## 4 Conclusions  
From the results of this investigation, a general conclusion can be drawn: concrete with addition of GGBS (fulfilling SS-SS-EN 15167-1 [6]) up to 25% of replacement of cement (maximum amount of replacement permitted by SS 13 70 03 for exposure class XF4) presents adequate salt-frost resistance.  
Other conclusions are summarized below:  
* The frost resistance of concrete generally decreases with an increase of the addition of GGBS, for concrete with 4% to 5% of air content by volume. This fact may be due to the slower hydration of GGBS when compared to Portland cement concrete, which yields a more porous concrete with a lower strength at the age of the start of the freeze/thaw test. The results showed, however, that it is possible to produce frost resistant concrete with GGBS amounts up to 50% (of the weight of CEM I), by changing some properties of the mix (such as increasing the air content), i.e., it is possible to produce salt-frost resistant concrete with percentages of GGBS replacement higher than the limit defined by SS 13 70 03 [9] for exposure class XF4 (25% of the weight of CEM I).
* For concrete with 50% of the weight of cement replaced by GGBS, the results for frost resistance after 112 cycles changed from not acceptable, for concrete with an air content of 4.5%, to good, for concrete with 5.6% of air. These results show that the beneficial effect of an adequate air pore structure in the salt-frost resistance of concrete is also valid for concrete with GGBS. The results also showed that amounts of GGBS up to 50% of the cement weight can be safely used in freezing environments where de-icing salts are used, as long as proper air entrainment is provided.
* Addition of GGBS in concrete significantly improves the resistance against chloride ingress, when compared to Portland cement concrete. The results show that the performance of concrete against chloride penetration increases with increased cement replacement levels (up to 100% of the Portland cement weight). The resistance of GGBS concrete against chloride is mainly related to its denser and more refined microstructure, which results in a less permeable concrete and, consequently, on a slower diffusion of the Cl' ions. On the other hand, GGBS has also shown to improve both the physical and chemical binding of chloride ions, which also contributes to a reduction of the free chlorides in the concrete paste.
* The results also show that GGBS concrete outperforms Portland cement concrete at all ages, and all percentages of replacement, even if the slag replaces the cement on a one-to-one basis (i.e., for a \(k\)-factor of 1.0). The efficiency factor concept is based on the water/binder ratio and compressive strength development of a concrete quality with additions, in comparison to Portland cement concrete, while no durability aspects are taken into account. The present results raise questions about the applicability of the efficiency factors when durability issues under concern.  
Conceptualization, J.G.F., L.T. and A.L.; methodology, V.C., L.T. and A.L.; validation, V.C. and L.T.; formal analysis, V.C.; investigation, V.C.; resources, L.T. and A.L.; data curation, V.C.; writing--original draft preparation, V.C.; writing--review and editing, J.G.F., L.T.; visualization, V.C.; supervision, J.G.F., L.T.; project administration, A.L.; funding acquisition, A.L. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
The authors grateful acknowledge C-Lab Thomas Concrete Group for the support given to the experimental campaign developed in this research.