Composition-driven microstructure refinement in geopolymers enabled by copper slag-based core-shell structure

Geopolymers face limited load-bearing applications due to low elastic modulus and high porosity caused by weak N-A-S-H gels. Traditional modification methods often fail to address interfacial inertness or porosity issues. This study develops a novel copper slag (CS) based core-shell structure featuring an iron-rich core and nano-silica (NS) coating to enhance geopolymer stiffness synergistically. High-purity NS (358 m 2 /g) synthesized via acid leaching and complexation forms a dense 0.6 μm shell using polyvinylpyrrolidone (PVP). Adding 2 wt% composite optimizes reaction kinetics, increasing 28-day compressive strength by 21.30% (54.3 MPa) and elastic modulus by 26.41% (12.11 GPa), surpassing ordinary Portland cement at equivalent strength. The iron-rich core reduces harmful macro-pores by 8.13%, while the NS shell promotes Al 3+ substitution for Si 4+ , promoting the preferential formation of dense C-A-S-H gels. This dual mechanism achieves high synergy (coefficient = 1.35), enabling advancing sustainable high-performance geopolymers with combined environmental and structural benefits. These findings suggest various promising applications in such as high-rise buildings and large-span bridges.