## 3 Results and Discussion
### Effect of Surfactant Type  
Figures 3 and 4 summarize the results of the effect of the surfactant type added to the chemically stabilized soil samples with a curing time of 7 days. With the exception of the higher concentrations of the surfactant Amber 4001, the addition of the surfactants has a positive impact on the geomechanical behavior of the stabilized soil, proving that the surfactant has the potential to better disperse the binder and/or the soil particles, thus, allowing the construction of a denser and stronger solid skeleton matrix. The best result was obtained with the surfactant Viscocrete (_IF_ = 1.55), a nonionic surfactant type applied in a concentration of 3%. The results obtained for the other nonionic surfactant (Glycerox) are also positive but with lower strength improvement factors (_IF_ ranging from 1.07 to 1.21). However, the IF increases with the Glycerox concentration, suggesting that for a higher Glycerox concentration better results could be obtained. This result may be explained by  
Figure 2: Stress–strain curves for the reference test conditions (soil chemically stabilized with Portland cement). the fact that Visocrete presents a smaller hydrodynamic diameter and molecular weight than Glycerox; thus, a smaller size of the surfactant molecules allows better adsorption on the surface of the solid particles (soil and binder), ensuring better dispersion. This is also valid when comparing the two cationic surfactants, justifying the better results with the Amber 4001, which has a smaller hydrodynamic diameter and molecular weight than Amber 2001. Moreover, the fact that the particle surface is not so much covered by surfactant when lower molecular weight and hydrodynamic diameter surfactants are used, may favor the cementitious reactions. When comparing the two cationic surfactants (Amber 2001 and Amber 4001) with the nonionic surfactants for equal concentrations, it may be seen that for lower concentrations (0.5% and 1%) the best results are obtained with the cationic surfactants. However, for higher concentrations, the results of the Amber 2001 (2% conc.) are of the same order as those for Glycerox while for Amber 4001 (2% and 3% conc.) the results are negative (\(I\!F<\!1.0\)) and lower than those for Glycerox and Visocrete, respectively, for a concentration of 2% and 3%. Thus, the surfactant choice is not so dependent on the surfactant charge but rather on the balance between the concentration and the hydrodynamic diameter/molecular weight. Figure 3: Stress–strain normalized curves (for samples with 7 curing days) for the stabilized soil with only a surfactant solution of **(a)** Amber 2001, **(b)** Amber 4001, **(c)** Glycerox and **(d)** Visocrete. As time evolves, the impact of the surfactant on the geomechanical behavior of the chemically stabilized soil may change since the surfactant presence may have effect on the time development of the physicochemical reactions of the Portland cement. To study this effect, samples were prepared with nonionic and cationic surfactants for the best concentrations, i.e., Viscocrete and Amber 2001 applied in a concentration of 3% and 1%, respectively. From the results (Figure 5), it may be seen that the cationic surfactant now has a negative effect while the nonionic surfactant still presents a positive but smaller effect on the geomechanical behavior of the chemically stabilized soil. Indeed, as the cementitious products are produced over time the solid matrix becomes denser, justifying the decrease in the importance of the surfactant.