Notice that the clay does not entirely fill the spaces between the grains. This is important both for stability and for good buffering. If many of the sand grains are touching, the mud cannot shrink. Instead, the clay cracks into microscopic fissures, which give it a large accessible surface. The correct mixing of sand and clay to give a slightly porous structure, with sand grains touching, will give the best moisture buffering and also the best building quality.
Cellular concrete has a rather different structure.
The gas bubbles are mostly far apart, with only occasional interconnections. The bulk of the structure is a mass of fibrous calcium aluminosilicate fibres, together with some etched quartz grains. The pictures to the right zoom in on the structure. The large, isolated voids in cellular concrete give it good thermal insulation but harm its buffer properties. The volume in the voids contributes no moisture capacity and the voids are not so extensively interlinked that they provide an easy diffusion path for moisture from the interior to reach the surface.