Here, we report on the first ever study of an innovative kind of cement composite incorporated with n-octadecane (OC)/diatomite shape-stabilized composite phase change material (PCM). Diatomite decorated with carbon nanoparticles and calcined at 800 degrees C for 3 h (DC) was considered as the optimum supporting matrix due to its hierarchical porous microstructure and attractive crystallization character. These suborbicular thermal storage media are well dispersed in the cement matrix with excellent compatibility, and do not obviously influence the apparent density and porosity of the cement mortar. Besides, the flexural and compressive strength of the cement mortar with 30% OC/DC could still reach as high as 3.5 MPa and 18.3 MPa, respectively. It is noteworthy that the inclusion of greater amounts of OC/DC resulted in lower thermal conductivity and higher thermal energy storage capacity, while the chemical, mechanical and thermal reliability of cement remained practically stable even when subjected to a 400 melt-freeze cycle. It is found that the prepared heat storage cement mortar is capable of reducing indoor temperature fluctuation and exhibits excellent potential for energy savings and thermal comfort in building applications. (C) 2017 Elsevier Ltd. All rights reserved.