In the study, changes in the mass of concrete filled with polyacrylamide/Fe3O4 nanoparticles in an amount of 1–2% of the cement mass in sea and formation water were studied. Besides, by performing IR and X-ray characterization of the prepared samples, the chemical state of the polyacrylamide (PAA) and the nanoparticles in the structure was also determined. Changes in mass of concrete samples containing PAA/Fe3O4 nanoparticles in highly mineralized seawater were shown to be highly dependent on the amount of nanoparticles compared to PAA. It revealed that at 2% PAA-Fe3O4/concrete, the ion penetration was limited and there was a lower weight gain as compared to the control concrete. IR spectroscopic analyses proved that the polymer was in a coordination band with cement and magnetite nanoparticles. X-ray diffraction studies showed a slight increase in the crystallinity of the concrete filled with up to 2% PAA/Fe3O4 magnetite nanoparticles