Superplasticizers
2.3 Effects on the water–cement system
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2. The cement composition affects the rheological behavior of the system; cement pastes having low C3S/
C2S and C3A/C4AF ratios have a higher viscosity when the superplasticizer addition is delayed [27].
3. There is a relationship between the amount of superplasticizer adsorbed on to the cement and the apparent viscosity [28, 29]. The second reference indicates that the relationship is linear, while the relationship in the first reference is linear over at least part of the curve (Fig. 2.5).
4. At a fixed addition level of SNF, the Blaine surface area of the cement is directly proportional to the apparent viscosity [30].
2.3.2 Zeta potential
The zeta potential is the difference in potential between that of the total dispersed system and that of the layer at the interface of the dispersed particles (in this case cement) and the dispersing medium (water). Many studies have been made of the effect of superplasticizers on the zeta potential of the cement–water system from which the following conclusions can be drawn:
1. Cement has a positive zeta potential which is diminished and eventually becomes negative on the addition of a superplasticizer [31].
2. With SNF and SMF there is a correlation between zeta potential and reduction in paste viscosity [32].
Figure 2.5 Dependence of viscosity on the adsorption of SNF in cement paste.
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3. Also with SNF and SMF, there is a correlation between dosage and zeta potential up to a maximum negative value, which corresponds to a minimum paste viscosity [33].
4. For the sulfonate-based polymer types of superplasticizer, the zeta potential increases as the molecular weight increases, up to a maximum, as shown in Fig. 2.6 [34].
5. The zeta potential of the polyacrylate-based materials is significantly lower than that for SNF or SMF as shown in Fig. 2.7 [35]. In fact for a similar lowering of viscosity, the zeta potential for the polyacrylate products can be half that for SNF or SMF [36].
2.3.3 Adsorption
It had been noted early in the study of water-reducing agents of all types, including superplasticizers, that these admixtures do not remain in solution but are substantially and strongly adsorbed by the hydrating cement. The adsorption is studied by adding the admixture to the cement paste and after a period of time, filtering the system.
The filtrate is analyzed to give the quantity of admixture left in solution, and the amount adsorbed is calculated by difference. The following observations can be made:
1. Earlier work [37] by one of the authors indicated that superplasticizers of the SNF and SMF type were less strongly adsorbed onto the hydrating cement than normal water-reducing agents and this was used to explain why there was less retardation by the superplasticizers. This
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Figure 2.7 Zeta potentials of pastes containing CAE, SMF and SNF as a function of polymer dosage.
theory was flawed, in that later work showed that the part of the SNF that was effective (the polymer) was very strongly adsorbed, and it was the lower-molecular-weight by-products that remained in solution.
When the polymer alone is studied [38], the isotherms shown in Fig. 2.8 for three different cements are obtained.
2. SMF and SNF superplasticizers are adsorbed rapidly onto hydrating cement but this net effect is made up of very rapid adsorption by C3A and slower adsorption by the silicate phases, as shown in Fig. 2.9. [39].
3. Desorption experiments have shown that SMF and SNF are irreversibly adsorbed on to hydrating cement.
4. The amount of superplasticizer adsorbed is dependent on:
(a) Cement type, where Type III > Type I > Type II [40].
(b) Fineness, where the finer the cement, the greater the adsorption.
(c) The ratios of C3S/C2S and C3A/C4AF, where the higher the ratio the greater the adsorption [41].
5. The adsorbed layer of SNF appears to be 20–50 nm thick [42].
6. The new polyacrylate-based superplasticizers are not as strongly adsorbed as the SNF and SMF types [43], as shown in Fig. 2.10.
2.3.4 Effects on the products and kinetics of hydration
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There is no doubt that superplasticizers affect the manner and rate in which the individual components in cement react with water and with each other.
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Figure 2.8 Adsorption isotherms of polymer fraction of SNF on different cement brands (polymer adsorption is refered to 1 g of cement).
Figure 2.9 SMF adsorption on cement compounds and cement during hydration.
The chemistry is very complicated, even in the absence of superplasticizers, but the following is a summary of what has been established by the many workers in this field.
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Figure 2.10 Relation between adsorbed amount and dosage
(a) Interactions with C3A
1. It is generally agreed that both SNF and SMF retard the hydration of C3A [44, 45]. Figure 2.11 illustrates the heat evolution of C3A hydrated in the presence of SMF. Tests up to 28 days with SNF [46] indicate that the C3A remains less hydrated.
Fig. 2.11 Conduction calorimetric curves of C3A hydrated in the presence of SMF.
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Fig. 2.12 Conduction calorimetric curves for C3A + gypsum + H2O containing SMF.
2. SMF [47] and [48] retard the C3A hydration to the hexagonal phase as well as the subsequent conversion to the cubic form.
3. In studies with SNF [38] of varying molecular weight, there is an indication that the higher-molecular- weight fraction causes less retardation of the hydration process than the lower-molecular-weight materials.
4. In the presence of sulfates, both SNF [49] and SMF [47] retard the C3A hydration, as shown in Fig. 2.12 for SMF and the conversion of ettringite to the monosulfate hydrate is delayed, as illustrated in Table 2.2.
5. Recent work [50] has shown that in the presence of sulfate ions, SNF forms an organo-mineral compound with C3A, allows a greater dissolution rate of the CaSO4, and alters the morphology of the hydration products.
(b) Interaction with the C3S phase
1. Both SNF and SMF retard the hydration of C3S [51–53] as shown in Fig. 2.13 for SMF [54].
Table 2.2 Amounts of ettringite and monosulfate hydrate formed in the C3A–CaSO4. 2H2O system
Ettringite (%) Monosulfate hydrate
6 17.6 24.4 5.5 3.5
24 12.3 — 13.2 —
48 10.0 18.8 15.0 9.9
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Fig. 2.13 Influence of SMF on the conduction colorimetric curves of C3S hydration.
2. The early retarded hydration is followed by a period of hydration proceeding faster until at 28 days the degree of hydration is equivalent to or ahead of a control [55].
3. The type and quantity of sulfate and alkalis influences the degree of retardation of the C3S phase.
4. The molecular weight of SNF influences the degree of retardation of C3S; the higher the molecular weight, the greater the retardation [38].
(c) Interaction with cement
The interaction of superplasticizers with Portland cement is the most complicated situation of all because of reactions between the various components of the cement and the competition, for example between the superplasticizer and gypsum for reaction with C3A. However, in general:
1. The hydration is retarded in a similar manner to the individual components as shown in Fig. 2.14 [54].
2. The C3S phase is not as strongly retarded as for the individual material because the C3A strongly adsorbs a large proportion of the superplasticizer preferentially.
3. The formation of ettringite is accelerated.
4. The higher the molecular weight of SNF, the greater the retardation of cement hydration.
2.3.5 Interpretation in terms of a mode of action
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Fig. 2.14 Conduction calorimetric curves for Portland cement hydrated in the presence of SMF.
In the case of SNF and SMF, the ionized sulfonate groups on the adsorbed superplasticizer molecules are
strongly negatively charged and the repulsion between the cement particles overcomes the weaker Van der Waal forces of attraction, resulting in a dispersed system. The polyacrylate materials are similarly adsorbed and cause dispersion of the cement particles in part by the same electrostatic mechanism through the ionized carboxyl groups. However, the long flexible side chains attached to these polymers, especially those which are
ethoxylated, act as physical barriers, preventing the cement particles from coming within the range of the Van der Waal forces. This steric hinderence mechanism acts as an additional means of causing and maintaining
dispersion.
There is some retardation of cement hydration but at 28 days the products of C3S hydration are essentially the same as in an unsuperplasticized cement system. The C3A/gypsum reaction products may be changed
morphologically to a cubic rather than a hexagonal form.