There are several forms. Some of the most popular ones: - Blocks. From a technical perspective, these are perfectly usable as structural wall material in most use cases. Excellent insulation, breathable, moisture resistant. It is more expensive compared to traditional brick though. That's a big reason its use is usually confined to lower layers in touch with moisture. - Aggregate. Used as a structural, insulative anti-capillary layer below concrete slabs, wooden floors or wooden (!) foundations. Moisture barrier and insulation around some moisture sensitive houses also. Very popular in some areas as excellent breathable insulation for renovation of historical buildings. Used below roads in Scandinavia too to prevent frost heave. - Granulate. Mixed into concrete or plasters to improve their insulation qualities. - Custom shaped inert insulation for pipes, ...