cement is just heat treated limestone that makes up Portland cement, theres always sand added for concrete mix and rock aggregate, I'm sure its just sand and cement on the paint.
OK that said the cement can be dissolved but not the sand,
You may use any acid mild of course, vinegar is mild or coffee pot scale remover, you can try CLR from the hardware store, ice machine cleaner discaler, bar keepers friend, all these are acid and will desolve or weaken it, start with the weakest product vinegar like jfelbab said keep it moist. don't rub it off its just going to scratch the hell out of the paint.