## 3 Results and Discussion
### Rapid Chloride Migration
#### 3.2.3 Influence of the Efficiency Factor  
The results presented in Figure 15 show a higher value of the migration coefficient for the concrete specimens with an efficiency factor of 1.0 (Mix 6), both at 28 and at 56 days of age, when compared with Mix 3, with an efficient factor of 0.6. These results were actually expected, considering, as previously referred, that the chloride ingress in concrete is closely related to the diffusivity of the paste, which in turn depends on the water/binder ratio. The equivalent water/cement ratio, determined according to Equation (1) is 0.45 for both mixes. However, for Mix 3 (with an efficiency factor of 0.6), the actual water/binder ratio is 0.39, whereas for Mix 6 (with a k-factor of 1) the actual water/binder ratio is 0.45. Lower water/binder ratios result in lower diffusivity of concrete, which explains why the migration coefficient is lower for Mix 3 when compared to that of Mix 6, although the ratio of GGBS to cement (50%) is the same in both mixes. The increase in the DRCM coefficient for a greater k-factor is more pronounced for concrete tested at 28 days, varying from \(11.1\times 10^{-12}\) m\({}^{2}\)/s for Mix 3 to \(16.6\times 10^{-12}\) m\({}^{2}\)/s for Mix 6 (variation of 50%), whereas for concrete tested at 56 days only a slight increase in the DRCM coefficient is identified, increasing from \(6.3\times 10^{-12}\) m\({}^{2}\)/s for Mix 3 to \(8.2\times 10^{-12}\) m\({}^{2}\)/s for Mix 6 (variation of 30%). On the other hand, the decrease in the Rapid Chloride Migration coefficient between 28 and 56 days is relatively similar for both concrete qualities (43% reduction for Mix 3 and 51% for Mix 6). It can thus  
Figure 15: Rapid chloride migration coefficient, DRCM, for mixes with 50% GGBS, 4.5% air content and different k-factor, cured at 20 \({}^{\circ}\)C.  
be concluded that the difference in the k-factor (and consequent difference in the water/binder ratio) does not significantly influence the evolution of the resistance against chloride ingress with the ageing of concrete. Comparing the results obtained for Mix 6, with 50% of GGBS replacement of the mass of Portland cement (Figure 15) and the results for Portland cement concrete (Mix 1, Figure 13), both with the same actual water/binder ratio (0.45), it can be concluded that, for the age of 28 days, the DRCM coefficient is similar for both concrete qualities. However, the tests performed at 56 days of age reveal a significant lower chloride migration coefficient for concrete with GGBS than for Portland cement concrete. These results are related with the slower hydration reactions of slag, which result in a higher capillary porosity of GGBS concrete at early ages. With continuing hydration, however, concrete with addition of slag presents a much higher resistance against chloride ingress than Portland cement concrete produced with the same w/c ratio. The results obtained in this investigation show, therefore, that the addition of GGBS in concrete results in a significant improvement of the resistance against chloride ingress in concrete, even for efficiency factors equal to 1, as long as sufficient hydration is ensured. These results also demonstrate that the efficiency factor concept is an effective way to predict the compressive strength of concrete qualities with additions, but not necessarily its durability. In fact, the parameters required for high-quality concrete are usually related with minimum cement content, minimum cement class, maximum w/c ratio, and compressive strength achieved at 28 days of age. It is usually assumed that concrete that fulfils these requirements will present adequate protection against the aggressive agents. This is partially true. The majority of the deterioration mechanisms depends on the penetration of fluids or gases inside concrete, as it is the case of carbonation, chloride ingress, and frost scaling. Furthermore, as previously explained, the penetration of these agents in concrete depends on its porosity, which in turn depends on the water/cement ratio.