Producers must realize that gray-water recycling will have significant effects on how they run their operations and the properties of their fresh and hardened concrete.
No single guideline for gray-water reuse exists. Differences in local raw materials and regional climates give fresh concrete batched with gray water varying qualities.
Hydrated cement particles in the gray water can increase the fresh concrete's set time. An accelerated set time may be desirable in cold or mild weather and can be mitigated through temperature control and/or admixtures in hot weather when faster set times are undesirable.
Potential problems include increased potential for shrinkage cracks, rapid slump loss, increased water demand to achieve the desired slump, and a corresponding reduction in compressive strength. In some cases, a specifying authority absolutely prohibits the use of gray water or restricts its use to a specified maximum percentage of the total batch water content.
Gray water must be treated like any new ingredient in the concrete mix design. Lab and truckload batches should be tested to evaluate the effect of gray water on fresh and hardened concrete properties.
Because plant conditions change daily, the producer needs to monitor gray water regularly for consistency and uniformity. Tests include checks for variability in specific gravity, temperature, and chemical composition.The article includes tips on operational changes that are necessitated by gray-water recycling.