To be considered concrete, the mixture should include coarse aggregate, which increases the density of the mixture and it should not flow as it did in the trap scene.
Concrete in its liquid form is not called concrete it is called cement. That is why they have cement mixer trucks and not concrete mixer trucks. When Oliver and Dig were in the room being filled with the liquid Oliver called concrete it was actually cement. And it looked very watered down at that because it flowed way more than normal cement.
At 13:56 when Oliver and Diggle were rescued from the concrete trap, the circular section that fell out of the metal ceiling floats.
When John and Oliver are trapped in a room being filled with concrete, Curtis uses his T-Spheres to cut out a circular hole in the ceiling. Right after the circular section of ceiling falls into the concrete, there is a second where we can see said section bob back up, revealing that it is made of Styrofoam or wood.
Oliver and Chase have a knock down drag out fight in the stairwell, including Oliver falling down the stairs. Yet neither one loses any arrows from their quivers.