Reinforcing of polylactide (PLA) with fillers can be an interesting solution to reduce its global price and to improve specific properties. Starting from calcium sulfate (gypsum) as by-product of the lactic acid fermentation process, novel high performance composites have been produced by melt-blending PLA and this filler after a previous specific dehydration performed at 500 degrees C for min. 1 h. Due to PLA sensitivity towards hydrolysis, it has first been demonstrated that formation of beta-anhydrite II (AII) by adequate thermal treatment of calcium sulfate hemihydrate is a prerequisite. Then, the modification of filler interfacial properties with different coating agents such as stearic acid (SA) and stearate salts has been considered. The effect of surface treatment on molecular, thermal and mechanical properties has been examined together with the morphology of the resulting composites. To take advantage of the improved lubricity and better wetting characteristics, the filler was coated by up to 2% (by weight) SA. The coating of the filler leads to PLA-AII composites that surprisingly exhibit thermal stability, cold crystallization and enhanced impact properties. Such remarkable performances can be accounted for by the good filler dispersion as evidenced by SEM-BSE imaging of fractured surfaces. As far as tensile proprieties are concerned, notable utilization of uncoated filler or filler coated by stearate salts leads to PLA-AII composites characterized by higher tensile strength and Young's modulus values. The study represents a new approach in formulating new melt-processable grades with improved characteristic features by using PLA as polymer matrix. (C) Koninklijke Brill NV. Leiden, 2009