Unlike other solidified materials, concrete has an internal structure that contains small pores that vary in size from several nanometers to the submicrometer range. These pores govern the strength of the material and hence the lifetime of concrete buildings. The size of the pores is governed by the presence of calcium silicate hydrate (CaO center dot 2SiO(2)center dot 4H(2)O, C-S-H). To identify how the pore size is determined during the concretion process, we used transmission electron microscopy to examine the early solidification of a simplified reaction system consisting of silica, portlandite [Ca(OH)(2), CH], and water. The silica particles, used to suppress the degradation of concrete, expanded and, consequently, embedded the pores through hydration before C-S-H was formed, contrary to the predicted decrease in pore size by C-S-H formation after simple dissolution of silica. Visualization of this type of solidification process should permit improvement in the mechanical strength of concrete.