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 µm. The wet foams are 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 (by up to 2000 times), approaching that of the sintered materials. A new theoretical expression for predicting the mechanical strength of such materials is derived and is used to explain the measured strengths of the produced materials through the van der Waals attraction between the particles in the final dry materials.