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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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附件 1 : 原4_Plank_J,_Hirsch_C._Impact_of_zeta_potential_of_early_cement_hydration_phases_on_superplasticizer_adsorptionJ._Cement_and_Concrete_Research,_2007,_37(4)__537-542..pdf

2016-03-20 17:48:49, 271.89 K

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