Thinking bit more generically, you might be really thinking about concrete with air pockets (though ensuring they're spherical might make for excellent load distribution through the side walls). There are already concrete variants called Aerated Concrete and Foam Concrete where bubbles are formed inside the concrete, resulting in a lighter product, though with different structural properties.
https://en.wikipedia.org/wiki/Autoclaved_aerated_concrete https://en.wikipedia.org/wiki/Foam_concrete