## 3 Results and Discussion
### Effect of MWCNTs  
Figure 6 presents the results of the chemically stabilized soil samples additivated with MWCNTs dispersed in aqueous solution (without surfactant) for curing times of 7 and 28 days. It is clear that independent of the curing time the addition of the MWCNTs has a negligible effect on the geomechanical behavior of the chemically stabilized soil. This result contradicts previous studies with carbon nanotubes where it was found that the addition of a small concentration of CNTs has a significant impact on the geomechanical properties of a soil [8, 32, 33, 34]. These results make clear that more important than the introduction of MWCNTs in a soil matrix is the need to ensure that they are properly dispersed in order to avoid the loss of their beneficial properties. Thus, it is crucial to add to the MWCNTs aqueous suspension a surfactant that may help in the dispersion process. Figure 7 shows several aqueous suspensions containing MWCNTs with and without surfactants, all dispersed with ultrasonic energy. The MWCNTs suspension without surfactant (Figure 7a) has aggregates clearly visible to the naked eye, whereas in the case where surfactant was added at the lowest concentration (Figure 7b,c) homogeneous suspensions are present. Figure 4: Strength improvement factor (for samples with 7 curing days) for the stabilized soil with only surfactant solutions. Figure 5: Stress-strain normalized curves (for samples with 28 curing days) for the stabilized soil with only a surfactant solution of Amber 2001 (conc. of 1%) and Visocortex (conc. of 3%). Figure 6: Stress-strain normalized curves for the stabilized soil additivated with MWCNTs and no surfactant for samples with (**a**) 7 curing days and (**b**) 28 curing days. Figure 7: Photographs of MWCNT suspensions immediately after ultrasonication for a MWCNT concentration of 0.01%: (**a**) sample without surfactant, (**b**) sample with Glycerox at concentration of 0.5% and (**c**) sample with Amber 4001 at concentration of 0.5%. Figures 8 and 9 summarize the results of the impact of adding MWCNTs (in concentrations of 0.001% and 0.01%) dispersed in a solution of surfactants to chemically stabilized soil samples for a curing time of 7 days. With the exception of the higher concentrations of the surfactant Amber 4001, the addition of MWCNTs for both concentrations (0.001% and 0.01%) dispersed in a solution of surfactants have a positive impact on the geomechanical behavior of the stabilized soil, proving that the addition of MWCNTs to a chemically stabilized soil can only be effective if the MWCNTs are \"properly\" dispersed. The best results were obtained with the surfactants Amber 4001, Viscocrete and Glycerox, applied in concentrations of 1%, 3% and 2%, respectively, independent of the MWCNTs concentration (\(IF=1.76\), 1.62 and 1.63 for a MWCNTs conc. of 0.01%; \(IF=1.66\), 1.63 and 1.48 for a MWCNTs conc. of 0.001%). The results prove that the size of the surfactant molecules (Table 3) have a major role on the MWCNTs dispersion process (Table 4) and, ultimately, on the geomechanical behavior of the composite materials. Indeed, the smaller the size of the surfactant molecules, the better the quality of the dispersion (Table 4), i.e., smaller surfactant molecules adsorb more easily on the surface of the solid particles (MWCNTs, binder and soil), ensuring better dispersion and geomechanical behavior. These results agree with previous findings regarding the surfactant effect when no MWCNTs were added, but now the presence of MWCNTs enhances even more the strength improvement factor. However, it should be emphasized that better geomechanical behavior is not always associated with better MWCNTs dispersion because the medium where dispersion occurs is different; the characterization of MWCNT dispersion occurs in an aqueous medium enriched with surfactant while for the UCS tests in the medium there are chemically reacting cement particles and soil particles.