The addition of graphene oxide (GO) and fly ash (FA) to SFRC (steel fiber reinforced concrete) increases frost resistance. Based on the analysis of the amelioration of GO on the frost resistance of SFRC, the improvement mechanism and the effects of GO and FA on the durability of SFRC were studied in depth. The test blocks’ compressive strength, relative dynamic modulus of elasticity, and mass-loss rate were tested through experiments. The pore distribution and morphological characteristics of concrete were captured by industrial CT scanning technology. The effects of GO and FA on the pore distribution and morphology of the SFRC are discussed. The results show that the compressive strength of GO-SFRC(graphene oxide and steel fiber reinforced concrete)with GO proportion of 0.03% is 28.20% higher than that of ordinary SFRC without freezing and thawing; after 100 freeze-thaw cycles, the compressive strength of the G_0.03S₂₅ increased by 31.70% compared with the compressive strength of the G_0.00S₂₅, and pore shape of G_0.03S₂₅ presents a spherical shape and an elliptical spherical shape; based on the data analysis of the strength loss, relative dynamic elastic modulus loss and mass-loss rate, it is considered that the properties of ordinary SFRC are the worst under freezing and thawing; as FA admixture increases, the porosity decreases; with FA of 30% and GO of 0.03%, the GO-FA-SFRC has the best frost resistance, as well as most of the pores are closed which resemble spheres and ellipsoids.

The poor early shrinkage and cracking performances of manufactured sand concrete, waste powder concrete, and recycled aggregate concrete are the main difficulties in engineering applications. To solve these problems, early shrinkage and cracking, strength, and impermeability tests were performed on high-volume stone powder manufactured sand concrete mixed with fly ash and slag powder (FS), a shrinkage-reducing agent (SRA), polyvinyl alcohol (PVA) fibers, and a superabsorbent polymer (SAP). Furthermore, the microstructures and pore structures of these concretes were revealed using nuclear magnetic resonance (NMR) and scanning electron microscopy (SEM). The results showed that the mixture of FS, SRA, PVA fibers, and SAP could effectively inhibit the shrinkage strain and cracking area of the concrete. The effect of the SAP on reducing the early shrinkage of the concrete is the greatest, and the shrinkage strain can be reduced by 76.49%. The PVA fibers had the most obvious effect on inhibiting the early cracking of the concrete, and the total cracking area was reduced by 66.91%. Significantly, the incorporation of the FS can improve the particle gradation and the pore structure and improve its compactness. The PVA fibers not only provide good carriers for cement-based materials but also enhance the bonding force between the particles inside the concrete, filling the pores inside the concrete, inhibiting the loss of water, and reducing the generation of internal microcracks. The FS and PVA can reduce the shrinkage and cracking risk and improve the strength and impermeability of the concrete. Although the SRA and SAP can reduce the shrinkage and cracking risks, it will lead to a significant decrease in the later strength and impermeability. The main reason is that the SRA leads to an increase in micropores in the matrix and microcracks near the aggregate, which are not conducive to the development of the strength and penetration resistance of the MS. Similarly, the SAP can promote the rapid formation of ettringite (Aft) at an early age and improve the early shrinkage, early cracking, and early strength of the concrete. However, with an increase in age, the residual pores, after SAP dehydration, will cause the deterioration of the concrete pore structure, resulting in the deterioration of the strength and impermeability.

This study aims to investigate the effects and underlying mechanisms of recycled sand replacement rate, additional water compensation factor, and recycled powder content on the strength, volumetric water absorption, and impermeability of recycled sand concrete. A total of 12 groups of concrete specimens with different mixtures were tested for their mechanical properties, volumetric water absorption, and chloride ion penetration. Furthermore, NMR and SEM analyses were conducted to reveal the microstructural mechanisms by which the additional water level and recycled sand content influence the mechanical performance and durability of the concrete. The results indicate that although recycled sand particles inherently contain numerous micro cracks, adhered porous cement paste, and pre-existing interfaces that enhance capillary water absorption and lead to reductions in strength and durability, these shortcomings can be mitigated by compensating for the additional water and controlling the recycled powder content. Increasing the additional water slightly reduces the strength of the recycled sand concrete. More importantly, appropriate amounts of additional water can reduce water absorption and improve the penetration resistance of recycled sand concrete. Furthermore, with an increase in recycled sand content, the strength, and impermeability of the concrete first increase and then decrease, reaching their maximum values at a recycled powder content of 4%. The water absorption of recycled sand concrete gradually increases with higher recycled powder content. Overall, recycled sand concrete can achieve satisfactory performance by optimizing the additional water amount and recycled powder content. It is recommended that the pre-saturation water compensation factor of recycled sand be maintained at 70%–80% of its 24 h saturated water absorption, and that the recycled powder content be controlled within 4%–8%.

To improve the performance of recycled sand concrete (RSC), this study investigated the synergistic effects of the slurry-coating mixing method and Nano-SiO₂ (NS). The strength, durability, and microstructure of concrete with different recycled sand replacement ratios (RSR), mixing methods, and NS dosages were systematically examined. The results indicated that the strength and durability of concrete generally decreased with increasing RSR. Compared with the conventional mixing method, the slurry-coating mixing method increased the 7-day compressive strength by 8.9–13.9% and the 28-day compressive strength by 4.1–9.3%. Similarly, the addition of NS substantially improved both the mechanical strength and durability of concrete. The maximum increases reached 24.4% for 7-day compressive strength, 9.7% for 28-day compressive strength, and 18.2% for 28-day splitting tensile strength. Meanwhile, the electric flux, the mass loss after freeze–thaw cycles, and the relative dynamic modulus loss were noticeably reduced when either the slurry-coating mixing method or NS was applied. When both modification methods were combined, a pronounced synergistic enhancement effect was observed. The 7-day compressive strength increased by up to 43.2%, while the 28-day compressive strength increased by approximately 21%. In addition, the electric flux value decreased by up to 29.2%, and the freeze–thaw resistance was significantly improved. Microstructural observations revealed that the slurry-coating mixing method promoted the formation of a dense slurry layer on the aggregate surface, while NS enhanced hydration through its filling effect, pozzolanic reactivity, and nucleation effect. The combined action effectively densified the interfacial transition zone (ITZ) and optimized the pore structure, thereby improving the mechanical properties and durability of RSC. These results demonstrate that the synergistic modification of slurry-coating mixing and NS is an effective strategy for enhancing the overall performance of RSC. This study provides a promising approach for improving the performance of RSC and promoting sustainable construction materials.