Everything in the load that is not cement, water or stone
A concrete mix has three main ingredients, and an admixture is anything beyond those three, added in small amounts at the plant to make the load do something it would not do on its own. None of it is exotic. These are ordinary tools mix design people reach for constantly, chosen against the strength, the exposure and the schedule of a particular job.
- Air-entraining admixtures, for concrete that has to survive freeze and thaw
- Accelerators, to speed up set time in cold weather
- Retarders, to slow set time down when the schedule calls for it
- Plasticizers, to improve workability without adding water
- Fiber, mixed through the load rather than tied in as steel
Admixtures change how a load moves, on the ground and through a hose
None of these are a problem to include in a mix that calls for them. What they do is change the material's behavior, and that behavior matters more once a load has to travel through a line pump instead of sliding down a chute. Set-time admixtures especially change the clock. Anything that speeds a pour up shortens how long the concrete can sit still in a hose, and anything that slows it down changes when the finishing crew gets to start.
Which admixture goes in is a plant decision, made against the job
Choosing the right admixture, and how much of it, is mix design work done at the batch plant, not a call to make from a glossary page or a supply house shelf. What matters on your end is telling us the conditions the pour is up against, cold weather, a tight schedule, exposed exterior work, and letting that information travel to the plant with the rest of the order.
When a plan already names one
Sometimes the decision is already made before we get the job. An engineer's plan or a spec sheet will occasionally call for a particular admixture by name, for a footing, a slab or a specialty finish. When that happens, we would rather read it before the concrete is ordered than find out about it once the load is already in the drum, since it changes what goes on the ticket.