# Resistance of Concretes to External Chlorides in the Presence and Absence of Sulfphates: A Review
## 2 Resistance of Concrete to External Chlorides
### 2.
However, the impact of temperature was minor compared to that of the cement composition. Meanwhile, [PERSON] [67] did not observe any significant difference in the chloride binding capacity of cement pastes exposed to calcium chloride solutions at 5 \({}^{\circ}\)C and 23 \({}^{\circ}\)C.  
Presence of other anions:  
The presence of other anions, e.g., sulphates and carbonates, may affect the chloride binding capacity of the cements. This has been attributed mainly to reactions between the various anions in the pore solution. In the case of sulphates, reduced chloride binding has been attributed to the preferential reaction of C\({}_{3}\)A with sulphates [68]. In the presence of additional sulphates, the C\({}_{3}\)A reacts with the sulphates to form ettrinite [69; 70], which is more thermodynamically stable than [PERSON]'s salt. This reduces the amount of residual C\({}_{3}\)A to bind the chlorides. Figure 3: Effect of varying concentrations of chloride ions on chloride binding capacity [58]. This has been reported by several researchers [68, 71, 72, 73, 74, 75, 76]; for example, Figure 4 shows the bound chloride content vs. free chloride content for slag blends, with or without anhydrite ($). For the slag blends with anhydrite (i.e., C2S1$ and C2S2$), additional anhydrite ($) was blended with the original slag blends (i.e., C2S1 and C2S2) so as to bring the sulphate contents of the slag blends to that of the plain cement. The presence of this additional sulphate lowered the bound chloride content. On the other hand, carbonation changes the nature of the hydration products, e.g., portlandite Ca(OH)\({}_{2}\) is converted to CaCO\({}_{3}\) and so reduces the pH of the pore solution. This reduction in pH can result in a destabilization of Friedel's salt [77, 78] and eventually a decrease in chloride binding. 7. Presence of SCMs:  
As mentioned earlier, SCMs primarily affect resistance to chloride ingress by modifying the pore structure, hence, reducing permeability. However, SCMs, such as fly ash, GGBS, metakaolin, silica fume, etc., also affect chloride binding, the extent of which depends on the type of SCM. * GGBS:  
The addition of GGBS to PC increases the chloride binding capacity. This has been attributed to several factors, such as: (1) the high alumina content of GGBS [13, 40, 58, 79, 68], (2) an increase in the amount of hydrotalcite (Ht) formed, especially when greater proportions of GGBS are used [80, 81], (3) a lower sulphate content of PC-GGBS blends [68, 72, 64] and (4) the formation of a C-A-S-H phase that is responsible for the binding, through physical adsorption, of about two-thirds of the chloride [82]. The chloride binding capacity of PC-slag blends is also dependent on several factors, such as the level of slag replacement, w/b ratio, chemical composition of the slag and the curing temperature. For example, [PERSON] [13] found that slags with higher alumina contents had greater chloride binding capacities, while [PERSON] [58] found that chloride binding capacity increased with the level of slag replacement. They established a model relating chloride binding capacity and GGBS content, as shown below:  
\[C_{b}=\ (-22.21G^{2}+39.45G+3.36)\ X+(6.84G^{2}-6.40G+3.64) \tag{1}\]  
Figure 4: Bound chloride content vs. free chloride content for different slag blends [76]. where \(C_{b}\) is the chloride binding capacity in mg/g of sample, \(G\) is the GGBS/total binder ratio and \(X\) is the chloride exposure concentration in mol/liter. However, their model considered just a single source of GGBS, excluding any impact of GGBS composition. * Fly ash: As with GGBS, the inclusion of fly ash in PC increases the chloride binding capacity [67; 83; 84]. Again, this is attributed primarily to fly ash's high alumina content, which facilitates the formation of more Friedel's salt. However, at very high replacement levels, fly ash has been found to reduce the chloride binding capacity [39].