sulphate attack and chloride attack

Many situations are faced wherein concrete is expected to be exposed to environments with more than one type of attack. Say, concrete exposed to sulphate attack and has aggregates which are reactive. Usually the solution to such problems would not have existed i we would have had choice of only one cement. but with the research and development works, with different additives in cement we have been able to figure out solutions for different problems exisiting in concrete.
but the most peculiar problem which was faced earlier was when, concrete was exposed to sulphate as well as chloride environent. Now for sulphate rich environment OPC is specified to have C3A content not higher than 5% (sulphate resisting cement). since sulphate reacts with C3A to form expansive ettringite and disrupt the concrete. but on the other hand C3A is very effective for resistance against chloride intrusion into concrete. thus concrete would get affected from chloride if not sulphate and vice versa. later on a very narrow boundary was drawn for uch cases, wherein, C3A content was specified to be between 5 to 8% in OPCs, ad it is said that this cement may prove effective against both threats.