Hi Oliver,
I’ve found this public template in the eTool library that explains how to model concrete with a certain % of recycled water content: Poured Concrete – External Works, Paving, 25MPa, 50% BFS, 50% Recycled Water, 30 year Service Life (m3).
Here is the link: https://etoollcd.com/Library/Template/Index/42272
In this template, recycled water is modelled as a -(ve) quantity of fresh water. Note that this is for reduced fresh water usage and not a carbon saving (carbon impact is not modelled accurately, but is not significant).
For more information about modelling custom concrete mixes, this support post may be useful: https://support.etoollcd.com/index.php/knowledgebase/modeling-custom-concrete-mixes/