Summary Cement‐based composites is a promising type of structural material, which has prospective applications in relieving the urban heat island effect in summer and melted snow with low energy consumption. However, the major drawbacks of cement‐based composites are heterogeneity, porosity, and brittleness. Porosity and microcrack have considerable influence on the thermoelectric of cement‐based composites applied in large‐scale concrete structures in future. This paper studied in detail the effect of porosity and crack on thermoelectric properties of the cement‐based composite. The proper pores and cracks in the cement matrix are advantageous to enhance the Seebeck effect, but meanwhile it also reduces the electrical conductivity. So combined with Seebeck effect, electrical conductivity and other factors, it can obtain a comparatively low electrical conductivity (0.063S cm−1) of expanded graphite/carbon fiber reinforced cement‐based composites (EG‐CFRC), but EG‐CFRC manifests the maximum thermoelectric figure of merit (ZT) has reached 2.22 × 10−7 when the porosity is 3.90%. With different porosity, the Seebeck effect of prepared EG‐CFRC was strengthened when the crack existed. The effect is most pronounced by a factor of 2 when the porosity is 28.90%. Therefore, based on stabilizing the conductivity, the crack is fittingly made to have a good effect on the Seebeck coefficient.

Summary Thermoelectric cement‐based composite materials can convert the temperature difference between indoor and outdoor into electrical energy in summer. And the energy can be reversed to remove ice and snow on the pavement in winter. However, how to improve its thermoelectric conversion efficiency is the primary technical problem currently facing. Carbon nanotubes are often used as a container for their unique tubular structure. In this study, lithium acetate was filled in carbon nanotubes to obtain lithium carbonate modified carbon nanotubes (after heat treatment) by utilizing its capillary force. XRD, SEM, and TEM were used to characterize the morphology and crystal structure of lithium carbonate‐modified carbon nanotubes. ST2722‐SZ semiconductor powder resistivity tester was used to characterize the change of electrical conductivity before and after modification of carbon nanotubes (48.22 S/cm reduced to 3.43 S/cm, ∅11.28 mm, 30 MPa). The thermoelectric power factor of lithium carbonate modified carbon nanotubes reinforced cement‐based composites reaches 0.039 μW m−1 k−2. The Fermi level calculated by the Mott formula fluctuates between 0.116 and 0.256 eV. And the order of magnitude of the carrier concentration is stable at 1020 cm−3.The composite material not only exhibits superior thermoelectric properties but also reduces the carbon nanotube content to achieve a more cost‐effective purpose.