Zirconia aerogels are made of connected fractal clusters composed of small crystalline particles. When heated at a low temperature (i.e. 0.13 times the melting temperature), the mass transport process which predominantly occurs is surface diffusion. Due to the local character of surface diffusion, fragmentation appears in the sample during thermal annealing. This particular evolution of the structure is numerically analysed and experimentally studied by small-angle x-ray scattering.