The influence of the synergistic effect of recycled micro-powder and different types of mineral admixtures on the hydration and microstructure of cement-based materials

This study investigates the influence of different proportions of recycled micro-powder (RP) mixed with fly ash and slag on the hydration process and mechanical properties of cement, establishing a correlation between hydration performance and strength. Various modern analytical techniques were employed to quantitatively characterize the hydration products and microstructural evolution of multi-component cementitious materials. The results showed that both RP and slag notably expedite the early hydration rate, whereas fly ash tends to retard the process. When RP is combined with either slag or fly ash, the types of hydration products remain unchanged, but their quantities increase. The primary hydration products include calcium silicate (aluminate) hydrate, calcium hydroxide, gypsum, ettringite, and quartz, in addition to unhydrated tricalcium silicate, dicalcium silicate, and carbonated calcium carbonate. At the 28 days, the consumption of calcium hydroxide in the RP - fly ash combination is lower than that in the RP - slag combination. Although the mechanical properties of the RP - fly ash decline initially, they show significant enhancement in later stage, with the optimal RP to fly ash ratio determined to be 1:1. In contrast, the RP-slag mixture exhibits improved early-stage strength and rapid strength development during mid-to-late stages, achieving maximum compressive strength at an RP-slag ratio of 1:2, which is 43.8 % higher than that of pure RP. Overall porosity decreases after mixing, as multi-damage and harmful pores are filled by secondary hydration products, transforming them into harmless or less harmful pores. The lowest porosity (16.58 %) is observed in the RP-slag mixture. This study provides scientific guidance for the high-value application of RP.