Calcination of excavation sludge at short residence times for performant cement blends

The sludges from washing excavated materials for aggregate production were studied for use as supplementary cementitious material (SCM). The performance was optimised using a calcination process with short residence time in a rotary furnace. The short residence time of 10–20 s provides a clear increase in R3 heat release with respect to a 1-hour box furnace calcination, from 70 J/g to 150 J/g, transforming the calcined sludge from an inert filler to a lowly reactive SCM. This transformation is not accompanied by substantial changes to the phase composition. The SCM showed a pozzolanic reaction and strength contribution after 90 days. To compose performant cements in the conventional 28 days conformity timeframe, synergistic behaviour was searched and found with limestone and calcined clay, which provided 94% relative strength at 28 days (compared to reference CEM I mortar) for a blend with 20 wt% calcined sludge, 5 wt% limestone and 5 wt% kaolinitic clay (of ∼75 wt% kaolinite purity), without significantly affecting mortar flow.