Hydration of Portland cement in the presence of clay minerals studied by ²⁹Si and ²⁷Al MAS NMR spectroscopy

The effects of the addition of clay minerals on the hydration reactions and kinetics of alite and belite are investigated for a white Portland cement (WPC)–kaolinite (20 wt%) blend and a mixture of 90 wt% WPC and 10 wt% bentonite using ²⁹Si MAS NMR spectroscopy. 29Si MAS NMR spectra, used to follow the hydration from 12 h to one year, demonstrates that the two clay minerals have an accelerating effect on the hydration of both alite and belite. The relative intensities for the resonances from the calcium–silicate–hydrate (C–S–H) phase in these spectra show that addition of kaolinite results in a C–S–H phase that on average has longer chains of SiO₄ tetrahedra as compared to the results for pure WPC. On the other hand, addition of bentonite results in a C–S–H with slightly shorter average SiO4 chain lengths. Furthermore, the ²⁹Si MAS NMR spectra demonstrate that the basic structure of the kaolinite admixture is not affected by the hydrating cement and that kaolinite is not consumed during hydration. This observation is supported by ²⁷Al MAS NMR which also shows that the rate of formation of ettringite is very similar for the pure WPC and the WPC–kaolinite mixture. Finally, the ²⁷Al quadrupole coupling parameters and isotropic chemical shift are reported for octahedrally coordinated aluminum in kaolinite.