This colloidal silica powder is a strong, dense thickener mixed into the epoxy when bonding tight wood-to-wood joints.  Typically used for gluing scarf joints, puzzle joints, sheerclamps, rubrails, knees, etc.  Both fillers can me mixed with other fillers such as woodflour.  White in color.  

An alternative choice would be Cell-o-Fil (included in all CLC/MAS epoxy kits).