Investigation of the effects of different sulfate types found in seawater on concrete containing microfiber and silica fume

Sulfate salts which available in seawater with high concentrations cause the formation of ettringite in hydrated structures which formed as a result of the hydration of cement. On the other hand, ettringite causes excessive volume expansions and eventually leads to cracking of the concrete due to the internal stresses in concrete since it is a large volume mineral structure. In this study, ultrafine cement and silica fume as mineral additive were used together for binder design. Besides, microfiber has been added to the binder systems produced in different proportions. The produced specimens were kept separately in water, in solutions containing 2% Na2SO4 and 2% MgSO4 by weight for 90 days. The compressive strength test was performed at 28th and 90th days on cured specimens. In addition to the compressive strength test, the solution samples were taken from the curing solutions every 10 days and the change of sulfate concentrations was followed in the solutions. According to the results, in Na2SO4 solution higher compressive strength values were observed up to 66 MPa while strength loss was observed in the specimens cured in the MgSO4 solution. In parallel to this result, the remaining concentrations of SO42- ions in the MgSO4 solution were lower than those in the Na2SO4 solution. It was inferred that in Na2SO4 solution, the fibers could compensate for the internal stresses. This situation shows that especially the microfiber additive can compensate for the expansion that will occur as a result of ettringite formation, and thus it can help the mechanical stability of the concrete.