Effect of CNFs on the mechanical properties and microstructure of early strength seawater sea-sand engineered cementitious composites

Carbon nanofibres (CNFs) are a promising material for improving the mechanical properties of cementitious composites. Thus, it is necessary to disperse CNFs for use in materials. In this study, three types of surfactants used in distilled water and seawater were investigated. To obtain the optimal dispersion conditions, the effects of different types and concentrations of surfactants and ultrasonic treatment times on the dispersion of CNFs were investigated quantitatively. The CNF suspensions were studied via visual examination, Transmission electron microscopy (TEM), and Ultraviolet–Visible (UV–Vis) spectroscopy. The compressive strength, tensile strength, and porosity were examined to evaluate the mechanical properties of CNF-modified early strength seawater sea-sand engineered cementitious composites (SS-ECCs). Moreover, field-emission electron Scanning microscopy (SEM) was used to study their microstructures. After dispersion with surfactants and ultrasonication, individual fibres were found in the suspension. Seawater adversely affected the stability of the suspensions. Well-dispersed CNFs improved the mechanical properties of the cementitious composites, with an increase in compressive and tensile strengths of 21–24% and 16–20%, respectively. The tensile strain capacity increased by approximately twofold. The porosity was reduced and the pores structure was refined. Microscopic examination proved that CNFs were also well dispersed in cement matrix. The results showed that the addition of CNFs could significantly improve the mechanical properties of cement-based composites.