Achieving sustainable hydrophobicity in alkali-activated concrete with MNC@PDMS core-shell nanostructures: Performance and interfacial mechanisms

Nanomaterial reinforcement is pivotal for reconciling the competing demands of moisture resistance and mechanical strength in low-carbon alkali-activated concrete (AAC). However, conventional separate-addition strategies are hindered by fundamental limitations such as phase segregation and competitive adsorption, which prevent the formation of a stable, high-performance microstructure within the caustic AAC matrix. To overcome these challenges, a novel core–shell nanostructure, comprising a polydimethylsiloxane (PDMS) core encapsulated by a modified nano-calcium carbonate (MNC) shell (MNC@PDMS), was designed and synthesized to induce a strategic interfacial bridging effect. Systematic characterization revealed that a dosage window of 0.3–0.6% catalyzes early hydration kinetics, significantly promoting the accumulation of C-(A)-N-S-H and AFm gels. Specifically, 0.3 wt% MNC@PDMS offset the dilution effect of the polymer core, yielding a 28-day compressive strength of 43.1 MPa. Concurrently, the strategic encapsulation of the PDMS core facilitated a transition to a highly hydrophobic state, evidenced by a 49% reduction in water absorption and a water contact angle of 119.4°. Notably, 0.6 wt% MNC@PDMS maximized chloride resistance, while 0.9 wt% enhanced carbonation stability by increasing phase crystallinity. Mechanistic analysis confirmed that the inorganic shell serves as a protective shield that prevents premature siloxane leaching. while suppressing detrimental oxidative cross-linking. However, thermogravimetric analysis revealed that excessive loadings (>0.9 wt%) persistently inhibit hydration development. Finally, a cradle-to-gate carbon footprint analysis verified that this integrated synthesis route aligns with the sustainability mandates for modern infrastructure.