Waste wash water recycling in ready-mixed concrete plants Franco Sandrolini*, Elisa Franzoni Dipartimento di Chimica Applicata e Scienza dei Materiali, Universita Á di Bologna, Viale Risorgimento 2, 40136 Bologna, Italy Received 8 August 2000; accepted 26 October 2000 Abstract 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. D 2001 Elsevier Science Ltd. All rights reserved. Keywords: Ready-mixed concrete; Wash water; Recycle; Mortar; Concrete plants 1. Introduction It has been calculated that a 9-m 3 ready-mixed concrete truck contains, at the end of each working day, approxi- mately 200±400 kg of returned plastic concrete [1,2]: this material can be left overnight in the truck with the addition of hydration control admixtures [3,4] or washed out. When washed out, with the addition of about 700 ± 1300 l of water, the material can be mechanically separated into aggregates ready for reuse and water containing amounts of suspended fine particles. According to Italian law [5], such water cannot be discharged into urban sewers because of its content of suspended matter (usually exceeding the max- imum allowed content of 2 ml/l) and pH value (usually higher than 9.5). Consequently, partial and complete recycle of waste wash water are usually adopted in the manufactur- ing plants. By the former method, water is collected in sedimentation basins: hence, clarified water is reused in the production, while sediment must be disposed of in authorised landfills; on the contrary, full recycle represents an environmentally safe and cheap procedure, because wash water is totally reused as mixing water in the production and no disposal procedure is involved. The use of recycled waste wash water for the production of new concrete is dealt with by prEN 1008 standard [6]: mixing water must meet severe requirements in composition and may be used only if concrete exhibits 7- and 28-day compressive strengths higher than 90% of the value exhib- ited by samples prepared with distilled water (in Table 1, the main requirements of pr EN1008 are shown in comparison with those of ASTM C94 [7]). In spite of the so far achieved requirements by prEN 1008, building companies often fear the use of ready-mix truck wash water in the production of fresh concrete, also because limited investigation has been carried out on this topic [2]: for this reason, the effect of waste wash water on the properties (strength, microstructure and durability) of mortars and concretes has been investi- gated in this paper, referring to a ready-mixed concrete plant where waste water recycling is daily carried out in the production after the removal of the aggregates > 0.15 mm from wash water by a reclaimer system. The plant is a * Corresponding author. Tel.: +39-51-2093205; fax: +39-51-2093213. E-mail addresses: franco.sandrolini@mail.ing.unibo.it, sandro@alma.unibo.it (F. Sandrolini). Cement and Concrete Research 31 (2001) 485 ± 489 0008-8846/00/$ ± see front matter D 2001 Elsevier Science Ltd. All rights reserved. PII:S0008-8846(00)00468-3