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Table 2 presents the zeta potentials and BET surface areas of the pure early hydration phases. With a zeta potential of + 4.15 mV, ettringite has by far the highest positive value for all phases. Its BET surface area was found to be 5.4 m2/g. Because of this positive zeta potential, ettringite should have the highest potential to adsorb negatively charged superplasticizers. A positive zeta potential of +2.84 mV, though not as high as for ettringite, was measured for monosulfate. Its surface area (5.3 m2/g) is comparible with ettringite. The zeta potentials for all other hydration products are either almost zero or negative, giving no favourable combination for the adsorption of negatively charged superplasticizers via electrostatic attraction. It should be noted that the absolute zeta potential values of the phases may vary during the precipitation process and also when formed from different cements. The reason being that it depends on the concentration of potential determining ions which will vary with cement composition and during the hydration process. The adsorption data of different superplasticizers on ettringite, monosulfate and syngenite is presented in Table 2.The adsorption of superplasticizers on portlandite and gypsum was negligible (b5%). The data allows the conclusions as follows: 1. The adsorbed amount of superplasticizer strongly depends on the existence of a positive zeta potential of the hydration phase. Therefore, ettringite is able to adsorb high quantities of negatively charged superplasticizers. Mineral phases with a zeta potential around zero or even a negative zeta potential do not adsorb significant amounts of superplasticizers. 2. High surface area by itself does not lead to adsorption if the zeta potential is negative or only slightly positive. Thus, a positive zeta potential is a key factor for adsorption. 3. At comparable specific surface area, ettringite shows 2–4 times more polymer adsorbed per surface area than monosulfate. 4. Generally, the adsorbed amount of polycondensates (in mg/g or mg/m2) is much higher than for polycarboxylates (approx. 20 vs. 3–5 mg/m2 on ettringite). This is in line with the experimentally determined anionic charge densities of the superplasticizers. It confirms that polycondensates achieve its dispersion capability mainly through electrostatic repulsion mechanism. 5. The adsorption ratio of polycarboxylates also depends on their anionic charge density: the higher this charge density,the stronger the adsorption. Therefore, PC-a showing high anionic character has much higher adsorption rate than PC-c. The very different adsorption behaviour of polycondensate and polycarboxylate type superplasticizers on early hydration products helps to understand the different dosages of superplasticizer required for cements of diverse composition. In many cases, these were attributed to variations in aluminate content in the cements [20–26]. Obviously, the ettringite formed from the aluminate phase of the cement and CaSO4·nH2O determines the dosage of superplasticizer to a great extent. Also, the rate of formation and the crystal size and shape (surface area) of ettringite will influence superplasticizer dosage. Monosulfate has less and all other early hydration phases show practically no effect on superplasticizer dosage. This underscores the importance of ettringite for cement–superplasticizer interaction. Only for ettringite, a big difference between the adsorbed amount of polycondensate added at the beginning or at the end of the crystallization process was observed. Fig. 2 shows the adsorbed amounts for all superplasticizers for addition at the beginning and after completion of ettringite crystallization. As can be seen, the adsorbed amounts of polycondensates (PMS, BNS) are approx. 50% less when these superplasticizers are added at the end of the crystallization process. This effect might be explained by a decrease in zeta potential, specific surface area or rate of formation of ettringite during the crystallization process. Hence, when polycondensates are present during the crystallization process, higher amounts of superplasticizer arerequired [27,28]. This result confirms the benefit of delayed addition known frompractical use of polycondensates in concrete. The adsorbed superplasticizers not only change the zeta potential, but also the size and morphology of the hydrate phases. Fig. 3 shows an environmental scanning electron micrograph (ESEM) of two different samples of ettringite. The first sample was precipitated in the absence of a superplasticizer, whereas the second one was prepared in the presence of PMS. In the presence of PMS, the ettringite crystals are much smaller. Also, the morphology changes from long and thin to short and compact needles. Similar effects were found also for BNS and polycarboxylate type superplasticizers. Most likely, the changes in size and morphology of the crystals induced by superplasticizers are either based upon a higher rate of nuclei formation or on preferred adsorption on specific crystal faces, thus preventing normal growth of the crystals. In contrast, crystal size and morphology of monosulfate and syngenite which show a very low zeta potential and do not adsorb any superplasticizer, were not changed in the presence of superplasticizers (see Fig. 4). |
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2016-03-20 17:48:49, 271.89 K
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