That was the major question when I visit marine structure... where the engineer need to know the scientific reason for don't using sulphate-resisting cement not used in marine concrete?
Because of the chances of corrosion in steel
Sulfide stress cracking (SSC) is a type of hydrogen cracking which presents a cathodic embrittlement mechanism. This term should not be confused with stress corrosion cracking (SCS), since this is anodic in nature.
SSC is a form of spontaneous brittle breakdown in occurring in high-strength alloys and steels when it comes in contact with humid hydrogen sulfide as well as sulfidic atmospheres.
marine concrete sulphate-resisting cement should not be used because tricalcium aluminate has high affinity for chloride ions. This is based on the possible reaction of chloride ions and tricalcium aluminate to form calcium chloroaluminate hydrate and the reduction of which may increase the rate of chloride attack to the concrete marine structure and result in faster corrosion of steel reinforcement in marine structures.
SRPC achieves sulphate resistance due to its low C3A content (thus reducing the risk of expansive etringite formation). However, C3A also binds chlorides as Friedel's salt. Therefore, the lower C3A content required to reduce damage due to sulphate attack also leads to reduced chloride binding, and so increases the risk of rebar corrosion.
In addition to given information,
Because porous structure is formed by dissolving and leaching of ettringite and free gypsum in sea water, aggresive salts of sea water can easily reach to hydration product of cement. Thus, ion exchange reaction between C-S-H and Mg salts produces M-S-H gel having no binding characteristics in addition to easy corrosion phenomena. This state can also be the reason of your question.
However, increment in C3A of cement may result in higher permeability of the matrix. I think permeability of matrix is more important issue to cease the deterioration. Such sulphate-resisting cements are produced by controling C3A in US as understood by ASTM standards. In Europe, blended cements can satisfactorily result thanks to their advanced durability aspects with reduced permeability characteristics if they are well cured.
This conference paper can be helpful:
Conference Paper Permeability Characterization of Concrete with Different Wat...
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