Understanding the role of carbon nanotubes in low carbon sulfoaluminate cement-based composite

Carbon nanotubes (CNTs) have excellent mechanical properties and outstanding aspect ratios, which could remarkably impact the concrete's properties. This research investigates the influence of four different aspect ratios of CNTs with two dosages (i.e., 0.09% and 0.15%, by weight of cement) on hydration, macro performance, and microstructure of low-carbon sulfoaluminate cement-based composite. The experimental results show that adding different aspect ratios of CNTs can enhance the macro performance of the composite. The SM2 (short-CNTs with 0.15% content) group with a small aspect ratio (50–100) has better flexural strength and compressive strength than M0 (plain group), which is increased by 32.7% and 37.0%, respectively, the hydration degree of C₄A₃Š (ye'elimite) and C₂S (belit) are increased by 19.9% and 57.3% than that of the M0 group. At low dosages (0.09%), LM1 (long-CNTs with an aspect ration of 250–1500) and LM2 (long-CNTs with an aspect ration of 125–750) improved the flexural strength of UHPC more than SM1 (short-CNTs with an aspect ration of 100–200). However, LM1 (long-CNTs with a large aspect ratio of 250–1500) has a better enhancement at a dosage of 0.09% than 0.15%. CNTs mainly accelerated the hydration process of the matrix and promoted the generation of more AFt, AH₃. AFt is a needle-like fiber, and the increase in content is beneficial to improve the toughness and flexural strength of the matrix. The increase in AH₃ content can improve the compressive strength of the matrix. The AH₃ phase can be filled into the pores of the matrix, thereby increasing the compatibility of the matrix and improving the pore structure. The increase in the content of the two hydration products can synergistically enhance the mechanical properties of the matrix. AH₃ is closely connected to CNTs, which can improve the bond strength between the matrix and CNTs. It is observed that AFt is closely associated with AH₃, filling the pores of the matrix, and the nanoscale AFt, AH₃, and CNTs work together to improve the strength of the matrix, which differs from the effect of CNTs in OPC. This research prepares UHPC from low-carbon cement, which effectively enhances the life cycle of concrete materials and provides a new idea for reducing carbon emissions. © 2023 Elsevier Ltd.