Microstructural Analysis of C-(A-M)-S-H Formation in the Portland Cement Paste through the Internal Mg2+ Incorporation

Abstract

Sulfate plays a critical role in portland cement. The internal gypsum (CaSO4·2H2O) 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 Mg2+ addition on the composition and microstructure of hardened portland cement paste. Notably, the test results demonstrate that internal Mg2+ does not show the same aggressive effect as external Mg2+ on the destabilization of portland cement. Instead, internal Mg2+ 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.