Results obtained with small angle neutron and X-ray scattering will be
presented of systems with various silica concentrations. Silica gels with
low silica concentration exhibit uniform mass fractal behaviour and after
prolonged aging, dual mass fractal behaviour. A zeolite (silicalite)
synthesis precursor gel with a high silica concentration was also
investigated and was found to exhibit surface fractal behaviour. A model is
presented, explaining the occurrence of dual fractality after prolonged
aging. The change-over from mass fractal behaviour to surface fractal
behaviour is explained, supported by scattering spectra calculated from
simulated aggregate structures.