The microstructural study was conducted on cement and cement-slag pastes immersed in different concentrations of Mg(NO3)2 solutions utilizing 29Si, 27Al NMR spectroscopy and XRD techniques. The results show that the hydration of both the cement and cement-slag pastes is delayed when the pastes are cured in Mg(NO3)2 solutions as compared to the pastes cured in water. Moreover, Mg2+ ions also exhibit an decalcifying and dealuminizing effect on the C-A-S-H in cement and cement-slag pastes, and thereby decrease Ca/Si and Al[4]/Si ratios of the C-A-S-H. The dealuminization of C-A-S-H is mitigated for cement-slag paste as compared to pure cement paste. The depolymerized calcium and aluminum ions from C-A-S-H gel mainly enter the pore solution to maintain the pH value and form Al[6] in TAH, respectively. On the other hand, Mg2+ions exert an impact on the intra-transition between Al[6] species, from AFm and hydrogarnet to hydrotalcite-like phase. NO3- ions are interstratified in the layered Mg-Al structure and formed nitrated hydrotalcite-like phase (Mg1-xAlx(OH)2(NO3)x?nH2O). Results from both 27Al NMR and XRD data show that ettringite seems not to react with Mg2+ ions.