What are admixtures and why are they used in concrete?

Admixtures are materials added to concrete besides cement, water, and aggregates to modify properties. Common types include: plasticizers/superplasticizers (reduce water, improve workability), retarders (delay setting for hot weather or long transport), accelerators (speed up setting for cold weather), air-entraining agents (improve freeze-thaw resistance), and water-reducing agents. They are used to achieve desired properties that cannot be obtained with basic ingredients alone.