## 3 Results and Discussion
### Effect of MWCNTs
It should be noted that as the surfactant concentration increases, there may be formation of micelles when the critical micelle concentration (CMC) is exceeded, an effect that potentially may happen more easily given a larger size of the surfactant molecules. As seen from Table 4, for the surfactants with largest molecule sizes (Glycerox and Amber 2001), the hydrodynamic diameter of the MWCNTs dispersion increases slightly for a Glycerox concentration above 1% (suggesting the CMC is somewhat between 1% and 3%), while for the surfactant Amber 2001 the hydrodynamic diameter always increases with surfactant concentration (suggesting the CMC can be less than 0.5%). However, this is compatible with good MWCNTs dispersion since the formation of micelles is not necessarily detrimental for particle dispersion [8]. Thus, as stated before, the surfactant choice should be based on the balance between the concentration and the hydrodynamic diameter/molecular weight. The surfactant Amber 2001 exhibits the worst performance since it has the largest hydrodynamic diameter, leading to MWCNTs dispersions of bad quality, as it may be seen from Table 4. The other cationic surfactant (Amber 4001) has an effect on the geomechanical behavior of the stabilized soil that depends on the surfactant concentration. For concentrations up to 1% (probably below the CMC), there is a significant beneficial impact that may be attributed to the cationic charge of the surfactant Amber 4001, which favors adsorption to MWCNTs and binder particles, thus promoting better dispersion and geomechanical behavior. On the contrary, for higher surfactant concentrations (2% and 3%, probably above the CMC) the MWCNT binder particle dispersion is of poor quality, producing a negative effect in terms of geomechanical behavior of the chemically stabilized soil additivated with MWCNTs. Generally, the Glycerox surfactant presented better results regarding the geomechanical behavior of stabilized soils additivated with MWCNTs, especially for concentrations of 2% or higher. This can be associated with better dispersion of MWCNTs obtained with this surfactant (see Table 4). Indeed, it was observed that for higher surfactant concentrations (2% and 3%) a nonionic surfactant type (Glycerox and Viscocrete) assures better geomechanical behavior as long as it has a good MWCNTs dispersion. Figure 8: Stress–strain normalized curves (for samples with 7 curing days) for the stabilized soil with MWCNTs suspensions for different surfactants applied in concentrations of (**a**) 0.1%, (**b**) 0.5%, (**c**) 1.0%, (**d**) 2.0% and (**e**) 3.0%. Generally, better results were obtained for higher MWCNT concentrations, which may be explained by the fact that the presence of a higher number of nanoparticles will nanofill the matrix voids, allowing the development of a denser stabilized matrix. At the same time, due to the extraordinary mechanical properties of the MWCNTs (very high strength and stiffness), their presence in a higher quantity in a stabilized matrix will promote the enhancement of the geomechanical behavior since MWCNTs are bonded to the cementitious products produced by the Portland cement. In summary, when the MWCNTs are dispersed in a soil-binder matrix they may act as a nanofiller and nanoreinforcement, promoting a denser and stronger stabilized matrix. As observed for the effect of surfactant, the impact of the MWCNTs on the geomechanical behavior of the chemically stabilized soil decreases as time evolves for all the surfactants and concentrations studied, independent of the quantity of MWCNTs (Figure 10). As the curing time evolves from 7 to 28 days, the cationic surfactant no longer has a positive effect on the geomechanical behavior, presenting a detrimental effect: for a MWCNTs concentration of 0.001%, the strength improvement factor decreases from 1.15/1.14 to 0.97/0.89 for surfactant concentration of 0.1%/1%, respectively; while for the case of a MWCNTs concentration of 0.01% the IF decreases from 1.06/0.99 to 0.93/0.78 for the same surfactant concentrations.