Mechanism of DR–PS in regulating early–stage microstructural evolution and strength enhancement of low–carbon alkali–activated concrete

The monolithic hydrophobic alkali-activated concrete (AAC) performs excellent long-term impermeability, while unavoidably accompanied with the lose of its mechanical strength. Microencapsulation can enhance hydrophobicity without significantly reducing compressive strength in cementitious materials; however, it still faces premature core material release issues, which inhibit the early alkali activation reaction of AAC. To overcome this limitation, a core-shell delayed release polysiloxane (DR-PS) was synthesized based on pH-time dual response coating. The effects of DR-PS incorporation on the mechanical performance and hydrophobicity of AAC were systematically evaluated, and the underlying mechanisms were elucidated through comprehensive microstructural characterization. The results revealed that, compared to the reference AAC, the 28-day compressive strength of DR-PS-AAC increased by 19.27 %, while the water contact angle (WCA) reached 90.5°, demonstrating a synergistic improvement in both mechanical strength and surface hydrophobicity. Mechanistically, the delayed release of polysiloxane avoided interference with the critical early-stage depolymerization and polycondensation of reactive Si–Al species, thereby facilitating the formation of a continuous gel network. In the later hydration stages, the released polysiloxane imparted durable hydrophobicity, while the degradation of the DR-PS shell contributed to pore structure refinement, evidenced by a 24.08 % reduction in pores larger than 10 nm compared to PS-AAC. This work presents a mechanistically informed strategy to reconcile early-stage structural integrity with long-term durability in AAC, offering a viable pathway for the development of high-performance, hydrophobic, and low-carbon concrete materials for aggressive service environments.