# Resistance of Concretes to External Chlorides in the Presence and Absence of Sulfphates: A Review
## 2 Resistance of Concrete to External Chlorides
### 2.
The chloride binding ability of PC-fly ash mixes seems to also be influenced by other factors, such as the curing technique, presence of carbonates (carbonation), source of chloride (whether internal or external), pretreatment of the fly ash and class of fly ash [56; 85; 86; 87]. [PERSON] [85] observed that the chloride binding capacity of PC-fly ash pastes increased when the pastes were cured for longer duration, presumably due to the increased presence of chloride binding to C-S-H. They also observed significantly decreased chloride binding when pastes were subjected to environments containing carbonates. [PERSON] [56] found that more chlorides were bound by PC pastes containing fly ash, when the chlorides were from internal sources as compared to when they were from external sources. [PERSON] [87] observed that pretreatment the fly ash with alkali solution before usage increased the chloride binding capacity of PC-fly ash pastes. Pretreatment with the alkali solution helped in depolymerizing the surface structure of the fly ash particles, thereby hastening the dissolution of ions and accelerating the pozzolanic reaction at an early age. As regards the influence of the class of the fly ash, [PERSON] [89] compared the durability performances of concretes incorporating Class C and Class F fly ash. They observed that the mixtures containing Class C fly ash performed better than those containing Class F fly ash, in terms of resistance to chloride ion permeability, and they attributed this to the higher chloride binding capacity of the Class C fly ash mixtures. * Metakaolin: Metakaolin has been widely reported to increase chloride binding [40; 90; 91] and its use is widespread in concrete exposed to chloride-rich environments. The addition of metakaolin to PC can also increase the rate of binding of internal chlorides [90]. Just like in GGBS and fly ash, the high chloride binding capacity is attributed mainly to metakaolin's high alumina content. * Silica fume: The impact of silica fume on chloride binding capacity is unclear. Most studies [38; 55; 56; 92; 93] report that chloride binding capacity decreases with increased replacement levels, but some [94; 95] have reported increased chloride binding capacity. According to [PERSON] [10], the addition of silica fume to concrete can affect chloride binding in three ways. The reduced portlandite content should reduce pore solution pH and dilution of the clinker should reduce the C\({}_{3}\)A content. Both of these factors decrease chloride binding. Conversely, the pozzolanic reaction leads to C-S-H formation and increased chloride binding. The significance of each of these three factors seems to be dependent on the replacement level. At higher levels (10% and above), the reduction in C\({}_{3}\)A appears to dominate and chloride binding is reduced, whereas, at lower replacement levels (5% and below), the addition of silica fume may increase chloride binding [96]. * Limestone: While not strictly cementitious, limestone is also increasingly used as a cement-replacement material in cements, with many standards allowing for an up to 20% clinker replacement [97]. Such systems offer a considerable reduction in carbon footprint. However, they also alter the hydration and microstructure of the hardened cement paste. The replacement of up to about 15% clinker with limestone leads to pore structure refinement due to the filler effect, limestone providing nucleation sites for the formation of C-S-H and encouraging hydration by increasing the effective water/cement ratio at the early ages. Thus, chloride ingress has been found to be hindered in limestone cements [98]. However, at higher replacement levels, these changes are insufficient to overcome pore coarsening due to clinker dilution. The addition of limestone also leads to the formation of carboaluminates via the reaction with C\({}_{3}\)A. It has been postulated that hemi- and monocarbonaluminate formation could inhibit chloride binding in [PERSON]'s salt. [PERSON] et al.