Series 400 may be constructed of 2 plies of FKM fluoroelastomer and one or 2 plies of reinforcing material (corrosion resistant wire mesh, fiberglass cloth, or other). Two plies of fluoroelastomer and one ply of reinforcing material are calendared and vulcanized to form rugged unit that is impervious to flue gases and has excellent resistance to acids. Non-metallic expansion joints are durable enough not to require internal baffles for protection against fly ash abrasion.