Or just a few pounds of sugar when the concrete is wet. Concrete needs to be able to bond to the water molecules to cure, but dissolved sugar prevents that. So the concrete will never actually cure, and will crumble back into dust when it dries instead of turning solid. Anyone with a hardhat, safety vest, and work boots could blend into a large construction site long enough to lob a bag of sugar into a concrete mixer.
I’m not saying you should use it to oppose police stations, prisons, roadways immediately surrounding capitol buildings, etc from being built or repaired. But you could.
You need enough sugar to bond to enough water molecules to prevent enough concrete from bonding to it. It’ll take a pound or more, because you need to “capture” the water molecules before the concrete can. The nice part is that you don’t need to be careful about it; You can just pop a bag or two straight into a concrete mixer, and let the mixer do its job.