Sandstone is a natural type of stone, available in many colours ranging from tans to brown to reds to greys. Building sandstone chimneys consists of mortaring pieces of sandstone together to form the chimney. Sandstone chimneys stand up to outdoor conditions, but can incur damage from accidents or crumbling mortar.
Place a cold chisel onto the centre of the damaged mortar line and strike with a hammer to crack the mortar. Or, install a diamond wheel into a grinder and follow the mortar line, chipping away old mortar. When using a grinder with a diamond wheel, move along the mortar line in one direction---do not go back and forth.
Brush out chipped, broken and loose pieces of mortar with a wire brush.
Mix mortar with water in a bucket to create a paste-like consistency. Wet the grout space with plain water from a spray bottle. Add wet mortar to a grout bag. Place the tip in between sandstone pieces and squeeze the bag, filling the space until it is slightly higher than the sandstone.
Press and push the mortar into the mortar joints with a trowel until the mortar sits even with the sandstones.
Mist the mortar with plain water and wrap the chimney tightly with a plastic tarp for three to four days. Check the mortar joints daily and mist with water if they dry out in order to prevent the mortar from drying out and crumbling.
Place a cold chisel into the middle of the mortar line and strike with a hammer. Move the cold chisel around the damaged piece of sandstone striking, cracking and breaking mortar from the perimeter of the damage. If you are using a grinder with a diamond wheel, move the grinder through the centre of the mortar joint in one direction only.
Place a cold chisel on the centre of the damaged piece of sandstone and strike forcefully with a hammer to break it. Remove the piece of sandstone from the chimney. Use a wire brush to brush out old mortar.
Fit a new piece of sandstone into the void space. Use a wet saw to cut a new piece to size if necessary.
Mix mortar with water in a bucket to form a paste-like consistency. Use a trowel to add a 1/2-inch layer of wet mortar to the void space. Apply a very thin coat, called a skim coat, of mortar to all sides of the sandstone except the front. Apply a 1/4-inch thick layer of mortar on top of the wet skim coat.
Push the new piece of sandstone into place forcefully. Mortar will ooze out from the mortar joints; use a trowel to press the oozing mortar tightly into the joints. Lightly mist with plain water and cover tightly with plastic. Keep the new mortar damp and covered for three to four days while the mortar cures.
Wear protective eyewear, dust mask and gloves. Sandstone will pull the water out of the mortar and ruin it if the mortar joints are not re-wet as needed.