This study aims to investigate the correlation between Al incorporation and Na fixation by C-(A)-S-H, which was derived from hydrated C₃S blended with and without 10 wt% metakaolin at 1, 2, and 3 wt% Na₂O content. C-(A)-S-H compositions were calculated based on mass balance method. NMR was used to calculate the fraction of different coordinations of Si and Al in C-(A)-S-H. Results show that the incorporation of Al into C-S-H enhances the polymerization of silicate chains, it also increases the Ca/Si of the formed C-A-S-H because more Ca ions are required to stabilize the incorporated Al(VI). The increased Ca/Si subsequently decreases the alkali fixation capacity of C-A-S-H compared to that of C-S-H. Increasing alkali concentration favors the polymerization of C-A-S-H by incorporating more Al(IV) in bridging sites and shows less effect on Al(VI)/Si and Ca/Si due to the different negative charges and calcium coordination environment regarding Al(VI) and Al(IV).