Most chimney masons when constructing a chimney simply use the same materials for the chimney crown as for laying the bricks or stone. This material is called a "mortar mix" and is only 20% as strong as "Portland Concrete" which should be used for Chimney crowns.
Masonry chimneys shall have a concrete, metal, or stone cap, a drip edge, and a caulked bond break around any flue liner in accordance with ASTM C1283. ( American Society for Testing and Material). The concrete metal or stone cap shall be sloped to shed water.