Effects of chloride content on early hydration performance of cement pastes

Journal article, 2023

In this paper, the effect of chloride ion content, ranging from 0% to 5%, on the early hydration performance of cement pastes was comprehensively investigated using isothermal calorimetry, low-field NMR, resistivity, and rheological tests. The results show that the hydration is accelerated, and the hydration peak is continuously advanced by increasing the chloride content from 0.5% to 2%. However, chloride content of 5% initially has an inhibitory effect, but after 19 h, it has an accelerating effect. The T2 spectrum exhibits a noticeable shift from 10 to 100 ms toward the range of 1–10 ms during the acceleration period, and the changing tendency of relative signal intensity (RSI) is consistent with the heat of hydration. The results of low-field NMR provide an explanation for the variation of early strength of mortars containing chloride. The effect of chloride ions on resistivity appears after mixing. The resistivity of chloride-containing pastes should be controlled by hydration and free chloride ions in the pore solution. Besides, the rheological properties are also affected by the chloride ion admixture and hydration process. The flowability is reduced with 0.5% and 1% Cl- but improved with 2% and 5% Cl-. The results provide a comprehensive characterization of the effect of chloride ions on the early hydration of cement pastes from various perspectives, and the results from different methods are consistent.