Plasma heating is a highly efficient industrial treatment for municipal solid waste incinerator fly ashes, converted into leach-resistant glass. The viscous flow sintering of glass powders appears a very promising way for the subsequent conversion of the vitrified waste, not refined and available as glass flakes, into valuable glass-ceramics. Because of the particular glass composition, the densification of glass-ceramics was much hindered by the strong tendency of glass toward surface crystallization. The crystallization of glass was successfully coupled to a low porosity, thus yielding strong gehlenite-based glass-ceramics (with a bending strength exceeding 100 MPa), by adopting a fast sintering treatment (direct insertion of glass compacts at 1050 degrees C, with an holding time of 1 h).