Experimental study on the effect of hot steam on the physicochemical properties and mineralization efficiency of blast furnace slag–CO2

To address the efficiency bottleneck caused by the passivation layer that envelops the active surface and obstructs the pores during the mineralization of CO 2 in blast furnace slag (BFS), this study proposed a hot steam (HS) activation method. The study investigated the effect of different HS activation stages on BFS mineralization efficiency. The results reveal the following: (1) The HS pre-activation stage demonstrating the highest performance. Specifically, pre-activation at 180 °C for 20 min resulted in a mineralization efficiency of 29.6 %. Efficiency decreased sequentially from the HS pre- activation stage to full-process, later, and initial activation stages. (2) During pre-activation, mineralization efficiency increased with rising HS temperature up to 220 °C, peaking at 34.9 % at 220 °C. (3) Scanning electron microscopy indicated that the extent of surface coverage and accumulation of CaCO 3 correlates directly with mineralization efficiency. In the HS pre-activation stage, HS facilitated BFS hydration, forming transition-zone macro-pores and gel pores. After mineralization, CaCO 3 extensively covered BFS surfaces and filled its pores, increasing the carbon mass fraction from 0 % to 5.27 %. (4) The HS-activated mineralized blast furnace slag (MBFS) consistently exhibited enhanced porosity, with 22 % mineralization efficiency identified as a critical threshold. Below this threshold, pore-blocking effects from carbonate precipitation remain negligible, while beyond 22 % efficiency, the pore-filling effect of precipitated CaCO 3 becomes increasingly pronounced, effectively moderating the rate of hydration-induced pore expansion. (5) With a pore size of 60 nm as the threshold, the porosity in the HS later activation stage first higher and then lower than that in the initial activation stage as pore size increased. • A method to improve the efficiency of steel slag-CO 2 mineralization by using hot steam (HS) is proposed. • HS increases the active surface area and optimizes pore structure, reducing product layer’s constraints. • Separating hydration and mineralization reactions maximize mineralization efficiency. • 220℃ is the optimal temperature for enhancing mineralization efficiency. • Porosity in HS-excited mineralization products is primarily influenced by pore expansion from hydration, followed by CaCO 3 filling.