Selection of dispersants for stabilization of unfunctionalized carbon nanotubes in high pH aqueous suspensions: Application to cementitious matrices

A main challenge in concrete reinforcement with carbon nanotubes is to overcome dispersion issues inherent to the high pH value of cement pastes. A two-step approach was envisaged where dispersion ability of nanotubes is verified in alkaline aqueous suspension, previously to incorporation in cement. Characterization in sequential steps using Raman spectroscopy, UV–Vis and electrophoresis allowed to achieve maximum nanotubes dispersion with minimum damage. Besides disclosing optimum dispersant type and concentration, sonication time and pH, this procedure provided understanding of the dominating dispersion mechanisms acting at basic pH, allowing knowledgeable selection of efficient dispersants. Afterwards, carbon nanotubes stable suspensions were introduced in cement pastes; flexural and compressive strength were evaluated after setting. Ionic dispersants were more efficient than steric, highlighting the importance of charge control to prevent nanotubes reagglomeration at high pH. Reinforcement with pristine nanotubes rendered small strength increase compared to reference pastes; the most efficiently dispersed suspension rendered the highest flexural and compressive strength increase, respectively 33.8% and 25.5%. Microscopy evidenced nanotubes bridging effect as the most relevant cement strengthening mechanism. Overall, results show that if a dispersant is efficiently stabilizes carbon nanotubes in high pH aqueous suspension, it will also be effective regarding dispersion in a cement matrix.