Calcium aluminate cements are used for special applications but are nowadays banned for general
structural purposes due to the calcium aluminate hydrate conversion, that has led, for concretes
fabricated with high water contents, to building collapses. The stoichiometries of these conversion
chemical reactions are relatively well established but the consequences in porosity, key to predict
durability, were unknown. Here, we have used hard X-ray ptychographic nanotomography to study
the hydration of CaAl2O4 at different temperatures and chiefly, at 4ºC and then at 50ºC to provoke
conversion similar to field conditions. The mass densities of the resulting Al(OH)3 gels were 1.94,
1.98 and 2.23 g.cm-3, for samples hydrated at 4, 20 and 50ºC, respectively. These values are lower
than that of gibbsite, 2.42 g.cm-3. Above all, this 3D imaging technique has allowed measuring the
secondary water porosity developed in the conversion, which has an average pore dimension close
to 140 nm.
