Effect of co-doped additives on the hydration and microstructure of high-volume fly ash cement composites

Fly ash, as a low-carbon material, can partially replace cement in concrete, thereby contributing to reductions in CO₂ emissions and energy consumption. However, the incorporation of high-volume fly ash (HVFA) typically leads to a significant decline in the early strength of cement-based composites, limiting its broader application. This study investigated the influence of binary systems composed of nano-silica (NS) combined respectively with silica fume (SF), metakaolin (MK), or graphene oxide (GO) on the strength development of HVFA cement composites. The underlying mechanisms of strength enhancement were elucidated through analyses of pore structure, phase evolution, elemental distribution in the interfacial transition zone (ITZ), and microstructure. The experimental results demonstrated that NS and MK reduced the proportions of small and large pores, respectively, while SF decreased the volume of large and medium pores. Additionally, NS, SF, and MK effectively accelerated the pozzolanic reaction by reacting with calcium hydroxide, which promoted the formation of hydration products, leading to a reduced Ca/Si ratio in the ITZ gel and a denser microstructure. In single systems, the incorporation of 7.5 % NS yielded the most significant improvement in 7-d strength, with an increase of 148 %, while 15 % SF showed the greatest enhancement at 28 d, reaching 74.2 %. In binary systems, the combination of 7.5 % NS and 15 % MK achieved the highest 7-d strength improvement of 221 %, whereas the combination of 5 % NS and 15 % SF exhibited the most pronounced 28-d strength increase of 115.9 %. These findings offer valuable reference for advancing the development of sustainable construction materials.