## 5 Conclusions  
Cementitious composites reinforced with MWCNTs were developed and applied to soft soil to improve its mechanical properties. Two surfactants with different hydrodynamic diameters were used to disperse the MWCNTs in water. A preliminary analysis of the results obtained showed that the mechanical properties of the soft soil chemically improved with the addition of MWCNTs is mainly related to three variables: the concentration and the hydrodynamic diameter of the surfactant and the MWCNT concentration. The improvement in the mechanical properties in relation with these three variables was assessed by determining \(\mathrm{q_{u\;max}}\) and \(\mathrm{E_{u\;50}}\).  
A PLS methodology was used to identify the influence of each variable on \(\mathrm{q_{u\;max}}\) and \(\mathrm{E_{u\;50}}\), and two models were constructed, namely a model for \(\mathrm{q_{u\;max}}\) and a model for \(\mathrm{E_{u\;50}}\).  
In both cases, the optimum model was composed of five components, and the model of \(\mathrm{q_{u\;max}}\) can explain 70% of the model response while the model for \(\mathrm{E_{u\;50}}\) is able to explain 89% of the model response. However, the conclusions regarding the influence of the variables on the response parameters are very similar for the two parameters. The MWCNT concentration is the most important factor and has a positive impact on the response (\(\mathrm{q_{u\;max}}\) and \(\mathrm{E_{u\;50}}\)). However, it was observed that the probability of agglomeration increases with the MWCNT concentration, meaning that there is an optimum value of MWCNT concentration that maximizes the response. The surfactant concentration and surfactant hydrodynamic diameter have a negative impact on the response, but, curiously, when combined, the impact becomes positive. It means that these variables depend on each other. Indeed, the MWCNTs are directly responsible for improving the mechanical performance of the stabilized soil, while the surfactant properties ensure a better performance of the MWCNTs.  
Considering the results obtained, it is possible to propose two models for both \(\mathrm{q_{u\;max}}\) and \(\mathrm{E_{u\;50}}\) when MWCNTs are added to the binder for soil stabilization purposes. These models can be helpful for further developments toward the application of carbon nanotubes on soil stabilization in real situations.  
[PERSON] participated in the conceptualization, validation, supervision, writing and editing of the manuscript; [PERSON] conducted the experimental investigation and prepared the first draft of the manuscript; [PERSON] was responsible for the conceptualization and participated in the validation and supervision of the research and in the review and editing of the manuscript. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Strategic Research Centre (CIEPQPF) Project UIDB00102/2020 and by the research project PTDC/ECI-CON/28382/2017, both funded by the FCT (Fundacao para a Ciencia e Tecnologia), Portugal.  
Figure 12: Surface plot of \(\mathrm{E_{u\;50}}\) vs. x2 and x3.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Some or all data, models, or code generated or used during the study are available from the corresponding author upon reasonable request.  
**Acknowledgments:** The authors acknowledge AquaTech, Switzerland, and Cimpor, Portugal, for supplying, respectively, the surfactants and the binders used in the work.  
**Conflicts of Interest:** The authors declare no conflict of interest.