Saw this here ; see if it will work:

http://www.howtocleananything.com/tipdetails.asp?tid=922

Cement and plaster are alkaline compounds. They are easily removed from clothes, woodwork, your body and cars using distilled white vinegar (a mild acid). Simply soak a sponge with the vinegar and lay it on the cement spots for a few seconds. This will loosen the cement so it can be hosed away. It may take a couple applications to remove the cement from the car. Whatever you do, don't try to wipe the cement off with the sponge, as this will scratch the paint.