Here we demonstrate thermal energy storage cement mortar (TESCM) fabricated by integrating ordinary cement mortar with a composite phase change material (PCM) based on n-octadecane and expanded graphite (EG). The mass percentage of n-octadecane in the composite PCM can reach as high as 90% due to the excellent adsorption ability of EG, which endows the composite PCM with large latent heat. SEM images of the composite PCM show that n-octadecane is adsorbed into the pores of EG and uniformly covers on the nanosheets of EG, which microstructure contributes to preventing leakage of melted n-octadecane after it changes phase from solid state to liquid state. The n-octadecane/EG composite PCM has a good compatibility with ordinary cement mortar, and does not obviously deteriorate the apparent densities of the TESCM samples. Based on the thermal energy storage performance evaluation, it is found that the TESCM containing the n-octadecane/EG composite PCM plays a role in reducing the variation of indoor temperature, which helps to decrease the energy consumption for buildings. (C) 2012 Elsevier Ltd. All rights reserved.