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.