Waste wash water recycling in ready-mixed concrete plants

Production of large amounts of waste wash water coming from ready-mixed concrete plants leads to problems of environmental impact. National laws usually prohibit the disposal of such types of water, due to their extremely high pH value and suspended matter amount, and require the water to be treated prior to discharge. prEN 1008 provides for recycling waste water in the production of new concrete, but gives some restrictions for its composition and use. In this paper, the use of waste wash water (coming from a medium-size ready-mixed concrete plant) in mixing water for concrete and mortars has been investigated: the effects on physical–mechanical properties and microstructure are investigated as a function of the characteristics of waste water used. The results have shown that mortar and concrete prepared with recycled water exhibit 28-day mechanical strength in no way lower than 96% of the reference materials (90% is the minimum allowed in prEN 1008) and, in some cases, even better. Moreover, the use of wash water in concrete leads to a reduction of the concrete capillary water absorption and mortar microporosity, which surely improves the durability of the material. This effect can be ascribed to the filling action of the fines present in the wash water and to the slight reduction of the actual water/cement ratio.