The method used to clean cement and stone fireplaces varies depending on if it is sealed or not. If a sealant was applied to the fireplace stones, the fireplace can be cleaned more aggressively than a fireplace with unsealed stones. However, because the sealant will protect the stones from the soot build up, it is usually only necessary to wipe the stones with a cloth dipped in warm water.
If the stones on your fireplace aren't sealed, you will have to use a hard bristle brush to remove any debris. Once you have removed all you can, you should wet the stones as much as possible. You can use a sponge to do this on the vertical stones. This will stop the stones and cement from absorbing the chemicals used to clean the stones.
Next, mix 1 cup of white vinegar, 3 tbsp of dish washing detergent and 6 cups of water in a bucket. Dip a soft bristled brush into the mixture and use it to scrub the stones and cement. If there are stains on the stones, you can treat them by rubbing baking soda on the stains. Make sure that you rinse the baking soda off using plain, warm water. After it is rinsed off, you can scrub the area again with the mixture and soft bristle brush.
Stains on a sealed stone and cement fireplace can be treated in a number of ways. The most environmentally friendly is to treat the stain with a lemon. To do this, slice a lemon in half lengthwise then rub the lemon on the stain. If this doesn't work, you can try to bleach the stain. However, if you do this, make sure that you test a small inconspicuous area before treating a large visible stain. Using a lime, calcium and rust removing product is another possibility. Should you choose to do this, you must thoroughly rinse the chemical off after using to avoid a fire hazard.