I recently made a plaster cast mold of a friends hand outside of my house, i used dental alginate, and plaster of paris.. here's the problem; I mixed the plaster of paris but used too much leaving extra mixed plaster, my friend disposed of it by pouring it down the street drain, only the majority of it just stayed i guess since i came out the next morning to find dried plaster on the concrete sidewalk.. how can I possibly get it off, any help would be appreciated, please.

well, aside from grinding it off with a grinder with a mason disk - you are plain outta luck.