Mechanical properties and fractal analysis of cement mortar incorporating styrene-butadiene rubber latex and carboxylated MWCNTs

•Positive effect of hybridizing carboxylated MWCNTs and SBR in mortar was demonstrated.•Carboxylated MWCNT-engineered SBR latex significantly improved the mechanical properties of mortars.•The formation of networks and the strong interfacial interaction were responsible for this enhancement.•The pore surface fractal dimension (Ds) is positively correlated with the compressive strength.•The incorporation of MWCNTs and MWCNTs/SBR increased the Ds value of mortar greatly. Styrene-butadiene rubber (SBR) modified cement-based material owns many advantages, however, reduction in compressive strength within cement composite has limited its application. This paper evaluates the effect of carboxylated multiwall carbon nanotubes (MWCNT-COOH) on the mechanical properties and microstructure of SBR modified cement mortar. MWCNT-COOH was firstly dispersed within SBR by using ultrasonic vibration, and then the MWCNT-COOH/SBR suspension was incorporated into cement mortar. Four different kinds of mortars were compared in this study, i.e., plain cement mortar, pure SBR modified mortar, pure MWCNT-COOH modified mortar, and MWCNT-COOH/SBR hybrid modified mortar. The strength and fracture toughness of these composites were firstly analyzed, in which the experiment results showed that a significant improvement was achieved by incorporating MWCNT-COOH and SBR hybrid nano-composites. The pore structures and surface fractal dimensions (Ds) of cement mortars were determined by using mercury intrusion porosimetry (MIP) and a fractal model, the relationships between the mechanical properties and the porosity as well as the Ds values were then revealed and discussed in this study. It was found that the adding MWCNT-COOH/SBR hybrid and mono MWCNT-COOH greatly refined the pore size and reduced the porosity of cement mortar, resulting in higher mechanical properties. Fractal analysis indicated that there existed a positive relationship between Ds and compressive strength, and the addition of MWCNT-COOH and MWCNT-COOH/SBR hybrid greatly increased the Ds values of cement-matrix composite. Fourier transform infrared spectroscopy (FTIR) and field emission scanning electron microscopy (SEM) were used to characterize the phase composition, mineralogy and microstructure. Results indicated that the homogeneous dispersion of MWCNT-COOH/SBR hybrid nano-composites, a strong interfacial adhesion, and the formation of load-transfer networks are responsible for the considerable enhancement.