## 1 Introduction  
Due to their high tensile strength of up to 60 GPa [1], carbon nanotubes (CNTs) are considered a superior material for nano-reinforcement of cement-based composites [2]. CNTs have been found to increase the mechanical strength of pastes, mortars, and concretes [3, 4, 5, 6] through four different mechanisms: crack bridging, matrix densification, pore filling, and C-S-H nucleation [7]. Additionally, due to their electric and magnetic properties [8], CNTs have the potential for developing novel construction materials with smart properties, such as self-sensing, self-heating, electromagnetic shielding [9], and self-healing [10].  
Many authors have reported a decrease in mechanical properties of cement-based composites due to the presence of poorly dispersed CNTs in the cement matrix [5, 11]. Due to their hydrophobic nature and Van der Waals interactions [12, 13], CNTs tend to agglomerate, forming bundles that induce weakness regions in the composites [2]. It is now known that appropriate reinforcement does not only depend on the intrinsic properties of CNTs, but also on how well they are dispersed throughout cement matrices [14]. Self-sensing applications, based on CNTs as a conductive filler, also rely on proper nanotubedispersion to form a continuous electrically conductive path, i.e., reaching the electrical percolation threshold [9].  
There are three main strategies to disperse CNTs: physical methods, chemical methods, and a combination of both. Some of the physical methods usually employed are sonication, mechanical stirring, and ball milling. It has been found that ball milling and mechanical stirring are not efficient enough [7], while controlled ultrasonic pulses have been found to be an efficient method for dispersing CNTs through induced cavitation [15; 16; 17]. Chemical methods employ dispersing agents that improve CNT wettability and induce electrostatic repulsion between individual tubes. Some of the most commonly used chemical dispersing agents for CNTs are surfactants [2; 18; 19; 20] and superplasticizers (SP) [21; 22]. Surfactants have been found to directly affect the cement matrix [23], delaying the hydration reaction and increasing the amount of entrapped air. Superplasticizers are considered a more promising dispersing agent due to their compatibility with cement; nevertheless, effective dispersion of CNTs without affecting the properties of the cement matrix still is a challenge.  
Superplasticizers can be seen as effective in performing dual function, dispersing both cement grains and CNTs. By means of electrostatic repulsion, SPs act at the cement grain surface, inducing repulsion between grains. At the same time, due to their structure, which combines hydrophobic and hydrophilic groups, SPs may adsorb on the surface of CNTs, increasing their wettability and maintaining individual nanotubes separated [24]. Due to this dual function, SPs have a potential to be used as a CNT dispersant agent in cement composites.  
UV-Vis spectroscopy and \(\zeta\)-potential have been found to be good techniques in characterizing CNT dispersions in water [25]. Individual CNTs can be identified through their characteristic absorption band around 253 nm [26], while \(\zeta\)-potential identifies the magnitude of colloidal particles electrostatic interaction and characterizes the stability of CNT-water colloidal systems [27]. This work uses results obtained from both techniques as a basis for optimizing a CNT ultrasonic dispersion process with four different commercially available SP, of two different chemical bases, as dispersing agents. UV-Vis absorbance and \(\zeta\)-potential values were used to characterize the efficiency of each SP in water, while the structural integrity of CNTs after sonication was verified using Raman Spectroscopy. Electrical resistivity was used to characterize the efficiency of each SP after CNTs were blended into the cement-based composites.