Post-functionalized roofing granules and process for preparing same

A process for preparing functionalized roofing granules, the process comprising:

• (a) providing base roofing granules having a first coating layer disposed on an inert mineral core, the first coating layer including at least one metal oxide colorant disposed in a binder based on an alkali metal silicate;
• then
• (b) coating the base roofing granules with a second coating composition to form a second coating layer over the first coating layer, the second coating layer being a sodium ion barrier formed from silica glass or crystalline silicaceous material;
• (c) curing the second coating layer on the base roofing granules to form intermediate granules;
• (d) coating the intermediate granules with a transparent or translucent outer coating composition, the outer coating composition comprising an outer coating binder and at least one functional material, the outer coating binder being one or more of colloidal silica and organic polymer, the outer coating not including a binder based on an alkali metal silicate, an alkaline earth metal silicate, or a metal phosphate, the at least one functional material being photocatalytic anatase titanium dioxide having an average particle size less than 0.2 microns;
• and
• (e) curing the outer coating composition to form a transparent or translucent outer coating layer on the base roofing granules.