Mechanical Response of EPDM Composites Partially Replacing N550 Carbon Black With FGD Gypsum‐Derived Calcium Sulfate Whiskers: Effects of Replacement Ratio and CSW Characteristics

To investigate the potential of FGD gypsum‐derived calcium sulfate whiskers (CSW) as a partial substitute for N550 carbon black in EPDM rubber, CSW were incorporated by mass replacement of N550. The replacement ratio was first evaluated by tensile strength, elongation at break, and Shore A hardness. The highest measured tensile strength was 12.96 MPa at 7% replacement, but it was not statistically different from the 0%, 5%, 9%, 11%, and 13% formulations. At 15% replacement, high‐aspect‐ratio CSW (HRCSW), anhydrous CSW (ACSW), and surface‐modified CSW were compared. HRCSW showed the highest tensile strength (12.68 MPa), while CSW‐Ti1 exhibited the highest elongation at break (489.39%). CSW‐Si and ACSW showed relatively high hardness. Surface modification altered the apparent water wettability and surface chemical environment of CSW without obvious bulk‐phase transformation. SEM observations suggested contributions from whisker bridging, pull‐out, crack deflection, and stress concentration. Overall, the mechanical response depends on the combined effects of replacement ratio, whisker geometry, crystalline state, and surface characteristics.