# Resistance of Concretes to External Chlorides in the Presence and Absence of Sulfphates: A Review
## 2 Resistance of Concrete to External Chlorides
### Factors Affecting Chloride Binding  
Several factors have been reported to affect the formation of bound chlorides, such as the cement type, alkalinity of the pore solution, cation type of the salt, chloride concentration, temperature, presence of other anions, e.g., sulphates and carbonates, and the presence of SCMs in the mix. These factors are discussed here as follows.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**S/N** & **Model** & **Law Based On** & **Suitable Areas of Application** & **Ref.** \\ \hline
1. & \(J=-D\frac{d\mathcal{C}}{dx}\) & [PERSON]’s 1 st law & Steady state conditions where there are no changes in flow of ions. & [30] \\ \hline
2. & \(C(x,t)=C_{s}\left[1-erf\left(\frac{x}{2\sqrt{D_{t}}t}\right)\right]\) & [PERSON]’s 2 nd law & Non-steady state conditions, usually suitable for saturated conditions such as laboratory ponding tests and field conditions where the samples are continuously immersed in seawater. & [26] \\ \hline
3. & \(J(x)=-D\left[\frac{\partial c(x)}{dx}+\frac{ZFE}{RH}c\right]\) & Nernst-Planck & Electrical accelerated tests. & [1, 30] \\ \hline
4. & \(\frac{\partial C_{c}}{dt}=\underbrace{\mathrm{div}\left(D_{c}w_{c}\frac{ \rightarrow}{\sqrt{}}\left(C_{f}\right)\right)}_{\text{diffusion}}+\) & [PERSON]’s 2 nd law & Unsaturated conditions. & [27] \\ \hline \hline \end{tabular} \(J\) is the flux, \(C(x,t)\) is the chloride concentration at a given time and position, \(C_{s}\) is the surface chloride concentration, \(D_{c}\) is the chloride diffusion coefficient, \(t\) is the time, \(x\) is the depth in the diffusion direction, \(orf\) is the error function, \(Z\) is the valence of the ion (\(-1\) for Cl\({}^{-2}\)), \(F\) is [PERSON]’s constant, \(E\) is the electrical field, \(R\) is the universal gas constant, \(T\) is the temperature, \(C_{c}\) is the total chloride concentration, \(w_{c}\) is the evaporable water content, \(C_{c}\) is the concentration of chlorides dissolved in the pore solution (free chlorides), \(D_{b}\) is the effective humidity diffusion coefficient and \(h\) is the relative humidity.  
\end{table}
Table 1: Typical chloride diffusion models used in chloride ingress studies.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Factor** & **Effect** & **References** \\ \hline Porosity & The finer the pore structure, the greater the resistance to ingress & [31, 32, 33] \\  & of aggressive species, including chlorides. & [31, 32, 33] \\ \hline Curing conditions & Prolonged curing reduces porosity and hence permeability, so enhances resistance to chloride ingress. Elevated temperatures, e.g., 40 \({}^{\circ}\)C and above, while increasing the degree of hydration, lead to a more porous network for a given degree of hydration. & [13, 32, 34, 35] \\  & This leads to higher rates of chloride ingress. & \\ \hline w/b ratio & At any given temperature, higher values of w/b will result in higher rates of chloride ingress & [29, 36, 37] \\ \hline \hline \end{tabular}
\begin{tabular}{c c} \hline \hline
**SCMs** & **SCMs** & **SCMs** \\ \hline \hline \end{tabular}
\end{table}
Table 2: Factors influencing the rate of chloride ingress into concrete.  
1. [ENDTABLE]