Strengthened thermoplastic materials use fly ash or cinders as extenders. The extended materials use fly ash or cinders that preferably contain no more than about 2% by weight of moisture. The thermoplastic materials are preferably foamed and extruded to form extruded closed cell foam articles, for example a window blind slat, shutter louver, shutter stile, or shutter frame. Thermoplastic materials formed with the fly ash or cinders can also be formed into non-foamed products or into pellets for use in later extruding to form products.