ITZ properties of concrete with carbonated steel slag aggregate in salty freeze-thaw environment

• Carbonated steel slag aggregate (CSA) was used as fully replacement of normal aggregate (NA).
• Freeze-thaw resistance of concrete with NA, SSA and CSA were investigated.
• The ITZ was investigated and presented by Vickers hardness, Ca/Si ratios and microcosmic morphology.
• The effects of resistance on freezing and thawing of concrete were explained by ITZ properties.

The concrete samples with carbonated steel slag aggregate (CSA), normal aggregate (NA) and crushed steel slag aggregate (SSA) with W/C of 0.42 were prepared. The properties of the resistance on freezing and thawing were investigated and compared by freeze-thaw test with the condition of fresh-water and salt-water. The rapid chloride migration test (RCM) and coulomb electric flux test (CEF) were made in order to test the resistance of the concrete to chloride ion penetration. The ITZ properties and freeze thaw resistance of concrete were investigated by micro hardness tests, SEM, Ca/Si ratios, weight loss and relative dynamic elastic modulus test. The results show that using CSA instead of NA increased the freeze-thaw cycles from about 30 cycles to 175 cycles in salt-water condition and from about 150 cycles to 300 cycles in fresh-water condition. The average value of the hardness of ITZ around CSA is more than 43, which may due to the low W/C ratio caused by high water absorption of the CSA. The range of interfacial transition zone (ITZ) of CSA from SEM and EDS is larger but denser than NA and SSA, and fewer cracks are observed. The resistance to freeze and thaw will be improved by the hydration of steel slag to bond cement matrix. The cambered surfaces of CSA are less likely to occur stress concentration.

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