculate and trap water between them. The admixture charges the particles so that they repel one another; the trapped water is released and the mix flows more freely.
Two grades
The European standard EN 934-2 for admixtures distinguishes them by the water reduction achieved:
- Plasticizer (water reducer) — reduces the water required by at least 5 per cent at equal workability.
- Superplasticizer (high-range water reducer) — reduces water by at least 12 per cent. In practice, modern polycarboxylate ether products achieve considerably more, which is the basis of self-compacting and high-strength concrete.
Older types are based on lignosulfonate; melamine and naphthalene sulfonate variants also exist.
What it is used for
The effect can be used in two ways — but not both at full value at once:
- Same workability, less water. This lowers the water-cement ratio, raising compressive strength, density and durability. By far the most important application.
- Same water-cement ratio, better workability. Useful with congested reinforcement, slender walls, complex formwork and pumped concrete.
Dosage is usually in the range of a few tenths of a per cent up to a few per cent of cement weight; the exact range is on the product data sheet.
Points to watch
- More water is not a substitute. Extra mixing water also makes concrete flow, but it lowers strength and increases shrinkage. Avoiding exactly that is the point of a plasticizer.
- Overdosing causes problems. Too much admixture can cause segregation (the coarse aggregate sinks), bleeding, entrapped air and severely retarded setting. Weigh the dose, do not estimate it.
- Working time. The effect of many superplasticizers falls away within tens of minutes. Dose at the plant and place promptly; redosing on site should only follow the supplier’s instructions.
- Not every admixture suits every cement. The combination of cement type, addition and admixture determines the result. On important work, trial the mix first, especially in cold weather.
- Never combine untested. Plasticizers, accelerators, retarders and air-entraining agents interact. Check compatibility with the supplier.
Related terms
- Admixture
- Water-cement ratio
- Consistence class
- Concrete
- Plasticity
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