I have conducted a few tests on concretes mixed and cured in sea water. I observed that the strength increased for the first 14 days but decreased there after. I also repeated the tests for OPC, Fly ash concrete and GGBS blended concretes and observed the same phenomenon. I need possible reasons for how this could happen.
Dear Ariade Tsambani,
I need reason or justification. The observations in your work are in line with mine. But i need the possible cause or reason.
The answer may depend on the type of cement used and the physical properties of the concrete (porosity, etc.). The possible explanation is that during the first 14 days, the dominant factor is the hydration of cement and strength gaining. After the 14 days, the formation of ettringite and its expansion may have started to show its effect on weakening the cement past.
Since the curing itself in sea water then even though you have used normal water for concrete mixing due to early stage higher porosity possible migration of harmful ions like mg,so4,na, to inside concrete, to balance ion concentration. Then as water consumed fir hydration sea water itself penetrates into it,to fulfill extra water needed for consumed water. To migrate and react with hydrated cement paste takes time as its time depending, concentration of ions, cement content, porocity, even temperature will affect the phenomenon. Then whatever the reaction happens in naso4 and mgso4 attack on concrete the same will happen. strength incremental is possible untill the decrease of strength is higher than the increase in strength. Chloride present in it can acts as accelerator too. Pure sea water and laboratory prepared sea water you find difference in strength reduction. Over all its a cumulative plus and minus strength, which ever is higher it comes in picture.
Yes, it's clearly a problem of cement resistance to agressive milieu (sea water: salt).
Simply because the the sea water contain salts such as chloride and sulfate which will react with the cement paste (calcium aluminate) and produce gypsum and ettringite. As a result, the concrete will soften and loose its strength due to disintegration of its particles since the binding material is destroyed.
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