The use of construction waste for concrete is an effective way to reduce the environmental burden while improving the sustainability of construction materials. Nano-SiO₂ (NS) has excellent volcanic ash activity, which can effectively improve the strength of concrete. In this study, the synergistic effects of NS and 30% recycled concrete aggregate (RCA) on the mass loss, compressive strength, ultrasonic sound velocity values and microstructure of RAC after 25, 50, 75 and 100 repetitions of a dry–wet cycle (DWC) of sulfuric acid were investigated. The results show that NS has a significant role in improving RAC performance. The RAC specimens showed the lowest mass loss rate, the highest compressive strength, the lowest ultrasonic velocity value and the best resistance to sulfate erosion when the NS doping was 4%. Meanwhile, under CT and SEM, RAC with NS has lower porosity and higher hydration, which can effectively inhibit the crack generation and has stable volume growth within 0–50 repetitions of DWC. In addition, the addition of NS improves not only the microstructure of the substrate but also the interfacial transition zone (ITZ). When 4% NS was added, the porosity was the lowest and the durability improvement was the best performance. This study not only improves the production performance of RAC but also provides a strong reference for the integrated application of nanomaterials in concrete.

The progressive collapse of buildings induces a variety of catastrophic consequences, such as casualties and property loss over the past few decades. The corner column is more prone to abnormal load events compared to the inner column and outer column; thus, it is easier to trigger progressive collapse. By considering the effects of floor slabs and adjacent bays on progressive collapse behavior, the pseudo-static loading method was used to study the progressive collapse test of a 1/3 scaled, one story, 2 × 2-bay cast-in-place reinforced concrete frame substructure under the removal condition of a corner column. The test results show that the flexural deformation principally concentrates upon the components of a directly affected part (DAP), and compressive arch actions are observed in members of the indirectly affected part (IAP). Moreover, the slab adjacent to the removed column and periphery elements contributes great resistance to a progressive collapse.

Cement-based materials used in China’s coastal and salt lake areas in the northwest are exposed to long-term chloride corrosion, which deteriorates the materials and substantially reduces the durability of the structures. This study investigates the chlorine ion erosion resistance in salt spray environments of cement-based materials made with iron ore tailings (IOTs) as an aggregate (namely, IOTCs). The compressive strength, mass loss, and relative dynamic elastic modulus (RDEM) macroscopic performance of IOTC undergoing different chloride diffusion times (0–180 d) were explored in detail. Chloride ion profiles at 0–180 d were analyzed via chemical titration, while X-ray computed tomography (CT) and scanning electron microscopy (SEM) were employed to characterize microstructural evolution. The results demonstrate that IOTC exhibited superior chloride resistance compared to conventional concrete (GC). While both materials showed early strength gain (<60 d) due to hydration and pore-filling effects, IOTC experienced only a 23.9% strength loss after long-term exposure (180 d) significantly less than the 37.2% reduction in GC. Chloride profiling revealed that IOTC had 43.5% lower free chloride ions (C_f) and 32% lower total chloride ions (C_t) at 1 mm depth after 180 d, alongside reduced chloride diffusion coefficients (D_a). The CT analysis revealed that IOTC exhibited a significantly denser and more uniformly distributed pore structure than GC, with a porosity of only 0.67% under chloride-free conditions. SEM confirmed IOTC’s more intact matrix and fewer microcracks.

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.