## 2 Experimental Work  
### Materials  
In this work, two surfactants were used, which differed in molecular weight and charge: Glycerox (glycerol cocoate) and Amber 4001 (polyamide), supplied by AquaTech, La Plaine, Switzerland. Their features are described in Table 1. The surfactants were selected based on the charge of both the CNTs and cement particles in order to favor the interaction between the surfactant and particles, aiming to ensure their good dispersion, as described in [19]. Thus, since both CNTs and cement particles possess negative charge, a quasi-neutral surfactant and a cationic one were selected (see Table 1). Glycerox acts essentially through the steric mechanism, while for Amber 4001, the predominant stabilization mechanism is based on electrostatic interactions. Additionally, these surfactants with quite different molecular weight, which affects the hydrodynamic diameter of the surfactant molecules, were selected in order to study the effect of this parameter on the stabilization process since it affects the CNT dispersion, which can have a potential influence on the spacing between particles (namely CNTs and cement), with repercussions on the mechanical behavior of the stabilized soil enriched with CNTs [34]. While the charge was provided by the supplier, molecular weight and hydrodynamic diameter were experimentally determined using light scattering methods. Molecular weight was determined by static light scattering and hydrodynamic diameter was determined by dynamic light scattering (DLS), both in the Nanosizer ZSN from Malvern, UK. Multiwall carbon nanotubes (MWCNTs) supplied by Nanocyl, Sambreville, Belgium, product NC7000, were used. The choice for MWCNTs was mainly due to economic factors; MWCNTs are significantly less expensive than single-wall carbon nanotubes and, so far, only MWCNTs are produced at an industrial level [34; 38]. According to the supplier information, the MWCNTs have an average diameter of 9.5 nm, an average length of 1500 nm, an average specific surface of 275,000 m\({}^{2}\)/kg, and their chemical composition is pure carbon (90%) with some metal oxides (10%), as described in Table 2. MWCNTs have a negative electrical charge (\(-25.2\) mV) as measured by electrophoretic light scattering in the Nanosizer ZSN from Malvern, UK [33]. According to [PERSON] et al. [38], the MWCNTs are selected for this study are classified as CNTs with low crystallinity, i.e., they exhibit a greater potential capacity to interact with the surrounding particles. The grain size composition of the natural soil is made of 66% of silt, 22% of sand, and 12% of clay particles. In general, it is a soft soil with low strength, low permeability, and high compressibility, and it presents a high value of water content (80.87%) and organic matter content (9.3%), which are reflected in its weak geotechnical characteristics. Table 3 summarizes the main geotechnical properties and the mineralogical and chemical compositions of the natural soft soil. More details can be seen in [PERSON] et al. [19] and in [PERSON] [33]. The binder chosen for this study was Portland cement because it is the binder material used in practice in the majority of soil stabilization cases [3]. A Portland cement type I was selected, which is a mechanical strength class of 42.5 R (CEM I 42.5 R), and its mixture with water spontaneously initiates a physico-chemical process known as hydration reaction. The binder is mostly composed of CaO (62.84%) and SiO\({}_{2}\) (19.24%) as stated in the technical data supplied by the producer (Cimpor, Coimbra, Portugal) and shown in Table 4. The cement particles are slightly negatively charged on average (zeta potential measured using electrophoretic light scattering, in the Nanosizer ZSN from Malvern, is \(-2.14\) mV), which is in accordance with [PERSON] et al. [39].