10 13 14

(Keppert et al. 2021)

Concrete sludge or slurry is a waste stream generated by ready-mix and precast concrete plants. It comprises of unused wet concrete washed from mixing equipment and trucks returning from site. Aggregates can typically be recovered in the washing process, leaving a residue comprising predominantly of fines material and water.

The MPA estimated UK readymix concrete production in 2019 of 24.7 million cubic metres, converting using 2.38 tonnes per cubic metre, this means an annual production of 58.79 Mt (MPA, 2021). Previous estimates in the literature for the waste are as high as 1-4% of the production of fresh concrete (Correia et al., 2009), and 0.8% when best practice is adopted and this residue is de-watered using a filter press (Xuan et al., 2016). If we assumed best practice was adopted across the field, which is unlikely, it would still yield a total in significant excess of published data, at 470,320 tonnes per year.

The discrepancy could arise from a high proportion of the material being re-used internally, or perhaps more likely, from the waste being received under an EWC code falling under chapter 17 construction and demolition wastes.