Effect of composite cement paste on the properties of recycled concrete aggregate and its underlying mechanism

To address the limitations in enhancing the performance of recycled aggregate concrete (RAC) due to inadequate enhancement of the interfacial transition zone (ITZ) when using single coating materials—and to further elucidate the underlying modification mechanisms—this study introduces a novel composite slurry formulated with polyacrylate emulsion (PAE), fly ash, gypsum, and P·O 42.5 ordinary Portland cement. This composite slurry was used to treat recycled coarse aggregates (RCA), aiming to enhance their properties and, thereby, improve the overall performance of concrete incorporating such aggregates. The study systematically evaluates the effects of various slurry mix proportions on the physical characteristics of the coated recycled aggregates. Furthermore, the mechanical properties, impermeability, and drying shrinkage behavior of concrete prepared with these treated aggregates were analyzed. Complementary microstructural and mechanistic analyses were conducted using X-ray diffraction (XRD), scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM-EDS), and mercury intrusion porosimetry to investigate how the composite slurry influences ITZ structure and behavior at the microscopic level. The experimental results show that surface coating of recycled aggregates with the proposed composite slurry significantly reduces their crushing value and water absorption. Specifically, the reductions exceeded 30% compared with untreated aggregates, indicating a marked improvement in strength and density. In contrast to traditional pure cement slurry coatings, the composite slurry exhibited a more pronounced effect in enhancing both mechanical and durability performance of RAC. The compressive strength of RAC increased by 54.5%, while axial tensile strength rose by 66.1%. Durability indicators also improved considerably: the permeability coefficient and electric flux—commonly used as proxies for water penetration resistance and chloride ion transport—decreased by 46.2% and 52.4%, respectively. Moreover, long-term dimensional stability was enhanced, with the 90-day drying shrinkage rate reduced by 24.7%. The underlying enhancement mechanisms can be attributed to the synergistic effects of the slurry's constituents. Fly ash, a well-known pozzolanic material, plays a dual role. First, it reacts with calcium hydroxide produced during cement hydration to form additional calcium silicate hydrate (C–S–H) gel, which fills microvoids and refines pore structure. Second, it serves as a nucleation site, promoting hydration product formation at the aggregate–paste interface, thus improving ITZ compactness. Gypsum contributes to early-age formation of ettringite (AFt), which fills micro-cracks and enhances the ITZ crystalline structure. This transformation improves bond strength and microstructural stability. A particularly notable role is played by the polyacrylate emulsion (PAE), which exhibits significant polymerization behavior in the alkaline cementitious matrix. The PAE releases long-chain polymer molecules that form dense, flexible films on the surface of the recycled aggregates. These films not only seal surface pores and microcracks but also bridge the interface between aggregate and cement paste, improving stress transfer, reducing interfacial stress concentrations, and increasing resistance to moisture ingress. The combined action of these materials results in a densified ITZ, effectively mitigating the inherent defects of recycled aggregates and facilitating better integration within the cementitious matrix. Importantly, the composite slurry showed particularly significant improvements when applied to low-strength recycled aggregates. In these cases, surface deficiencies and high porosity make the aggregates especially responsive to modification. The composite slurry more effectively compensates for mechanical and durability limitations than single-component or cement-only coatings, highlighting its practical potential for diverse recycled aggregate applications.

　　In summary, this study presents a comprehensive approach for enhancing the performance of recycled aggregate and recycled aggregate concrete through the use of a multifunctional composite slurry. The combined chemical and physical effects of the incorporated components yield substantial improvements in mechanical strength, permeability resistance, and shrinkage control, while microstructural analyses confirm densification and stabilization of the ITZ. These findings offer both theoretical insight and practical guidance for the effective application of recycled aggregates in structural concrete, especially within the context of sustainable construction and circular material reuse.