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Washwater admixtures

There is a growing problem with disposal of washwater from cleaning the inside of truck mixer drums at many ready-mixed concrete plants. The Landfill Tax and the prohibition on disposal through the drainage system have increased the cost of disposal. One solution is to recycle the water by the use of a truck washwater admixture.

Washwater admixtures stabilise concrete washwater in the drum of a ready-mixed truck overnight or over a weekend by retarding the cement hydration. The washings are then incorporated in the first mix when work resumes. This process eliminates or reduces the volume of solids/water deposited into a plant wash pit. Research has shown that the correct use of proprietary admixtures does not impart any deleterious effect on the fresh concrete. Washwater admixtures coat the cement particles and prevent hydration for a specified time. The delay depends on the dosage, which is itself related to a number of other factors. With some systems, an accelerator (‘activator’) may be needed to restart hydration.

Technically, the admixtures may also be used to stabilise returned loads, which may be either downgraded to a lower specification and/or made up to new loads. However, such concretes could only be used with the agreement of the supplier, customer and regulating body.

Acknowledgement: The Concrete Society

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TR18 A guide to the selection of admixtures for concrete