A particulate sealant for forming plugs in selected cells of honeycomb structures and consisting essentially, by weight, of about 70 to 90% ceramic blend, the ceramic blend being raw ceramic materials selected to form a composition consisting essentially of in percent by weight about 12 to 16% MgO, about 33 to 38% Al2O3, and about 49 to 54% SiO2, which will form cordierite (2MgO.2Al2O3.5SiO2) on firing, and about 10 to 30% binder system comprising a thermoplastic polymer capable of forming a gel or a thermosetting resin.