  The empirical data in conjunction with the quantum mechanical calculations
show that the strength enhancement in the cement-carbon nanotubes (CNTs)
composites is the courtesy of electrostatic potential energy. This is contrary
to the general belief that the CNTs form bridges between the adjacent grains to
slow down the breaking process. The yield point for the cement paste is
improved up to 25% when prepared with 0.2 % by weight of various types of CNTs.
A significant strength enhancement is observed with carboxyl functionalized
(COOH) CNTs compared to other types. Further, an increase in the concentration
of CNTs up to 0.4 wt% has a negative effect on the strength of the matrix. The
electrostatic potential energy is mapped by using density functional theory
(DFT) with {\omega}B97X-D functional. At lower concentration of CNTs,
ion-dipole interaction in the cement paste and the CNTs creates a very strong
long range intermolecular force. Due to the increased entropy resulting from
the exothermic hydration process, these forces augment the strength of the
cement paste.
