## 3 Results and Discussion
### Rapid Chloride Migration
#### 3.2.1 Influence of the Amount of Portland Cement Replacement by GGBS  
The results plotted in Figure 13 show a lower chloride migration coefficient for the specimens tested at 56 days of age than for those tested at 28 days of age, for all concrete qualities. The resistance against chloride ingress depends on the diffusivity/penetrability of concrete, which in turn depends on the degree of hydration of concrete. The flow of liquids (and gases) through the concrete paste is facilitated through the larger capillary pores than through the smaller gel pores. With progress of hydration, the capillary pores in concrete are gradually filled with the C-S-H gel that is formed during the hydration reactions, i.e., the pore structure changes continuously, and the diffusivity of the paste is reduced with progress of hydration. Since the degree of hydration (amount of cement that has reacted, relative to the total amount of cement present in the mix) increases with time, the diffusivity will thus be reduced and, consequently, also the chloride migration coefficient. However, the effect of the hydration degree of concrete between 28 and 56 days is more obvious for concrete mixes with GGBS addition than for the reference Portland-cement only concrete. For Portland cement concrete (Mix 1), the DRCM coefficient decreases only 17% between specimens tested at 28 and 56 days of age, whereas for all concrete mixes with GGBS (Mixes 2, 3 and 4), a decrease between 43% and 50% was obtained. These results are related with the lower hydration rate of GGBS, which leads to a larger average pore size at early ages [7]. However, unlike the hydration of Portland cement, whose rate is greatly reduced after 28 days of age, the hydration of GGBS continues at a high rate, which leads to the continuous formation of gel and consequent reduction of the diffusivity of concrete with time [11]. The results also display a clear decrease in the average chloride migration coefficient with the increase in the amount of GGBS replacement in the mix. That is the case for concrete specimens tested at both 28 days and 56 days of age. The positive influence of the increase in GGBS content in the improvement of the resistance against chloride migration is widely reported, and is usually attributed  
Figure 13: Rapid chloride migration coefficient, D\({}_{\text{RCM}}\), for mixes cured at 20°C with different additions of GGBS. to the following factors: on the one hand, an increase in the slag content results in a denser and less permeable concrete, which reduces the diffusion of chloride ions and slows down capillary suction, therefore reducing the transport of the chloride ions inside the concrete, and lowering the concentration of available chloride ions in the paste [4; 12; 13; 14]. On the other hand, the lower concentration of hydroxyl ions in the pore solution of the GGBS concrete reduces its capacity to exchange anions, which also contributes for the reduction of the diffusion rate of the Cl- ions. Moreover, the presence of GGBS in the concrete mix increases the chemical and physical binding of chlorides, contributing to a reduction of the free chlorides in the paste [11]. However, the influence of the amount of GGBS addition seems slightly different for concrete tested at different ages. In fact, the chloride penetration of the specimens tested at 28 days of age decreases gradually as the amount of slag increases. It seems that, at 28 days of age, a significant reduction in the chloride ingress in concrete with GGBS is only achieved for amounts of replacement of about 50% of the Portland cement content. On the other hand, for the specimens tested at 56 days of age, the results show a markedly decrease (47%) in the DRCM coefficient between Portland cement concrete (Mix 1) and concrete with 25% of slag replacement (Mix 2). As the percentage of slag replacement increases, the chloride migration coefficient is not so significantly affected.