The present invention is a fly ash filler or filler blend having a particle size distribution with at least three modes that can be combined with a polymer at higher filler loadings to produce a filled polymer for polymer composites that, in many cases, can produce improved mechanical properties for the polymer composites over polymer composites using conventional fillers. As a result, superior polymer composites (e.g. those used in carpet backing) can be produced at a lower cost than conventional polymer composites. The present invention also includes a method for producing a polymer composite, comprising the steps of combining a polymer with a fly ash filler or a filler blend having a particle size distribution with at least three modes to produce a filled polymer and producing a polymer composite with the resulting filled polymer. The present invention further includes a method of determining what fly ashes can be used as fillers for polymer composites.