a b s t r a c tThe discovery of the unique properties of carbon nanotubes (CNT) did grow interest to its application in nanocomposites, for a wide variety of purposes. However, the greatest challenge for its application is associated with the natural tendency to aggregate, resulting in the loss of its beneficial properties. To overcome this problem it is common the use of surfactants and/or ultrasonic energy to promote their dispersion in suspension. This work is focused on the influence of surfactants' properties on the dispersion of carbon nanotubes and on the influence of the quality of the dispersions on the mechanical properties of stabilized soil. Two surfactants (Glycerox and Amber 4001 differing in molecular weight and charge) were fully characterized, followed by the study of which surfactant concentrations were more efficient on the dispersion of the CNTs. The characterization methods were based on light scattering techniques: Dynamic Light Scattering (DLS) for the hydrodynamic diameter and Static Light Scattering (SLS) for the molecular weight. Suspensions of CNTs were prepared in solutions of the aforementioned surfactants, with different concentrations, and further dispersion was promoted using ultra-sounds (20 kHz during 5 min). The dispersions of CNTs in these two surfactants were then fully characterized using again DLS. Finally, the dispersions of carbon nanotubes were added to the main agent responsible for soil stabilization, the binder (Portland cement type I 42.5R), and the mechanical behavior of the new composite material was studied by unconfined compression strength (UCS) tests. The results of the UCS tests led to conclude that the introduction of CNTs in the binder can have huge impact on the mechanical properties of the stabilized soil. Furthermore, the quality of the dispersion of CNTs has got a very high impact on the performance achieved. It was verified an improvement up to 77% on the compressive strength of the material and 155% on Young's modulus, referred to the reference test where no carbon nanotubes nor surfactant were added, fundamentally dependent on surfactant type and concentration used.

Chemical stabilization is one of the most successful techniques that has been applied to improve the geomechanical behavior of soil. Several additives have been studied to be a sustainable alternative to traditional additives (Portland cement and lime) normally associated with high cost and carbon footprint. Nanomaterials are one of the most recent additives proposed. This work is focused on one type of nanomaterial, multiwall carbon nanotubes (MWCNTs) with unique characteristics, applied to chemical stabilization of soils and aiming to identify the key-parameters affecting the stabilization improvement. It was found that a surfactant should be added in order to oppose the natural tendency of MWCNTs to aggregate with the consequent loss of benefits. The surfactant choice is not so dependent on the charge of the surfactant but rather on the balance between the concentration and the hydrodynamic diameter/molecular weight due to their impact on the geomechanical compression behavior. As time evolves from 7 to 28 days, there is a decrease in the geomechanical benefits associated with the presence of MWCNTs explained by the development of the cementitious matrix. MWCNTs applied in a proper concentration and enriched with a specific surfactant type may be a short-time valid alternative to the partial replacement of traditional additives.

The incorporation of carbon nanotubes (CNTs) in the process of chemical stabilization of soft soil is only possible when they are dispersed adequately in the medium. The maximum compressive strength (qu max) and the secant undrained Young’s modulus (Eu 50) are usually used to characterize the behavior of soil stabilized with Portland cement. In the present study, soft soil was additivated with a CNT dispersion prepared in a surfactant solution. This information was then used to produce a model based on an experimental design strategy, which allowed us to relate qu max and Eu 50 with the CNT concentration and the surfactant hydrodynamic diameter and concentration. The Partial Least Squares (PLS) regression method was selected to perform the regression, given the significant collinearity among the input variables. The results obtained lead us to conclude that the CNT concentration is the most important factor and has a positive impact on the responses (qu max and Eu 50). The surfactant concentration and hydrodynamic diameter have a negative impact on the responses, but, curiously, when combined, the impact becomes positive. It means that these variables depend on each other. The results obtained show that it is possible to produce a statistical model for these parameters with good correlation coefficient (R2).