Set retarders and accelerators: adjusting the setting time of mortar

What is setting?

Once cement or gypsum is mixed with water, at some point it starts to harden. That is the set. If it comes too early, the applicator can't use up the batch in time. If it comes too late, work slows down and early strength is delayed. Setting additives move that point to where you want it.

Set retarders

Retarders slow down the hydration reaction between cement and water. A well-chosen retarder extends working time without lowering final strength. It also reduces shrinkage of the mortar and limits segregation and bleeding.

• Citric acid: Common in cement-based products; minimises the effect of temperature on setting.
• Sodium gluconate: Gives controlled retardation without harming the cement.
• L(+) tartaric acid: The preferred retarder for gypsum-based products.

Typical uses: tile adhesives and renders in summer, screeds, gypsum skim coats.

Set accelerators

Accelerators do the opposite. They speed up the reaction between cement and water, so the gel that binds the aggregate forms sooner. Setting time drops and early compressive and tensile strength go up.

• Calcium formate: Raises early bond strength, especially in cold weather. It speeds up setting without forming a skin on the surface.

Typical uses: winter work, repair mortars, concrete that has to take load quickly.

Choosing the right one

Dosage depends on cement type, ambient temperature and the open time you are aiming for. We can help you work out which additive, and how much of it, suits your formulation.