Phil-
My father was a bricklayer and I've been a cement mason for over thirty years. Occasionally we have to clean splashes or smudges off various surfaces. The thing you have going in your favor is that porcelain is'nt very porous.But it is brittle so dont flex it to much or cracko!You'll probablly want to try to break down the cements bond to the sand particals and surface with some muriatic acid. You maybe can get a gallon of it at home cheapo or lowes. For sure at a builders supply(masonary). They will tell you to start off with like 10to1 mixture. I'm much to impatient for that.Mix it so when you wipe or brush it on the porcelain it doesnt bubble--Try on the back first probablly about 4or5 to 1.Make sure your not etching(taking the gloss off) if not wipe it on the cement. It should bubble a bit-dont breathe the fumes-try not to get on your skin definatly not in your eyes LOL. dont let it sit in puddles on the porcelain-maybe stand it up.repeat untill it corrodes or disolves and comes off rinse it good and then wipe lemon juice on it to neutralize the reaction on the exposed metal or it will rust like crazy. Good luck-I'd like to see before and after pictures. Jim