Study on the Structure of C-S-H Gels of Slag–Cement Hardened Paste by ²⁹Si, ²⁷Al MAS NMR

In this paper, the ²⁹Si and ²⁷Al MAS NMR techniques were utilized to investigate the structure of C-S-H gels of slag cement-hardened paste. The results show that in the hardened paste of Portland cement, the silicate tetrahedron [SiO₄]⁴⁻ exists in the Q⁰, Q¹ and Q² states, and the C-S-H gel mainly exists as the dimer (Q¹). After hydration for 3 days, Al³⁺ replaces some of the Si⁴⁺ in the tetrahedra and generates an aluminum-containing C-A-S-H gel. As the hydration proceeds, the content of C-A-S-H gel increases gradually. With an increase in the replacement of cement with slag, the content of Q² (1 Al) in the C-A-S-H gel increases significantly, and the mean chain length of the C-(A)-S-H gel increases. For the slag–cement blend with 50% slag replacement, the mean chain length reaches 4.3, and the Al/Si ratio reaches 0.11. Throughout the hydration stage, three coordination states (AlO₄, AlO₅, and AlO₆) exist in the hardened paste of the slag–cement blends.