# Resistance of Concretes to External Chlorides in the Presence and Absence of Sulfphates: A Review
## 2 Resistance of Concrete to External Chlorides
### 2. Alkalinity of the pore solution:  
The free chloride to hydroxide ratio is an indicator of susceptibility to reinforcement corrosion, with a higher ratio leading to increased risk of corrosion. However, also, the pH of the pore solution can influence the stability of bound chlorides. Several studies [47,50,51,52, 50] have shown that chloride binding capacity decreases with increasing pore solution pH (as shown in Figure 2). Meanwhile, [PERSON]'s salt solubility increases at pH > 12, thereby releasing chloride to the pore solution and reducing the amount of chemically bound chloride [53]. Figure 1: Chloride profiles for mortar samples with different C\({}_{3}\)A contents after exposure to a 4.0 M NaCl solution for 100 days [49]. Cation type of the salt:  
The source of the chloride ions or the associated cation affects the amount of bound chlorides [55; 56; 57]. [PERSON] et al. [56] observed greater chloride binding when samples were exposed to calcium and magnesium chlorides, compared to sodium chloride (see Table 3). This has been attributed to the cation's influence on chloroaduminate solubility, the accessibility of the chloride ions to the adsorption sites and the influence of the cation on the pH of the pore solution [53]. Na\({}^{+}\) ions raise the pH more effectively than Ca\({}^{2+}\) or Mg\({}^{2+}\), and since chloride binding reduces with increasing pH, chlorides associated with Na\({}^{+}\) will experience lower binding rates than those associated with Ca\({}^{2+}\) or Mg\({}^{2+}\). 4 Concentration of chloride solution:  
[PERSON] [58] and others [9; 40; 55; 59] observed that the chloride binding capacity of cement pastes increased with increasing chloride concentration (see Figure 3). This was attributed to the sensitivity of the chloride binding capacity on the Cl\({}^{-}\)/OH\({}^{-}\) ratio in the pore fluid. Figure 2: Effect of hydroxyl ion concentration on chloride binding for OPC pastes with water–cement ratio of 0.40 [54]. \begin{table}
\begin{tabular}{c c c c} \hline
**Cl Content (\% by wt. of Cement)** & **NaCl** & **CaCl\({}_{2}\)** & **MgCl\({}_{2}\)** \\ \hline Free & 0.831 & 0.765 & 1.480 \\ Bound & 0.804 & 1.408 & 2.347 \\ Total & 1.635 & 2.173 & 3.827 \\ Bound/Total (\%) & 50 & 65 & 61 \\ \hline \end{tabular}
\end{table} 
Table 3: Influence of cation type on chloride binding for 2-day-old PC paste samples (w/c = 0.5) immersed in 20 g Cl/L solution for 28 days [56]. Temperature:  
There is some controversy on the effect of increasing temperature on chloride binding, especially at temperatures above 0 \({}^{\circ}\)C. Some studies [9; 56; 60; 61] have reported increased chloride binding with increasing temperature, while others [62; 63; 64; 65] have reported the opposite. [PERSON] [61] observed an increase in the amount of bound chlorides as the temperature was increased from 5 to 22 \({}^{\circ}\)C, for neat pastes and pastes containing 25, 50 and 60 wt% GGBS. [PERSON] [60] observed increased chloride binding from 0 to 40 \({}^{\circ}\)C. In both studies, increased chloride binding was attributed to faster reaction rates. [PERSON] [9] also observed increased binding with increased temperature upon exposure to 3 M chloride solution, but observed decreased chloride binding with increasing temperature at lower chloride concentrations (0.1 and 1.0 M). [PERSON] and [PERSON] [13], meanwhile, attributed increased chloride binding at higher temperature (38 \({}^{\circ}\)C as opposed to 20 \({}^{\circ}\)C) to greater degrees of slag hydration. Conversely, [PERSON] [62] observed that the amount of unbound chlorides was significantly higher at 70 \({}^{\circ}\)C than at 20 \({}^{\circ}\)C. Similar trends were reported by [PERSON] [63] over a temperature range of 25 to 70 \({}^{\circ}\)C for admixed chlorides. This may be due to a partial release of the bound chlorides occurring at the higher temperature, as observed elsewhere [66]. In a study by [PERSON] [64], the bound chloride content calculated from the equilibrium solution decreased with increasing temperature.