## 1 Introduction  
The growing socio-economic development in modern societies has led to an increase in soil requirements in terms of stability and safety. Thus, new technical solutions have been proposed to improve soils characterized by poor mechanical properties (unsuitable strength and deformability). One of the techniques to make construction possible on such soils is chemical stabilization, which has been used with success during the last decades [1; 2; 3]. However, in order to maximize feasibility, it is crucial for the development of techniques to optimize the performance of cement-stabilized soil, namely to improve its mechanical behavior. The performance of chemically stabilized soil can be considerably improved by the incorporation of nanometric particles in the modified soil-binder matrix since they have the ability to fill the pores of the matrix at the nanometric scale, which can be reflected in a more solid and strong matrix [3; 4; 5]. However, direct application of such materials is not possible due to the natural tendency for aggregation of these particles, which causes a loss in the beneficial effects associated with their incorporation [6; 7; 8; 9]. Carbon nanotubes (CNTs) have unique properties, making them very attractive for application in composite materials [3; 9; 10; 11; 12]. Currently, the most widespread use of CNT nanocomposites is in electronics. These nanocomposites could be used to shield electromagnetic interference and as electrostatic-discharge components [13]. The removal of heavy metals from wastewaters [14], catalyst supporters [15; 16], and chemical sensors [17; 18] are other areas where CNTs are widely applied. However, as mentioned above, carbon nanotubes have a high tendency to aggregate when used in suspension, which results in a loss of its properties [19]. This problem can be solved by the addition of surfactants that promote the dispersion of the particles in the suspension or through the functionalization of CNT surface [9; 12; 20; 21; 22; 23; 24]. The mechanical behavior of soil can be improved by the mixture of chemical materials (binders with cementitious properties) with the natural soil [25; 26; 27; 28]. The cementitious products resulting from the physico-chemical interactions originate a new composite material with a better mechanical behavior than the original soil [19; 29; 30; 31; 32]. There is the possibility to improve this behavior even more by the addition of carbon nanotubes due to their unique properties, namely a high specific surface (high capacity to interact with other particles in the surrounding environment), and extremely high yield strength and moduli of elasticity (making them an excellent reinforcing material) [3; 33; 19; 34]. If carbon nanotubes are properly dispersed in a soil-cement matrix, they may act as a nanofiller and a nano-reinforcement agent, thereby promoting a denser and stronger stabilized matrix [3]. In order to disperse CNTs (applied in different concentrations), ultra-sounds were applied to aqueous solutions enriched with surfactants (varying in type and concentration). The quality of the CNTs' dispersion was evaluated through dynamic light scattering (DLS). Afterward, the suspensions were mixed with the binder and soil, and laboratory samples were prepared. At the end, the samples were submitted to unconfined compression strength (UCS) tests [26; 35; 36] in order to characterize the mechanical behavior of the new soil-binder-CNT material. The samples were compressed until failure while automatically recording the load and the vertical displacement. The unconfined compression strength (\(\mathrm{q_{u\max}}\)) was characterized by the maximum load recorded during the UCS test, while the stiffness (\(\mathrm{E_{u\,50}}\)) was described by the secant undrained Young's modulus evaluated at 50% of the compression strength [26; 37]. The parameters studied in this work are the type and concentration of surfactant, and the concentration of CNTs.