## 4 Conclusions  
The present work is a contribution to identify the most important or fundamental parameters that control the geomechanical behavior of a chemically stabilized soil additivated with MWCNTs dispersed or not in a surfactant solution. Based on the laboratory tests performed, it was found that a simple addition of MWCNTs to the stabilized soil does not produce any improvement of its geomechanical behavior since the nanoparticles are in an aggregate condition, which inhibits its ability to take advantage of the extraordinary properties of MWCNTs. The introduction of a surfactant to the chemically stabilized soil promotes the dispersion of the cement and soil particles, allowing the development of a denser and stronger solid skeleton matrix, which is reflected in an improvement of the geomechanical behavior up to 155% compared with the reference test. When the MWCNTs are combined with a surfactant solution and good MWCNTs dispersion is achieved, there is an enhancement of the geomechanical behavior of the stabilized soil up to 176% or 185% (for a MWCNTs concentration of 0.001% or 0.01%, respectively) compared with the reference test. Thus, for the concentrations examined, the MWCNTs concentration seems not to be a fundamental parameter since similar improvements can be achieved for a concentration that is 10 times lower. On the contrary, the characteristics of the surfactant seem to be a fundamental parameter affecting the geomechanical behavior of the stabilized soil enriched with MWCNTs. Indeed, the surfactant choice should depend on the balance between the concentration and the hydrodynamic diameter/molecular weight. A smaller size of the surfactant molecules allows better adsorption on the surface of the solid particles (MWCNTs, binder and soil), ensuring better dispersion without interfering with the cementitious reactions. Figure 11: Optical and SEM images of the leachate at 7 days from a chemically stabilized soil samples additivated with MWCNTs (conc. = 0.01%) dispersed in a solution of Amber 2001 (conc. = 0.1%) with 7 curing days: (**a**,**b**) optical images and (**c**,**d**) SEM images. As time evolves from 7 to 28 curing days, a relative decrease of the effect of surfactants and MWCNTs on the geomechanical behavior of stabilized soil samples was observed, explained by the fact that the physicochemical reactions of the Portland cement develop with time, producing a greater quantity of cementitious products responsible for making the stabilized matrix denser and stronger and, as a consequence, the additives importance diminishes. Thus, time seems to be a fundamental parameter since better results are achieved for shorter times. In summary, the addition of MWCNTs, \"properly\" dispersed, with the objective to contribute to the chemical stabilization of a soil, has potential to improve its geomechanical behavior. Thus, for the same level of unconfined compressive strength, MWCNTs applied in a proper concentration and enriched with a specific surfactant may provide a quick and valid alternative to the partial replacement of Portland cement. Moreover, it was demonstrated from the leaching tests that no traces of MWCNTs were observed in the leachate, proving that the MWCNTs are entrapped in the stabilized soil matrices, thereby not presenting a risk for people, animals or plants. [PERSON], [PERSON] and [PERSON] conducted the experimental investigation; [PERSON] prepared the first draft of the manuscript and was responsible for the conceptualization, validation and supervision of the research, and review and editing of the manuscript; [PERSON] was co-responsible for the conceptualization, validation and supervision of the research and participated in the review and editing of the manuscript. All authors have read and agreed to the published version of the manuscript. This research received financial support from the Strategic Research Centre (CIEPQPF) Project UIDB00102/2020, funded by FCT (Fundacao para a Ciencia e Tecnologia), Portugal. Not applicable.