Hollow glass microspheres are commonly used as fillers where lightweight high-quality materials are targeted. They are used in polymer-based, cementitious, and even for ceramic syntactic foams where they are utilized as a means to introduce air voids into the materials. The thin-shelled hollow glass microspheres allow the material scientists to better control the volume fraction and the size of the air voids in composite materials. With their average diameter of about 20 μm, which is about one-fifth of the thickness of a hair strand, hollow glass microspheres are truly microscale. Their wall thickness is on an even smaller order of magnitude, which is on average even less than a micrometer, similar to the wavelength of the visible light. Contrary to their small size they are useful in achieving great tasks.
A study on the mechanical properties of cementitious syntactic foams with hollow glass microspheres can be found in the following link. In this study, scientists investigated methods of manufacturing lightweight but strong cementitious materials with the help of hollow glass microspheres.