The best type of concrete sealer to use on brick chimneys is a Silane-Siloxane water repellent sealer. Silane-Siloxane water repellent sealers will help to reduce staining and deterioration caused by water absorption. They will also help to reduce the formation of mold, mildew, and efflorescence. Silane-Siloxane water repellents can be used to seal the entire brick chimney, grout included.
Siliconate water repellent sealers and silicate concrete sealers are two other types of penetrating sealers, but they aren't the best type of sealer to use on a brick chimney. Siliconate water repellents can leave behind a white residue when applied to brick, and silicate sealers will densify, but they won't repel water. If over applied, silicate sealers can also leave behind white surface crystals. Silane-Siloxane sealers are the best type of sealer to use on brick chimneys.
In most cases, a brick chimney sealer is all that your chimney needs, but if your chimney has issues with deterioration, cracking, moving, or leaking, you may need more than just a chimney sealer. A chimney is made up of a few different parts, and each part should be analyzed: