On the other hand, it is well known from the literature that
the chloride ions can be present in concrete in two forms [7]:
– free or weakly bound chloride ions;
– chloride combined in compounds such as calcium chloro
aluminate.
It is the free and weakly bound part of the chloride which
promotes corrosion [7] even if bound chloride may participate
in corrosion initiation when the establishment of pH gradients is
required to sustain passive film breakdown [8]. Nevertheless,
the chemical combining of chloride ions is frequently
considered as a beneficial effect in that it reduces the rate of
chloride penetration in concrete [9].
It is also well documented that a higher C3A content gives
cement greater chloride complexing ability resulting in the
formation of an “insoluble” calcium chloro aluminate
compound.
According to Metha [10], chemical binding of penetrating
chlorides cannot be expected unless the C3A content is higher
than about 8%.
Recent experimental observations have shown that a C3A
content as high as 8.6% does not effectively reduce chloride
penetration when concrete is immersed in seawater [11] and
other studies have found more corrosion-induced distress
associated with high C3A content [12].
It has also been reported that the majority of chloride binding
takes place during the first 28 days of curing [13].
Whatever the explanation, the C3A content may change the
electrochemical treatment efficiency.
In addition, it is believed that the components with bound
chlorides and free chlorides in the pore solution are connected
by a chemical equilibrium [14,15]. This implies that if the free