Comminuted (less than or equal to1.6 mm) spent-tire rubber scrap (Cd, Hg, As less than or equal to0.00025%, Sb, Cr less than or equal to 0.002, Ba < 0.001, Pb < 0.0125, S 1.7%) was co-extruded with LDPE and PP in a (i) single or (ii) twin-screw unit. Extrudates (i) had higher notched impact strength and, up to 20% scrap, higher bending strength; extrudates (ii) were mechanically inferior to the unmodified polyolefin. The extrudates were heterophasic. Free -OH found by IR on the scrap surface advised to use (a) 2-mercaptoethyltrimethoxy- and (b) N-2-(aminoethyl)-3-aminopropyltrimethoxy-silanes, 3 phr, as rubber-polymer compatibilizers. Correct formulation, processing mode and parameters gave satisfactory solid, or micro or macro open-pore (60-200 mum) materials. Heat-pretreated (20-90 min, 150degreesC) phosphogypsum (60 mum; Cd, Hg, As less than or equal to 0.0001%, Pb, Cr less than or equal to 0.001%, Ba < 0.01%), 10-40%, co-extruded with PP precoated with 0.5 wt.% (composite basis) of ethanolic 10% silane a or b, gave products with the bending and lzod impact strengths slightly lower, but with the higher rigidity. With no silane, the products were heterophasic. 44 refs.