Concrete continues to cure and harden for up to three months depending on the blend. Stone can be harder, but generally is set in mortar which is softer than concrete. Brick is generally softer than concrete as well. Many fireplaces have been done with concrete instead of stone or brick going back to Bay Area architect Bernard Maybeck. Check out hese concrete fireplaces from the Houzz archives.[houzz=][houzz=][houzz=][houzz=]