Characterisation of calcium aluminate cement hydration: comparison of low-field NMR and conventional methods

The early hydration process of calcium aluminate cement (CAC) was characterised by low-field nuclear magnetic resonance (LF-NMR), isothermal calorimetry (IC), X-ray diffraction (XRD) and scanning electron microscopy (SEM). It was revealed that LF-NMR characterisation of the hydration process gives results very similar to those of IC; both show that the hydration reaction is faster in about 10–17 h. In addition, the hydration process of CAC could be quantitatively divided into four stages according to the second derivative of LF-NMR signal amplitude. LF-NMR is also used in the study of CAC microstructure. The results of the T₂ distribution curve prove that there are mainly two kinds of pores of different sizes in the slurry, and they gradually become smaller, especially in more than 10 h. The weighted average T₂ and specific surface area decreased and increased, respectively, due to the gradual densification of the slurry. Owing to the conversion of CAH₁₀ and C₂AH₈ to C₃AH₆, the solid specific surface area decreased, resulting in the LF-NMR specific surface area curve slowing down or even decreasing in about 20–50 h of hydration. XRD and SEM results further validate the characterisation of hydrated CAC microstructure by LF-NMR.