I believe that is waterproofing material that would be being inserted along the edges of the excavation - kind of like a mini-shoring and waterproofing combination - in advance of breaking through the Maple Leaf Square / Longo's wall - to prevent water from running from the tunnel being built into the Longo's store.
A similar rig, complete with the bags of grey powder, being mixed and inserted into the hole, was being used for the Metrolinx Ontario Line ground sampling on Wellington Street West at Bathurst Street, where waterproofing was required because of the number of under the road utility services in the area.