Abstract: Aiming at the unsoundness problem of steel slag aggregates for asphalt roads,the volume expansion rate of the asphalt mixture for carbonated steel slag was studied.The variation and relationship between f-CaO content and autoclaving pulverization rate in carbonated steel slag were investigated.The micro-morphology of carbonated steel slag was analyzed by BSE(Back Scattered Electron),SEM(Scanning Electron Microscope) and EDS(Energy X-ray Dispersive Spectroscopy).The results show that the active f-CaO in steel slag is mainly digested by carbonation below 90 °C under 1.0 MPa CO₂ initial pressure,and the cubic CaCO₃ particles are big size and uniform.The content of f-CaO in carbonated steel slag decreases linearly with its autoclaved pulverization rate.The active f-CaO in 4.75~9.5 mm steel slag can be completely digested by the carbonation system with temperature higher than 90 °C and time longer than 9 h,and the f-MgO or RO phase can be gradually digested,resulting in a linear decrease in the autoclave pulverization rate of carbonated steel slag.It also promotes the splitting and rearrangement of large aggregate minerals to small size and promotes the formation of CaCO₃ with short clusters of inclusions such as MgCO₃ and FeCO₃.Finally,the unsoundness problem of steel slag can be fundamentally solved when the carbonation temperature is higher than 90 ℃ and the time is longer than 9 h.The volume expansion rate of asphalt mixture cured for 3 d (about 0.97 %) is lower than 35 % of the standard value (1.5 %),and the expansion rate of 6 d (about 1.16 %) tends to be stable and lower than 23 % of the standard value,which achieves the purpose of long-term good stability of steel slag asphalt concrete.

Key words: steel slag aggregate,carbonation system,volume stability of asphalt mixture,soundness,microstructure