A building product incorporating synthetic microspheres having a low alkali metal oxide content is provided. The synthetic microspheres are substantially chemically inert and thus a suitable replacement for cenospheres derived from coal combustion, particularly in caustic environments such as cementitious mixtures. The building product can have a cementitious matrix such as a fiber cement product. The synthetic microspheres can be incorporated as a low density additive and/or a filler for the building product and/or the like.