Both concentrated and diluted sodium silicate solutions have been investigated by combining Si-29 NMR spectroscopy and SAXS experiments The chemical nature of the entities responsible for the high siliceous species solubility observed in such alkaline concentrated sodium silicate solutions and their evolution according to dilution have been identified For the most concentrated solution ([Si] = 7 mol/l pH = 11 56 Si/Na atomic ratio = 1 71) the results evidence the preponderant presence of neutral Si7O18H4Na4 complexes which behave like colloids of about 06-0 8 nm able to form very small aggregates with an average size lower than 3 nm Addition of distilled water to this initial concentrated solution leads on one hand to a doubling of the colloid size i e 12-15 nm and on the other hand to a progressive decrease of the aggregate size until their total disappearance Such a behavior could be explained by considering first the dissociation of the neutral Si7O18H4Na4 complexes present in the concentrated solution into Na+ ions and charged (Si7O18H4Na4-n)(n-) complexes (with 1 <= n <= 4) and second the condensation of these siliceous charged species in order to form larger (Si7O18H4y-2zNa(4-n)y)(ny-) colloids The mean size of these colloids suggests that the condensation occurs between 2 and 8 (Si7O18H4Na4-n)(n-) groups (C) 2010 Elsevier Inc All rights reserved