Effect of fineness on the hydration and microstructure of cementitious materials with high-volume steel slag and blast furnace slag

In this work, steel slag (SS), blast furnace slag (BFS), ultrafine SS (USS) and ultrafine BFS (UBFS), with fineness of 407, 450, 748 and 760 m2/kg respectively, were prepared. The influences of SS and BFS fineness on cementitious systems were studied from perspectives of the hydration process, microstructure formation as well as fluidity and strength. The results show that the BFS with a high fineness exhibits a high reactivity while an adverse effect on the fluidity. A high fineness of SS could have a negative impact on the hydration of blended cement. The addition of BFS, regardless of the fineness, substantially increases the number of small pores below 20 nm and enhances the compressive strength of cementitious systems. The combination of SS and BFS can decrease the Ca/Si ratio and raise the Al/Si ratio of the main hydrates C(-A)-S-H, while the fineness does not have a great impact on the chemical composition of C(-A)-S-H. Compared to plain cement, the gap-graded ternary cement prepared with 25 wt% cement, 39 wt% SS and 36 wt % UBFS is featured with a lower embodied energy and CO2 emission without compromising the early and late performances.