Strengthening of water glass and colloidal sol based silica gels by aging in TEOS

Previous work demonstrated that gels can be strengthened and stiffened by providing new monomers to the alcogel thus giving xerogels ambient pressure dried with properties similar to aerogels with porosity up to 90%. That work is extended to increase the strength and stiffness of wet gels obtained from cheaper water soluble sodium silicate (water glass) and colloidal silica sols (3 and 6 nm). During aging the water glass based gels in a TEOS solution, a more than 10-fold increase in shear G modulus was obtained within 27 h, showing a very high reactivity of this system compared to alkoxide based gels. Water glass based gels generally show a lower xerogel density compared to the gels prepared from colloidal sols and a xerogel density of 0.22 g/cm(3) was obtained. Starting from colloidal sols, a smaller increase in G was observed for the same increase in initial density, however, the larger particle size and hence pore size of these gels reduces the capillary pressure during drying, giving low density xerogels. A lower density was obtained for the xerogels prepared from 6 nm sol compared to 3 nm sol for the gels aged in TEOS. A large decrease in xerogel density was observed when the colloidal sol based gels were aged in TEOS. (C) 1998 Elsevier Science B.V. All rights reserved.