The water resistance of silicate foams that were produced by the alkaline activation of an amorphous silica source was improved by the incorporation of aluminum into the silicate network. The aluminum was introduced by metakaolin, which showed a similar reactivity in alkaline solution as the silica source used. Depending on the metakaolin content, a transition from a silicate into an alumosilicate network occurred. The structure of the (alumo-)silicate gel, its foamability, and the strength and water resistance of the foamed granulates were determined. The incorporation of aluminum lead to a higher connectivity of the (alumo-)silicate gel. This (alumo-)silicate gel was still foamable at about 500 degrees C down to an Si/Al ratio of 6. The strength and the water resistance of the granulates increased with the aluminum content in the gel. A lower Si/Al ratio resulted in a stronger alumosilicate network with dominantly Q(4) building units of the silicate tetrahedrons. This stronger network inhibited a viscous flow at a temperature of 550 degrees C.