Silica Thickener
This material is also referred to as colloidal or fumed silica, and it's an excellent thixotropic agent used to control the viscosity of mixed epoxy systems.
Silica can be used alone for a very smooth, non-sagging, high-strength mixture or combined with fillers or fibers to make them non-sagging. A 1-quart quantity of Silica Thickener, when combined with 3/4-quart of mixed epoxy/hardener, will produce about 1 quart of a non-sagging firm paste.