In-situ monitoring of hydration kinetics of cement pastes by low-field NMR

Low field NMR technique was applied to investigate the hydration of cement pastes with different water to cement ratios or addition of superplasticizer. As a nondestructive method, this technique can be used to monitor the hydration kinetics process by following the changes of longitudinal relaxation time (T1) of water constrained in the pastes. The experimental results indicate that the T1 distributions of water in the fresh paste normally exhibite bimodal distribution, where the large peak is corresponding to the free water while the small one is contributed by the water stored in the flocculations. Time dependence of the weighted average T1 has a good agreement with the hydration process and could be divided into four stages, i e, initial period, dormant period, accelerated period and steady period. The hydration mechanism of each stage was described based on the theory of cement chemistry. In addition, the total signal intensity, which is proportional to the content of the physically bound water in the samples, decrease successively during the hydration reflecting the consumption of physically bound water by hydration reactions.