A concrete admixture is a substance added in small quantities to concrete or mortar to deliberately influence one or more properties. The quantities involved are small, usually less than 5% of the mass of the cement. Despite the small amount, an admixture has a clear effect on the fresh or hardened properties. Admixtures for concrete fall under the standard NEN-EN 934.
Common admixtures
- Plasticiser / superplasticiser — improves workability without extra water, or allows the same mix with less water (stronger concrete)
- Retarder — slows down setting, useful in warm weather or for long transport times
- Accelerator — speeds up setting and hardening, for example in cold conditions
- Air-entraining agent — introduces small air bubbles, which improves freeze-thaw resistance
- Water-repellent admixture — reduces the water absorption of the hardened concrete
Distinction from other additives
An admixture is not the same as an addition (such as fly ash or limestone powder), which is added in larger quantities and counts as part of the composition. An admixture also differs from the binder (cement) and the aggregate (sand, gravel).
When using admixtures, always observe the prescribed dosage: too much admixture can be counterproductive, for example through segregation or strongly delayed setting. Follow the supplier’s specification.
Related terms
- Concrete
- Cement
- Plasticiser
- Air-entraining agent
- Fly ash
- Concrete mix
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