The mutual interaction between geopolymerization and hydration in producing aluminosilicate geopolymer-OPC composites: Interface characterizations and mix-design regulation

Aiming to meet the demand for carbon peak and neutrality, the combination of alkali-activated metakaolin (AAMK) and Ordinary Portland cement (OPC) presents a promising solution. However, the lack of a comprehensive understanding of their synergistic effects poses challenges for the mix-design of geopolymer-OPC composites. This study investigated the macro-mechanical properties and microstructural characteristics of metakaolin-based geopolymer-OPC composites with varying compositional oxide ratios. The results demonstrate that adjusting the SiO₂/Al₂O₃ and H₂O/Na₂O ratios significantly affected the compressive strength alongside the initiation of plastic deformation. In contrast, the Na₂O/Al₂O₃ ratio modulated both the elastic modulus and the transition from elastic to plastic behaviour. Microstructural analysis reveals that the above oxide ratios may differentially influence the polycondensation of N-A-S-H at the interface compared to the geopolymer matrix alone, owing to the migration of mineral compounds and the variation of flowability at fresh state. Optimal compressive strength, surpassing that of pure OPC systems, was achieved at SiO₂/Al₂O₃ = 3.2, Na₂O/Al₂O₃ = 0.8–1.0, and H₂O/Na₂O = 8.75–10.75, highlighting the potential of these composites to reduce cement usage and carbon emissions in construction applications.