Zero field cooling curves (ZFC) under a relatively big magnetic field have been simulated in order to study the influence of the nanoparticle concentration on the rate of increase of the blocking temperature and its dependence with the nanoparticle size. Results show that for all nanoparticle concentrations the blocking temperature increases linearly with the nanoparticle size. The rate of increase of the blocking temperature is larger for larger nanoparticle concentrations, although it tends to a constant value for very large interactions between particles.