# Resistance of Concretes to External Chlorides in the Presence and Absence of Sulfphates: A Review
## 3 Resistance of Concrete to External Chloride in the Presence of Sulphate
### Effect of Chloride on Sulphate Deterioration  
The effects of chloride on sulphate attack of cementitious materials are inconclusive, with various studies suggesting that chlorides may mitigate or accelerate sulphate attack or have no significant effect [131]. The general consensus is that chlorides mitigate sulphate attack. However, differences in material compositions between different studies have made comparisons difficult. As early as 1956, [PERSON], as cited in [124], reported that chlorides reduce the rate of sulphate attack in Portland cement systems. However, this was countered by [PERSON], as also cited in [124], who argued that chlorides aggravate sulphate attack. A few other studies are highlighted below:  
[PERSON] et al. [131] investigated the effect of mineral admixtures, including natural pozzolana, fly ash and GGBS, on the deterioration of concrete due to thaumasite sulphate attack (TSA) and concluded that the presence of chloride reduced the deterioration of concrete due to sulphate attack when the binder was only limestone cement but was aggravated when limestone cement was combined with mineral admixtures.  
Meanwhile, [PERSON] [125] studied the effect of chlorides present in Portland cement mortar and concrete mixes on their sulphate resistance. Sodium chloride did not have any substantial effect on sulphate attack, while calcium chloride increased the rate of sulphate attack. However, recent findings [15], indicated by Figure 6, showed that sulphate expansion was significantly reduced in the presence of sodium chloride. This discrepancy can be explained by the duration of exposure, with the more recent study showing that the effects of chlorides on reducing sulphate-induced expansion were only visible beyond 65 weeks. This agrees with [PERSON] [136], in that the concomitant presence of chloride ions in a sulphate environment mitigates sulphate attack of plain and blended cements, attributed to increased solubility of ettringite and gypsum in chloride solutions [132]. This was slightly refined by [PERSON] et al. [123], who reported that chlorides mitigated sulphate deterioration in plain cements, but the effect was not great for blended cements incorporating GGBS, fly ash and silica fume.  
Further, [PERSON] et al. [128] studied the effect of chloride on CEM I and CEM I blended with limestone filler exposed to sulphate at low temperature (5 \(\pm\) 0.5 \({}^{\circ}\)C) for 630 days and showed that 0.5% chloride in sulphate solution increased damage but that the damage reduced when the chloride concentration was increased to 2%.  
Figure 6: Comparison between expansion of CEM I mortar prisms exposed to pure sulphate [137] and combined chloride–sulphate solution [15].  
Figure 7 shows the changes in compressive strengths between exposure for 1 and 12 months to different combined chloride and sulphate solutions, as extracted from [18]. The chloride solution was kept constant at 3%, while the concentration of sodium sulphate was increased from 3% to 10%. Exposure for 1 month led to no discernable change in compressive strength. However, there was a clear trend, with increasing loss of strength with increasing sulphate concentration. This agrees with [129], where samples exposed to lower concentrations of combined solutions suffered less diminution in compressive strength than specimens exposed to higher concentrations.