Abstract: The present invention describes the preparation of nanotubes made from portlandite, the naturally occurring form of calcium hydroxide, Ca(OH)2. Portlandite nanotubes are obtained by a process comprising the following steps: a) reacting calcium chloride with calcium oxide in aqueous solution, thus obtaining an aqueous dispersion; b) feeding as such the aqueous dispersion obtained in step a) to a hydrothermal reaction, thus obtaining portlandite nanotubes. The invention also concerns the use of the portlandite nanotubes as a component for cementitious compositions to provide reinforced mortar or concrete.