## 4 Conclusions  
In this study, the durability and microstructure of concrete containing water-cooled slag (WS) and air-cooled slag (AS) subjected to freeze-thaw cycles were experimentally examined. The following conclusions can be drawn.  
(1) When the compressive and flexural strength characteristics of concrete were investigated, the strength of OPC concrete was found to be slightly higher at early ages, but the strength development of concrete containing AS (WS35 AS05 and WS30 AS10) was found to be superior to that of OPC concrete as the age increased, because of the influence of larnite, a \(\beta\)-C\({}_{2}\)S hydrate.  
(2) When the air-void system of hardened concrete was measured, OPC concrete with high air content exhibited the largest spacing factor and the smallest specific surface area. On the contrary, WS40 concrete exhibited the smallest spacing factor despite its relatively low air content.  
Figure 13: SEM images of concrete after the 210 cycles of freeze–thaw test.  
(3) The three blended cement concrete types subjected to 300 freeze-thaw cycles exhibited RDME values of ~88% or higher due to the influence of the latent hydraulic properties of AS and WS, suggesting relatively higher frost resistance than that of OPC concrete (~55.4% after 240 freeze-thaw cycles). The DF of the concrete subjected to freeze-thaw cycles exhibited an increasing tendency as the spacing factor increased, and the specific surface area decreased.  
(4). The surface electrical resistivity of OPC concrete was found to be lower than that of blended cement concrete regardless of the number of freeze-thaw cycles. Although the surface electrical resistivity loss of OPC concrete was ~44.8% at 210 cycles, that of WS30 AS10 concrete was ~24.5%. Furthermore, the compressive strength loss of concrete due to freeze-thaw cycles exhibited a tendency similar to that of the surface electrical resistivity, confirming a relatively superior frost resistance of blended cement concrete.  
(5) When the microstructure of the concrete subjected to freeze-thaw cycles was analyzed, OPC concrete exhibited highly wide cracks near the ITZ, indicating relatively high damage caused by freeze-thaw cycles. On the contrary, WS40 concrete exhibited very dense microstructures near the ITZ, and concrete containing AS also exhibited excellent frost resistance with relatively small damages.  
(6) Blended cement concrete containing WS and AS based on an appropriate mix proportion design will exhibit improved durability in regions experiencing harsh winters and very low temperatures. More studies are needed to determine the appropriate mix proportions and percentages of blended AS and WS.  
Conceptualization, S.-TL, methodology, S.-TL. and S.-H.P; software, S.-H.P.; validation, S.-TL., D.-G.K. and J.-M.K.; formal analysis, D.-G.K. and J.-M.K.; investigation, S.-H.P. and D.-G.K.; resources, S.-T.L.; data curation, S.-T.L.; writing--original draft preparation, S.-T.L.; writing--review and editing, S.-T.L. and S.-H.P.; visualization, S.-T.L.; supervision, S.-T.L.; project administration, S.-T.L.; funding acquisition, S.-T.L. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the Ministry of Land, Infrastructure and Transport (MOLIT) and the Korea Agency for Infrastructure Technology Advancement (KIA) for a project on \"Development of anti-frost concrete pavement incorporating high volume air-cooled slag and its application (No. 21 CTAP-C156959-02)\".  
Not applicable.  
The author declares no conflict of interest.