###### Abstract  
Many studies have been conducted using carbon-based nanomaterials (CBNs) for improving the electrical conductivity and mechanical properties of cementitious composites, but their practical use is yet to be achieved. Several methods have been attempted to secure the dispersibility in the cementitious composite matrix of CBNs, such as multiwalled carbon nanotubes (MWCNTs) and graphene nanoplatelets (GNPs). In this study, MWCNTs and GNPs were noncovalently functionalized using melamine, a low-cost chemical, and ball milling, a simple process commonly used in industrial fields. Additionally, MWCNTs and GNPs having one- and two-dimensional shapes were mixed with the cement paste to examine their effect on electrical conductivity and compressive strength. Following the experiment, it was shown that the electrical conductivity was improved via the noncovalent functionalization of MWCNT and GNP. The compressive strength increased up to approximately 0.30-0.60% of the CBNs content; however, for CBN content higher than 0.60%, the compressive strength decreased. The hybrid MWCNT and GNP mixture had a negligible effect on the electrical conductivity and compressive strength.  
MWCNTs; GNPs; functionalization; conductivity; compressive strength; cement paste +
Footnote †: journal: Applied Physics Letters  
1
Footnote 1: email: kwang.mo.l.m.  
paste [8; 9; 10; 11; 12; 13; 14; 15]. In most cases, the strength increases as the content of MWCNT increases but decreases above a certain content ratio. However, the optimal incorporation and strength-increasing rates were reported with different results by various researchers [8; 9; 10; 11; 12; 13; 14; 15]. Conversely, several studies indicated its negative effects on mechanical performance [16; 17; 18; 19; 20]. However, finding a clear correlation between the incorporation rate of MWCNTs and decreasing strength was difficult. Many studies used GNPs to improve the mechanical properties of cementitious composites [21; 22; 23; 24; 25; 26], whereas other studies found that the use of GNPs had little to no effect on strength [27; 28; 29].  
It is a well-known fact that through the incorporation of CBNs, e.g., MWCNTs and GNPs, electrical conductivity can be imparted to cementitious composites. However, while the optimal mixture ratio and electrical conductivity were reported differently, there is uniform agreement that dispersing mixed CBNs evenly in the cement matrix is an important factor for improving electrical conductivity. CBNs tend to agglomerate owing to van der Waals forces; therefore, researchers have experienced difficulty dispersing them uniformly in a cement matrix. To date, no established standard exists for the dispersing method of CBNs; these include noncovalent approaches, such as surfactant and ultrasonication, and covalent approaches, such as acid etching. Many researchers concluded that CBNs, such as MWCNTs or GNPs, should be functionalized for the best performance when mixed into cementitious composites [5].  
[PERSON] et al. [30] proposed a method for the noncovalent functionalization of GNPs using a ball-milling process with melamine to improve their interfacial adhesion and dispersion in an epoxy matrix. In this study, the method suggested by [PERSON] et al. was used to homogeneously disperse the MWCNT or GNP in a cement paste matrix and compared this approach with ultrasonication, which is the most common dispersing method [30]. Most of the existing studies investigated the properties of cementitious composites by individually incorporating MWCNTs or GNPs. As only a few studies exist on hybrid cementitious composites in which two CBNs are simultaneously mixed, the compressive strength and the electrical conductivity of the hybrid cement pastes mixed with MWCNTs and GNPs were investigated in this study [31; 32].