Nanotechnology is one of the promising directions in improving the structure and properties of building materials. The paper presents the results of an experimental study on the use of carbon nanomaterials as a modifying agent for building composites with the aim of assessing the effect of carbon nanostructures on the physicomechanical characteristics of lightweight concrete. We propose a solution for the uniform distribution of carbon nanotubes in the composite matrix by introducing a dispersion of carbon nanotubes into the mixing water after ultrasonic treatment. The test results of the physicomechanical properties of the composite showed an increase in compressive strength of lightweight concrete by 68%, as well as a decrease in water absorption and porosity. The dispersed CNTs were found to improve the porous structure of the cement matrix, providing additional nucleation centers for the formation of calcium silicate crystalline hydrates, which leads to a denser microstructure. There is a difference in pore size and pore distribution in unmodified and modified lightweight concrete, in which the structure is more organized. It was revealed that even an insignificant concentration of CNTs (Taunit) (0.0004‒0.0012%) in proportion to the total weight of cement can affect the pore microstructure and the properties of the composite and, accordingly, significantly improve the physicomechanical characteristics of lightweight concrete.