This paper aims to study the effectiveness of adding waste polypropylene fibers into recycled aggregate concrete (RAC) on shrinkage cracking. The influences of fiber properties (length and content) on the shrinkage performance of RAC are investigated. Firstly, through the plat-ring-type shrinkage test and free shrinkage test, both of the early age and long-term shrinkage performance of waste fiber recycled concrete (WFRC) were measured. Then, X-ray industrial computed tomography (ICT) was carried out to reflect the internal porosity changes of RAC with different lengths and contents of fibers. Furthermore, the compressive strength and flexural strength tests are conducted to evaluate the mechanical performance. The test results indicated that the addition of waste fibers played an important role in improving the crack resistance performance of the investigated RAC specimens as well as controlling their shrinkage behaviour. The initial cracking time, amount and width of cracks and shrinkage rate of fiber-reinforced specimens were better than those of the non-fiber-reinforced specimen. The addition of waste fibers at a small volume fraction in recycled concrete had not obviously changed the porosity, but it changed the law of pore size distribution. Meanwhile, the addition of waste fibers had no significant effect on the compressive strength of RAC, but it enhanced the flexural strength by 43%. The specimens reinforced by 19-mm and 0.12% (volume fraction) waste fibers had the optimal performance of cracking resistance.

This study investigates the effects of slag and silica fume on the mechanical properties, transport behavior, and pore structure of cement-based mortars. Mortars incorporating different proportions of supplementary materials were evaluated by compressive and flexural strength, saturated water absorption, chloride permeability, and mercury intrusion porosimetry (MIP). Fractal analysis was further applied to assess pore structure complexity. At 28 days, the slag–silica fume blend SG20SF10 reached 46.5 MPa in compressive strength and 5.8 MPa in flexural strength, exceeding OPC. MIP showed a decrease in total porosity from ~14.5% to ~11.3% (about 22% lower) with a marked reduction in pores larger than 100 nm. Consistently, SG20SF10 exhibited the lowest water absorption and chloride permeability at both ages. These results indicate that the slag–silica fume synergy refines capillary porosity and increases pore-network complexity, thereby reducing directional connectivity and transport.