In order to increase the suitability of coarse recycled concrete aggregates and improve the frost resistance of recycled coarse aggregate concrete, this study aims to investigate the effects of an antifreeze-type water-reducing admixture, air-entraining admixture, and antifreeze admixture on the frost resistance of recycled coarse aggregate concrete. The effectiveness of these admixtures is gauged by the mass loss rate and the relative dynamic modulus of elasticity (RDM). Mercury-impressed porosimetry (MIP), super depth of field microscopy, and scanning electron microscopy (SEM) were employed to characterize the hydration products, microstructure, and pore structure of recycled coarse aggregate concrete, with a view to establishing a connection between the microstructural characteristics and the macro properties and analyzing the micro-mechanism of the improvement effect of frost resistance. The test results demonstrate that the admixtures have a significant impact on the frost resistance of recycled coarse aggregate concrete. In particular, the recycled coarse aggregate concrete with an antifreeze admixture (dosage of 1%) and a water–cement ratio of 0.41 exhibited a mass loss of only 1.23% after 200 freezing and thawing cycles, a relative dynamic modulus of elasticity of up to 93.97%; however, the control group had reached the stopping condition at 150 freeze–thaw cycles with more than 10% mass loss. The recycled coarse aggregate concrete with added antifreeze admixture had a tight connection between the aggregate and the paste and a more pronounced improvement in the pore structure, indicating excellent resistance to frost damage.

The sustainable recycling of construction waste is a critical challenge in civil engineering. Recycled powder (RP) shows great potential as a supplementary cementitious material, but the deterioration in frost resistance it causes limits the application of recycled powder concrete (RPC) in cold regions. This study screened the polycarboxylate superplasticizer (PC) with optimal compatibility with RP through adsorption tests and systematically compared the improvement effects of antifreeze water-reducer (AR), air-entraining agent (AE), and antifreeze agent (AF) on frost resistance. Macroscopic properties were evaluated via freeze–thaw cycle tests, and microscopic mechanisms were analyzed using scanning electron microscopy (SEM), super depth of field (SDF) imaging, and mercury intrusion porosimetry (MIP). Results indicated that after 200 freeze–thaw cycles, the frost resistance improvement ranked as 1% AF > AR > 5% AE. Quantitative analysis revealed that AF optimized the pore structure, reducing the proportion of harmful pores > 200 nm by 8.73%, significantly increasing hydration products, and effectively inhibiting frost heave damage. The study confirms that AF effectively enhances the durability of RPC, providing technical support for the resource utilization of construction waste in concrete engineering in cold regions.

This study addresses the technical challenges of using construction residue sand (CRS) with high mud content in 3D printing by developing a novel sustainable mortar system. The key novelty lies in optimizing the mix design with calcium sulfoaluminate cement (CSA) and defining a quantitative printability window. Results show that at a CSA dosage of 10–15% and a sand–binder ratio of 1.4–1.5, the mortar achieves excellent printability, with a fluidity of 165–195 mm and a slump of 10–40 mm. Mechanistically, CSA promotes AFt formation, refining pore structure, and enhancing mechanical strength. This work provides a high-value pathway for recycling construction waste and advancing green intelligent construction.