Introduction
In the concrete industry, both Cl- and Na+ ions are substances which are strictly limited due to their negative influence on the durability of concrete, i.e., alkali-silica reaction (ASR) or chloride attack. However, in some particular cases, as for example areas where there is a lack of natural freshwater, i.e., islands, seacoasts and places near the sea, using seawater instead of freshwater can be not only inevitable, but also the only alternative way to make the cement-based materials without using the valuable resource of fresh water. Lacking effective and in particular direct methods, the study on Cl- and Na+ ion binding during hydration has rarely been reported. In this study, a specially developed Nuclear Magnetic Resonance setup was employed to study the Cl- and Na+ ion binding of cement pastes during hydration. In this study we also looked at the influences of compositions of the cement paste and cement paste samples with the different replacement of blast furnace slag as well as fly ash were prepared.

    Results
As an example in Fig 1 the measured normalized Cl- signals as a function of time during first 48 h of hydration in the cement paste mixed with different slag replacement are shown. As can be seen in all cases, the normalized Cl- signals are at first almost constant and then decrease as a function of time as the hydration starts. Indeed, the time at which the Cl- ions start decrease and hence start to bind is found to coincide to the time of hydration. The addition of slag in cement has minor impacts on the Cl- ions binding of cement paste during the hydration process, as can be seen in Fig. 1. At the 48 h, the remaining normalized Cl- signal of the reference sample is found to be slightly lower than that of the cement paste mixed with slag.



Fig 1. The normalized Cl signals as a function of time in the cement paste mixed with different slag replacement

The normalized Na signals as a function of time in the cement paste mixed with different slag replacement are shown in Fig. 2. Similar to the Cl- signals, the normalized Na signals first remain almost constant then continuously decreases at the hydration start. It can be observed that there is a delay in the decrease of the normalized Na signals, i.e., seen as a shift to the right (from to 20 h), when more slag was used in binder. This suggests that the addition of slag results in later Na+ ions binding during this period. At the end of the measuring time (48 h), the normalized 23Na signals of all the measured samples reached approximately the same value, i.e., 0.20.



Fig 2. The normalized 23Na signal intensity as a function of time in the cement paste mixed with different slag replacement


In order to study the binding preference between Na and Cl ions during the cement hydration process we can look at the ratio of Na+ to Cl- ions in the sample. The ratio of the normalized Na to Cl signals as measured by NMR for the cement paste made with different slag replacement and fly ash replacement as a function of time are shown in Fig. 3. In general, a similar shape of the Na to Cl ratio curves could be observed for the samples. The ratio of normalized Na to Cl signals at first remains constant for a few hours, after which an increase is observed up to maximum ratio in the order of 1.4. Afterward, the ratio of normalized Na to normalized Cl signals continually decreases as the function of time. As can be seen this maximum ratio is obtained as the hydration starts and the hydrogen signal decreases.
 




Fig 3.The measured ratio of Na to Cl obtained from the NMR measurements as a function of time in cement paste mixed with different slag replacement

It is widely accepted that the C3A will produce ettringite (Aft), which will hinder the further hydration of the cement particle when sufficient sulfates are in the pore solution [7,28]. It can be observed that in the first few hours, for both Na+ and Cl-, there is no binding and also no preferred binding. As the sulfates run out, the unreacted C3A can bind the Cl- ions forming Friedel salt. As a result, during the 28 h, more Cl- ions could be bound during the hydration compared with Na+ ions and the ratio will change. As the hydration progress, the reaction of another mineral phase, i.e., C3S, becomes dominant. Hydration products like C-S-H gels and portlandite (CH) are generated during hydration, which can bind Na+ ions and Cl- ions to the surface by adhesion. Besides the basal surface of the C-S-H gels carries negative charges. In order to maintain the charge neutrality, Na+ ions in the pore solution will leave the pore solution and enter the C-S-H gel. As a result, there will be a preferred binding of Na+ ions in the later stage.


    Conclusions
In this study it has been shown that using a specialized NMRsetup the 1H, 35Cl, and 23Na can be measured during early hydration providing direct insight into the dynamic binding of Cl- and Na+ ions and the microstructure development of the cement pastes. The consumption of the Cl- and Na+ ions is closely linked to the hydration, but there can be a clear time leg in the binding of both ions.

     

Yanliang Ji, Leo Pel, Xiaoxiao Zhang, Zhenping Sun, Cl- and Na+ ions binding in slag and fly ash cement paste during early hydration as studied by 1H, 35Cl and 23Na NMR, Const. Build. Mat. 266B,121606 (2021).


Yanliang Ji, Leo Pel, Zhenping Sun, NMR study on the early-age hydration and ion binding of the cement paste prepared with NaCl solutions, Cement and Concrete Composites 129, 104489 (2022).