Preparation of low density water glass based silica gels by conventional drying

To reduce shrinkage and the possibility of fracture during ambient pressure drying it is of great importance to increase the strength and stiffness of the wet gels. In this paper is presented the strengthening and stiffening of wet silica gels prepared from sodium silicate (water glass) as well as properties of the corresponding xerogels. By washing gels containing different initial silica contents in water solutions at elevated pH, a maximum in shear modulus of ~4 MPa was obtained. The maximum stiffness enabled xerogels with bulk density of 0.28 g/cm3 to be made regardless of silica content and washing conditions. However, by aging the wet gels in a solution providing fresh monomers to the gel network, a shear modulus of 20 MPa was obtained after 27 h. By this method monolithic xerogels with a density down to ~0.2 g/cm3 was prepared. The results are compared to alkoxide based gels.