Volumetric Expansion Characteristics of Steel Slag during Immersion and Its Effects on the Low-Temperature Performance of Asphalt Mixtures

This study investigates the volumetric expansion behavior of steel slag under immersion conditions and its subsequent effect on the low-temperature performance of asphalt mixtures. To evaluate this, stability tests on steel slag and density measurements of compacted asphalt mixtures were performed, aiming to determine the immersion expansion rates of steel slag aggregates from various regions, as well as the expansion characteristics of asphalt mixtures with different gradation types. The free calcium oxide (f-CaO) content in the aggregates was quantified using the ethylene glycol method, and a detailed analysis was conducted to examine the correlation between the immersion expansion rates of steel slag aggregates and mixtures with respect to immersion time and f-CaO content. Additionally, considering the relationship between immersion time and expansion rate, the low-temperature performance of steel slag asphalt mixtures was investigated using a restrained specimen thermal stress test. The results demonstrated that the f-CaO content, expansion rate, and stabilization time of steel slag aggregates were substantially affected by both the aggregate gradation and the source of the steel slag. Notably, fine aggregates with higher f-CaO content led to a more pronounced increase in the expansion rate of steel slag mixtures. Type I steel slag aggregates with particle sizes of 10–15 and 5–10 mm respectively contributed 46.13% and 35.06% to the relative expansion rate of Baotou SMA-13 (SMA I), while Type II steel slag aggregates with a particle size of 0–3 mm contributed 46.38% to the relative expansion rate of Shandong SMA-13 (SMA II). Fine aggregates with higher f-CaO content led to a more significant increase in the expansion rate of steel slag asphalt mixture. A comparison of the expansion rates of Type I steel slag asphalt mixtures with different gradations, including SMA I [with an asphalt content of 5.5% (81.67 g of asphalt) and a void volume of 3%] and Baotou AC-13 (ACI) [with an asphalt content of 4.3% (65.33 g of asphalt) and a void volume of 4.44%], revealed that the change in void volume due to the asphalt content was one of the factors affecting the expansion rate of steel slag asphalt mixtures. Steel slag asphalt mixtures derived from different sources but with the same gradation exhibited comparable low-temperature performance under identical expansion rates and soaking durations. Following 6 days of immersion, the low-temperature performance of SMA I and SMA II decreased by 21%–44% and 18%–60%, respectively. Notably, the hydration-induced expansion of the 0–3 mm aggregates in SMA II resulted in a more pronounced reduction in low-temperature performance compared to the coarser aggregates (10–15 and 5–10 mm) in SMA I.