Ever clean a cement woodstove mantle ? What did you use?

Mantle or hearth? Some mantles are removable. 

If the mantle is dyed or painted concrete you can try mixing a 1/4 cup of Pine-Sol with a gallon of water and scrubbing with a rag. Let your solution soak before scrubbing.

If the mantle is not dyed or painted you could try the same solution of Pine-Sol and water or a 1/2 cup of baking soda with 1 gallon of water with a hard stiff scrub brush. Be sure to let the solution soak for 5 - 10 minutes before scrubbing.

If the mantle is removable and not dyed or painted, you could use a pressure washer or muratic acid diluted with water and a hard scrub brush. But be sure to wear heavy rubber gloves and eye protection with the muratic acid mixture.