set retarding admixture is used to delay the initial setting of concrete or to offset the accelerating effect of warm weather on the cement composition. It can also be used to increase the pumping time of a concrete composition and prevent premature thickening or setting before placement in a subterranean zone.
Uses for set retarding admixture include concrete pavement construction where it allows longer hauling distances, eliminating the need for moving a central mixing plant to the job site, and allowing more time for texturing or plastic grooving of concrete. It can also help to eliminate cold joints that occur when two batches of concrete are applied over a period of time and fail to mix.
Retarding admixtures slow down the initial reaction between concrete and water by reducing the rate of penetration to the cement particles and preventing the growth of hydration products. This results in a longer period during which the concrete is workable, minimizing bleeding and providing time for the strength to develop.
The length of this period depends on the temperature, consistence class, and water-cement ratio of the concrete as well as on the amount of retarder added. Retarding admixtures also reduce the initial water demand of concrete, resulting in improved workability and higher strengths.
Retarders are typically based on molasses, sugar or gluconate and often have a limited dosage range as they temporarily stall further hydration. They are effective under low dosages but overdosing can result in large extensions of set time.