A modified composition is disclosed for compounding with a polyolefin to produce a resin from which cement adherent polyolefin fibers may be produced by conventional fiber forming methods. The modifier composition comprising a polyolefin resin modified to contain a reactive Si(IV) moiety. A preferred embodiment comprises the reaction product obtained from the high shear dry blending of an alkylamino alkoxysilane with an acid modified polyolefin, preferably gamma-aminopropyltriethoxysilane dry blended with meleated polypropylene. Such modifier composition when compounded in from about 1 to about 10 weight percent with a polyolefin, preferably polypropylene, provides a resin from which cement adherent fibers may be produced.