Structural evolution of cement paste modified with silicate-based surface impregnant under carbonation: Insights from 27Al, 29Si, and 23Na NMR analysis

Although silicate-based surface impregnants have been used to enhance the durability of concrete structures, their long-term performance remains unclear. In this study, the chemical changes that occur when cement paste is treated with a silicate-based impregnant and subsequently carbonated were examined using X-ray diffraction and solid-state NMR spectroscopy, based on a powder-based model system simulating the treatment process. As the carbonation progressed, the decalcified regions of C–A–S–H underwent structural rearrangements. Both Al 3+ and Na + were incorporated into these Ca-depleted domains, and the resulting phases exhibited characteristics of aluminosilicate hydrates, including structures similar to C–(N, A)–S–H. Increases in tetrahedral Al and polymerized silicate species were evident in the 27 Al and 29 Si spectra, whereas 23 Na 3QMAS data showed that Na + migrated from regular C–S–H interlayer sites to less symmetric Si–O– and Al–O–related environments. These results suggest that silicate treatment influences the chemical processes during carbonation by facilitating Na + incorporation and promoting the formation of less-ordered aluminosilicate phases.