Hydration superplasticizers

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... 

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],... 

Superplasticizers operate by adsorption onto the initial hydrates of C3A, C2S and C3S. In the case of C3A there is evidence that this is more than just a physical effect, and that an organo-mineral compound may be formed. 

Modem concretes often incorporate a mixture of chemical and mineral admixtures, each of which may interact with the various constituents of cements and influence cement hydration reactions. The admixture-cement interactions may in fact be viewed as the reaction between two complex chemical systems - the multicomponent, multiphasic inorganic materials in the cement and the organic compounds of multicomponent admixture systems. For example, lignosulfonate water-reducers are intrinsically complex mixtures of chemical compounds derived from the chemical degradation of lignin, while synthetic admixtures such as superplasticizers contain species with a broad distribution of molecular weights, reaction products, or other chemicals added for a specific purpose [125]. The performance of an admixture in concrete is highly dependent on many... 

First-generation superplasticizers are primarily anionic materials. They create negative charges on the cement particles, causing them to repel each other, thereby reducing surface friction. They have little or no effect on the hydration process. Because of the short period of workability produced, first-generation superplasticizers are normally added at the job site. Second-generation superplasticizers are more adsorbed on the cement particles. This action not only... 

The fact tliat both conventional water reducers and superplasticizers are more efTective if added some time after mixing provides a strong indication that adsorption probably occurs at least in part on the hydrated phases, as the anhydrous surfaces have by that time become covered with hydration products. Chiocehio cl al. (C57) found that the optimum time for addition was at the start of the induction period. More of the admixture seems to be taken up by the early hydration products, especially of the aluminate phase, if it is added before the early reaction has subsided. 

Costa cl al. (C58) showed that superplasticizers increase the fluidity of C,S pastes much as they do that of cement pastes. Studies on individual anhydrous and hydrated compounds in aqueous and non-aqueous media indicate that calcium lignosulphonate and superplasticizers are adsorbed by C -S-H, AFm phases or CH but not by C,S, C,A or C,AH (R54, R55,R56,C58,M 105), though they appear to be taken up by unhydrated P-CiS (C59). The admixtures also enter interlayer sites of C4AF1. and perhaps also of C -S--H (R55). Intercalation of organic molecules in C4AH, is a well-established effect (Section 6.1.1),... 

Banfill (B140) noted that the amounts of superplasticizers taken up by cement were sufficient to form a layer on the anhydrous grains some 60 nm thick. He concluded that multilayer adsorption occurred and that steric hindrance was the major effect. This argument is weakened by the evidence that uptake is largely by the hydration products and that significant amounts of material are absorbed as well as adsorbed. The bulk of the evidence indicates that increase in -potential is the major effect. 

H53.H62). The properties were attributed to a combination of effects. The particles of microsilica, being much finer than those of the cement, partially fill the spaces between the cement grains, and this, together with the superplasticizer, allows the latter to pack more uniformly. They also provide nucleation sites for hydration products, undergo pozzolanic reaction and probably improve the paste aggregate bond. 

Furthermore two types of polycarboxylate ether superplasticizer (PCE) with varying charge density of the backbone were chosen for the investigations. The backbone charge determines the adsorption velocity of superplasticizers on clinker phases and early hydration products and thus influences the time dependent rheological performance as well as the setting. Table 1 provides a detailed overview of the used admixtures. 

There was a decrease in hydration heat at all ages for all the HPC mixes with SCMs, and FA was more pronounced in decreasing the hydration heat than GGBS. In addition, the inclusion of set retarding-type polycarboxylate superplasticizer further decreased the hydration heat release rate and could delay the occurrence time of exothermic peak. 

Kinetics of hydration can be followed by estimating free lime. Figure 13.2 shows that control (without superplasticizer) has substantially more free lime than superplasti-cized composition.This shows that the superplastidzer decreases hydration by formation of a thin layer around particles of cement This affects the rate of liberation of free lime. " Figure 13.3 shows that substantially more water was used when the concrete did not contain the superplasticizer. Figure 13.4 shows that the concrete is stronger when the superplasticizer is incorporated. 

To achieve good space-filling between the cement particles, the particles of microsilica (or mtile) must be effectively dispersed in the liquid phase. This may be achieved by introducing a superplasticizer to the system, in addition to cement and the microfiller. Superplasticizers based on melamine or naphthalene may be used for this purpose. Even better dispersion can be achieved with polyacrylate based dispersing agents (Richard and Cheyrezy, 1995), but these tend to retard the hydration process. To be sufficiently effective, the superplasticizer must be added in relatively large amounts, typically 1.5-2.0% per weight of cement. 

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