## 1 Introduction  
Cement, the major component for concrete, consists of calcium silicates (C\({}_{3}\)S, C\({}_{2}\)S), calcium aluminate (C\({}_{3}\)A), and calcium alumino-ferrite (C\({}_{4}\)AF). The cement compounds usually react with water, in a process called cement hydration, to produce cement hydrates such as calcium silicate hydrate (C-S-H), and calcium hydroxide (Ca(OH)\({}_{2}\)). The mechanical properties of microstructures with strong bonding between hydration products can be enhanced as time progresses. In the last several decades, various methods to improve strength and durability of concrete have been examined by many researchers, such as the use of fly ash (FA) or ground granulated blast-furnace slag (GGBFS)--by-products from the fossil-fuel power and iron industry--and the use of mineral additives such as silica fumes [1; 2; 3].  
Recently, the additions of carbon nanotubes (CNTs) or graphene to cement composites or concrete have been shown to improve the mechanical properties and durability of concrete as well as its electrical and thermal conductivity. [PERSON] et al. [4] and [PERSON] et al. [5] reported that the addition of CNTs improved the early age strength of concrete, and CNTs with a large surface area caused the quick formation of cement hydrates and an increase in the ratio of the C-S-H gel. A total of 0.5% of CNT addition enhanced the compressive strength of concrete by 26%. [PERSON] et al. [6] improved the strength by adding 0.1% CNTs to mixtures of cement and silica fume. [PERSON] et al. [7] focused on the thermal conductivity of CNT-embedded cementitious composites. [PERSON] et al. [8] reported that 1% of CNTaddition enhanced the compressive strength by more than 50% compared to ordinary Portland cement (OPC).  
In order to develop surface treatment of CNTs for better dispersion [9; 10; 11], many researchers conducted researches on CNT surface treatment. [PERSON] et al. [12] worked for the evaluation on wettability before and after oxidation treatment in cement composites. Most research on CNT addition has been conducted to reveal the mechanical properties as well as the microstructures of CNT-cement composites [13; 14; 15; 16]. In addition, several studies have been conducted on the durability of CNT-cement mortars [17; 18].  
In this study, the mechanical properties such as the compressive strength and flexural strength were tested with respect to the proportion of multi-walled CNTs (MWCNTs) in cement composites. Micro-hydration heat analysis was conducted to evaluate the effects of CNT content on the released cement hydration heat by isothermal calorimetry. Scanning electron microscopy was used to observe CNT distribution and types of hydration products inside the composite pastes. Thermal gravimetric analysis was used to observe the effects of CNT addition on the change in hydration products such as calcium hydroxide and calcium carbonates. Finally, thermal conductivities of CNT-cement composites were measured to investigate the heat transfer capacity. The amount of heat generated between two stainless-steel meshes in the CNT-cement composites was measured to determine the optimal amount of CNT content in the composites.