Concrete wash water, also referred to as concrete wastewater, is generated during the cleaning of concrete mixer trucks and is one of the major challenges confronting the concrete industry. Since wash water is classified as a hazardous waste, it must be neutralised before disposal. It is not only an environmental issue, but also an economical constraint for ready-mix concrete producers. Among the proposed techniques, recycling wash water is considered as one of the most effective methods of dealing with it. However, the negative effects associated with the reuse of wash water in concrete manufacturing limit its use. The main issue was that when wash water is used as mixing water, it affects the fresh properties of the concrete such as workability and setting time. The reason for this is the ongoing hydration of unreacted cement particles in wash water. Therefore, a proper treatment mechanism is required prior to utilising wash water as mixing water in concrete.

Carbon dioxide sequestration, also known as carbonation, could eliminate the deficiencies with wash water. This research study was conducted to investigate the efficacy of carbon dioxide sequestration in preventing the ongoing hydration of wash water. Wash water specimens with a specific gravity of 1.10 were used in this study, and the specimens were allowed to partially hydrate for 3 hours to simulate the actual scenario of wash water returned to concrete batching plants. Several levels of CO2 sequestration (based on the extent of CO2 uptake) were considered in order to determine the maximum CO2 uptake and to identify the required level of CO2 sequestration to completely stop the hydration of wash water. A CO2 uptake of 23% or higher by weight of cement (bwc) successfully inhibited further hydration of wash water. This was confirmed by X-ray diffractograms (XRD) and thermogravimetric analyses (TGA). Furthermore, the concentrations of chloride and sulphate ions, as well as the alkali content (Na2O equivalent) of the carbonated wash water specimens, were within acceptable levels for mixing water. Based on the findings, it is possible to conclude that carbon dioxide sequestration is a promising treatment technique for wash water in order to improve its suitability for use as mixing water in concrete. This method is also an effective way to recycle tonnes of CO2 produced as an industrial by-product, particularly from cement manufacturing plants, that significantly contributes to global warming.