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Carbon nanotubes are used to improve both the mechanical properties and electrical conductivity of cement, allowing the preparation of a strengthened and toughened cement that can be used for self-monitoring applications. Functionalization by reaction with acid is necessary to guarantee both a good dispersion in water and a strong interaction with cement. Sulfonitric acid (a solution of sulfuric acid and nitric acid) is the best oxidation treatment to decorate the surface of carbon nanotubes with polar groups. The time of treatment influences the mechanical and electrical properties of the composites, and in this work, the effect of the length of the treatment on both CNTs and composites was thoroughly analyzed. It was shown that a long-time oxidation treatment (90 min) was very effective to obtain well-dispersed carbon nanotubes that allow us to obtain cement-based composites with a large improvement of mechanical properties with respect to plain cement: 213% for flexural strength, 90% for fracture energy, and 20% for compressive strength. Furthermore, the electrical resistivity of cement-based composites was reduced to only 3% of the value of plain cement.

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