Microstructural Analysis of C-(A-M)-S-H Formation in the Portland Cement Paste through the Internal ${\mathrm{Mg}}^{2+}$ Incorporation

Sulfate plays a critical role in portland cement. The internal gypsum (${\mathrm{CaSO}}_4·2{\mathrm{H}}_2\mathrm{O}$) incorporating with clinker controls the hydration of cement, while the external gypsum can attack the hydration products. Magnesium sulfate is renowned for sulfate attack on concrete. This study aims to investigate the impact of the internal ${\mathrm{Mg}}^{2+}$ addition on the composition and microstructure of hardened portland cement paste. Notably, the test results demonstrate that internal ${\mathrm{Mg}}^{2+}$ does not show the same aggressive effect as external ${\mathrm{Mg}}^{2+}$ on the destabilization of portland cement. Instead, internal ${\mathrm{Mg}}^{2+}$ infiltrates into the calcium-silicate-hydrate (C-S-H) gels, displacing the Al-O tetrahedra on the Si chain, forming C-(A, M)-S-H gels by converting Ca octahedra to Mg octahedra or Al-O tetrahedra to Mg-O tetrahedra. This process contributes to a decrease in the Al/Si ratio and mean chain length of the C-S-H gels. Additionally, the displaced Al-O tetrahedra from the C-S-H gels react with AFt, resulting in the AFm formation. Consequently, these structural changes in the C-S-H gels, along with additional pore filling through AFm precipitation, play a significant role in refining the pore structure.