Effect of sodium sulfate/nano C-S-H composite on steel slag-cement hydration under steam-curing conditions

In this paper, the influence of sodium sulfate/nano-C-S-H (NC) composite on the compressive strength of a steel slag (30 wt.%)-cement system is explored under the conditions of steam curing, with the influence mechanism being analyzed by XRD, DSC-TG, MIP, SEM and FTIR. Results show that sodium sulfate can significantly improve the early compressive strength of the steel slag-cement system at 12 h, and the best effect is achieved when the dosage is 1.5% (mass fraction),  the strength being increased by 88.8%.The incorporation of NC and sodium sulfate together can further increase the compressive strength, and the larger the NC dosage, the more obvious the improvement, mainly because the nucleation effect of NC can improve the hydration degree of pastes and refine the pore structure. However, at 3 d and 28 d, the strength of the single and compound mixing sample is lower than that of the control sample, which indicates that the hydration of cement and steel slag is promoted and the hydration degree of the system is improved at 12 h under steam curing after adding the early strength agent, but it is not conducive to the strength development in late ages. However, the crystal nucleus effect of the compound doped NC increases the nucleation site and promotes the hydration reaction, which makes the strength of the co-doped system still higher than that of the single doped system.