Abstract—Mobile grinders could be used for instant recycling of concrete waste, eliminating the need for transport of the demolition waste, storing, and possibly also landfilling. Even though partial replacement of aggregate by recycled one has become established practice, incorporation of concrete fines composed of fine sand and stripped mortar into concrete mix has been considered harmful. However, these fines could contribute to the concrete matrix and ITZ strengthening by the microfilling effect and through activation of residual anhydrous clinker (RAC). This study focuses on the content and possible reactivation of RAC from various sources in cementitious pastes using microscopy, calorimetry, and macroscopic testing of mechanical properties. The results indicate that proper sorting of the recycled material before grinding is crucial.