To improve the high brittleness of recycled aggregate concrete containing iron ore tailings (TRAC), the feasibility of adding polypropylene fiber (PPF) to TRAC was studied by performing a compression stress–strain curve test, scanning electron microscope characterization, and a freeze–thaw cycle test. The results indicated that PPF had a beneficial impact on reducing the brittleness of TRAC. With the increase in PPF content, the peak strain increased, the elastic modulus decreased, and the peak stress and energy absorption capacity increased at first and then decreased. Furthermore, the microstructure investigation revealed that the interface friction between the PPF, aggregate, and cement matrix was the main source of energy dissipation. When the load acted on the concrete, the stress was dispersed to the fiber monofilaments, thus effectively enhancing the peak stress and peak strain of concrete and suppressing the generation and development of cracks in the concrete. In terms of freeze–thaw resistance, adding a small amount of PPF could reduce the negative effects of the freeze–thaw process on the cement matrix. On the premise of ensuring strength, the waste utilization should be as high as possible. Therefore, it was suggested that the content of PPF in fiber-reinforced tailings recycled aggregate concrete (TRAC-PP) should be 0.6%.

With the advantage of superior crack width control ability, stress transmission capacity and energy dissipation capacity, engineered cementitious composites (ECC) show ultra-high strain hardening characteristic, and its ultimate tensile strain can reach 3% to 7%, which is hundreds of times that of the normal concrete when cracking occurs. As a new kind of building material, ECC need to get more sustainable through adding vicinal materials. In the present research, we investigate the influence of ternary binder system of bentonite, fly ash (FA) and cement (C) on the mechanical behaviours and microstructures of ECC. The ratio of FA/ C were 1.2, 1.8, 2.4 and the amount of bentonite by mass of ternary binder system were 0%, 5%, 10%, 15%, and 20% in each FA/C ratio, respectively. The flexural strength, flexural deflection capacity and compressive strength were tested at 14, 28 and 90 curing time. The results show that ECC with combined supplementary cementitious materials have the behavior of strain hardening. Moreover, it was found that higher FA/C ratio and higher bentonite content can improve the flexural deflection capacity. In addition, there is a decrease in flexural strength and compressive strength with the increase in FA/C ratio and bentonite content. Furthermore, it could be seen that fly ash and bentonite are beneficial to the long-term strength development of ECC.

To investigate the feasibility of using molybdenum tailings (MTs) as fine aggregate in concrete production and the impact of carbonation on the performance of molybdenum tailings concrete (MTC), this study employed mechanical property testing, strain field analysis based on digital image correlation (DIC), industrial CT, and scanning electron microscopy (SEM) to systematically analyze the evolution of carbonation depth, compressive strength (f_cu), splitting tensile strength (f_sts), strain field distribution, pore structure, and microstructure of MTC at different carbonation ages. The relationship between porosity and f_cu and the two-parameter strength model of porosity and average pore volume were established. The results show that the carbonation depth gradually increases with the increase in carbonation age and also increases with an increase in the substitution rate of MTs. The carbonation depth of MTs completely replaced is up to 13.72 mm after 56 days of carbonation. Carbonation significantly improves the f_cu of concrete, but as the carbonation age increases, the rate of strength growth generally slows down. The 2D slice visualization of the pore structure shows that the pores of the concrete are gradually filled by carbonation products from outside to inside, the edge macropores gradually disappear, and the proportion of mesopores gradually increases. The total porosity is reduced (up to 37.3%). Microscopic observations confirmed that the carbonation-generated CaCO₃ filled the concrete pores and optimized the structure of the interfacial transition zone (ITZ), but the DIC strain field distribution showed that the filled part increased the brittleness of the concrete. Therefore, it is inferred that excessive carbonization may cause microcrack propagation (which will be discussed in subsequent carbonization time extension studies). The study revealed the dual effects of carbonation on the mechanical properties and pore structure of MTC, providing an experimental basis for the application of tailings in the laboratory.