A concrete retarder is a chemical admixture that slows the hydration reaction between cement and water, delaying initial and final set times. By extending the plastic state of the concrete, retarders give your crew more time to place, consolidate, and finish the slab — which is critical in hot weather and high-evaporation conditions when the concrete wants to set faster than your crew can work it.
Retarders do not change the ultimate strength of the concrete. They simply shift the set time curve to the right, giving you a longer working window. The concrete will still reach its design strength at 28 days — it just takes longer to reach initial set.
A concrete accelerator does the opposite: it speeds up the hydration reaction, reducing the time to initial set and accelerating early strength gain. Accelerators are used primarily in cold weather when low temperatures slow hydration to the point where the concrete may not gain enough strength before freezing.
There are two types: set accelerators (which speed up initial set) and hardening accelerators (which speed up early strength gain). Calcium chloride is the most common and cost-effective accelerator, but it cannot be used in reinforced concrete or post-tensioned applications due to corrosion risk. Non-chloride accelerators are available for those situations.
The decision between retarder and accelerator comes down to conditions — and evaporation rate is the variable most contractors underweight.
High evaporation rate (above 0.15 lb/ft²/hr): You almost certainly need a retarder. High evaporation conditions mean the concrete surface is losing moisture fast, and the concrete wants to stiffen before your crew can finish. A retarder extends the window and gives your crew a fighting chance. Combine it with evaporation retarders (spray-on surface films) for maximum protection against plastic shrinkage cracking.
Cold weather (below 50°F): You likely need an accelerator. Cold temperatures slow hydration, and if the concrete does not reach adequate strength before freezing, the water in the pores expands and damages the matrix permanently. An accelerator ensures the concrete gains enough early strength to survive.
Moderate conditions: In the sweet spot — mild temperatures, reasonable humidity, light wind — you may not need either. Check your evaporation rate to confirm.
Most retarders are based on lignosulfonates, hydroxylated carboxylic acids, or sugar-based compounds. Common products include BASF Pozzolith, GCP DARASET, and Sika Plastiment. Typical dosing ranges from 2 to 6 oz per 100 lbs of cement, depending on the desired set delay and ambient conditions.
Always follow the manufacturer's dosing recommendations and adjust based on trial batches. Over-retarding — adding too much — can delay set time by many hours and create a soft surface that is difficult to finish. In hot weather, start at the lower end of the dosing range and increase only if needed. Communicate with your ready-mix supplier — they can adjust the dose based on the conditions you report.
Calcium chloride (CaCl₂) is the most widely used accelerator, typically dosed at 1–2% by weight of cement. It is effective and inexpensive but restricted in reinforced and post-tensioned concrete due to chloride-induced corrosion risk.
Non-chloride accelerators — such as BASF Pozzutec, GCP DCI, and Sika Rapid — are used where chloride is prohibited. They cost more per yard but eliminate corrosion risk. Dosing varies by product; follow manufacturer recommendations and test in trial batches when possible.
In extreme cold (below 25°F), accelerators alone may not be sufficient. Combine with heated enclosures, curing blankets, and hot water in the mix to ensure adequate early strength gain.
The time to decide on admixtures is before the pour — not when the truck is backing up to your forms. Run the numbers with our free evaporation rate calculator, or let PourDay show you the real-time rate and forecast for your job site so you can see whether conditions call for a retarder, an accelerator, or neither. Check the night before and again at pour time, then communicate with your ready-mix supplier to get the right admixture in the right dose for the conditions you are actually facing.
Make the retarder-or-accelerator call with data, not guesswork. PourDay is free for every contractor.
Understand the number that determines whether you need a retarder, an accelerator, or neither.
Read articleThe standard behind hot weather precautions — including when retarders become mandatory.
Read articleHow temperature and admixtures affect workability — and why slump alone is not enough.
Read articleReal-time evaporation rate. 16-day forecast. Pour logging. Free for contractors.