AbstractA novel calcium silicate filler can be made from fly ash. This new filler, known as fly ash based calcium silicate (FACS), has a highly porous surface structure, high brightness (91% ISO), low bulk density (0.31 g/cm3), and high specific surface area (121 m2/g). In this paper, its potential application as a paper filler was studied and its effect on drainage, retention, and paper properties was investigated. The results from dynamic drainage tests showed that FACS had similar drainage and retention behaviors to the conventional precipitated calcium carbonate (PCC). Physical tests indicated that FACS-filled paper had higher tensile, burst, and tear indices, but lower brightness and opacity than those loaded with PCC. A more important finding was that the bulk of paper can be increased by 56.4% with 20% FACS content in paper relative to the control (no filler addition).