Effects of carbon nanotubes on the toughness and microstructure of steel fiber reinforced ultra-high performance geopolymer concrete

Ultra-high performance geopolymer concrete (UHPGC) prepared with low-carbon and high-early-strength geopolymers has great potential for rapid construction. High toughness is a key performance indicator for UHPGC. The widely used centimeter-scale steel fiber can only bridge and resist macro-cracks, while the micro-scale fibers such as carbon nanotubes (CNTs) act on micro-cracks. The combination of fibers of different scales is expected to further promote the toughness of UHPGC. In this paper, multi-scale fibers including steel fibers and CNTs were used to toughen UHPGC, and the mechanisms of CNTs were explored in hydration reaction, pore structure and fiber-matrix interface. The results showed that CNTs reduced the paste fluidity but improved the orientation and distribution of steel fibers. At 28 d age, the addition of 0.1 vol% CNTs into the UHPGC sample containing 2 vol% steel fibers increased the compressive strength by 23 %, the flexural strength by 66 %, the bending strength by 20 %, the flexural-compression ratio by 34 %, and the equivalent bending toughness by 22 %. Results of XRD, TG and ²⁹Si-NMR indicated that CNTs increased the C-(A)-S-H gel products by promoting the hydration of slag and fly ash, respectively, and affected the composition and structure of gel by increasing the chain length and Al/Si ratio. The CNTs could mitigate the adverse effects of steel fibers on the pore structure through micro-filling effect and increasing products. The CNTs raised the bonding of steel fiber-matrix interface by promoting the generation of hydration products around the interface, especially C-(A)-S-H gels with high elastic modulus. It was observed by SEM that CNTs and steel fibers can form a synergistic effect to jointly inhibit crack propagation at micro and macro scales. This study confirmed that CNTs can further toughen steel fiber reinforced UHPGC, and the mechanism was mainly the mechanical structural modification and chemical hydration reaction enhancement at the micro-scale.