  Preparation of particle-loaded foams, followed by drying, sintering and/or
cross-linking are widely explored routes for developing lightweight ceramics
with high mechanical strength. The non-sintered dry ceramic foams are less
studied due to their intricate production and the assumed poor mechanical
strength of the obtained "green" materials. Here we produce lightweight
ceramics from foamed particle suspensions containing spherical silica particles
with radii varied between 4.5 nm and 7 {\mu}m. The wet foams were prepared in
the presence of cationic surfactant and were dried at ambient conditions to
obtain porous materials with mass densities between 100 and 700 kg/m3. The
materials containing smaller particles exhibited much higher strength (up to
2000 times), approaching that of the sintered materials. A new theoretical
expression for predicting the mechanical strength of such materials was derived
and was used to explain the measured strengths of the produced materials
through the van der Waals attractions between the particles in the final dry
materials.
