The restoration of contaminated sites is attracting increasing attention because of environmental considerations. Barrier walls using cement-based solidified soil are a popular and effective method for enclosing contaminated sites. However, the working environment of cement-based solidified soils is often complicated and results in concern about the durability of the walls. Hence, the deterioration behaviour of solidified soils under a working environment should be investigated. A type of marine soft clay deposited in the coastal Lianyungang region of east China was solidified by a cement–metakaolin composite; then, the specimens were cured under standard, saline solution and cycled freeze–thaw environments. When the specimens reached the designed curing periods, the unconfined compressive strength, mass loss, pH value and mesostructure were determined. The results show that (a) both saline soaking and cycled freeze–thaw environments unavoidably reduce the durability of cement-based solidified soils and (b) metakaolin incorporation can effectively resist the deterioration behaviour of solidified soils under such environments. In this case, the optimal incorporation ratio is approximately 3·0%, and (c) although strength enhancement by chloride (Cl − ) ions was primarily inhibited by the water medium, metakaolin incorporation suppresses cracking on the surface and strengthens the integrity of the solidified soils.