Paving stones are generally laid without mortar on a carefully tamped and bordered base. As large flat surfaces, it is tempting to lay out other projects on them like mixing cement or mortar. If you get mortar or cement on paving stones, you can remove the stains without replacing the paving stones. Simple tools and chemicals will take care of most of these stains without discoloring your paving stones.
Removing Bulk Material
Remove material on the surface of the stone. Mortar and concrete spatters and spills often do not set into the stone. Try brushing the material away.
Tap the spatter with a hammer or cold chisel. If it is set, try tapping it gently with a cold chisel directly on top and in the center of the spot. If you are lucky, it will split in two and come cleanly off the stone. Apply measured extra force if the spot does not come off--you want to try to leave the surface of the paving stone undamaged.
Brush as much leftover material as possible. Use the wire brush in all directions. Tap remaining spots with the cold chisel and brush again.
Sand the spot. Use a grinder, sander, or sandpaper to get down to the level of the paving stone if the prior steps haven't worked.
Removing the Stains
Apply a phosphoric-acid based cleaner. Test on an unused paving stone or inconspicuous area to make sure it does not discolor the stone. Neutralize excess acid with garden lime, baking soda or ammonia. Use exactly what the manufacturer suggests, and use it at the appropriate concentration. Repeat if necessary. Sometimes the mortar will flake off with a brush after being treated with acid-based cleaners.
Apply diluted muriatic acid to stains that did not come off with the phosphoric acid. Muriatic acid is concentrated hydrochloric acid--dilute it with water exactly as the manufacturer specifies and use appropriate protective clothing and eyewear. Let the muriatic acid work on the stain until it is dry.
Neutralize the surface. Follow the manufacturer's exact instructions for diluting, neutralizing and rinsing. Flush generously with water.
Wash with TSP (trisodium phosphate) if necessary. This can help make the color more consistent with the unstained paving stones. Rinse thoroughly.
Re-apply any sealer that was originally on the paving stones. Allow the stones to dry and apply the sealer according to the manufacturer's instructions.