Superplasticizers setting time

The effect that superplasticizers have on the setting times of concrete depends on a number of factors including the type of superplasticizer, cement composition, and particularly whether there is a simple addition of the admixture to the concrete or if a reduction in water-cement ratio is made. In general it can be stated that ... 

With a direct addition of superplasticizer to obtain highly workable concrete, initial and final setting times are invariably increased in the order SMF < SNF < polyacrylates. At normal dosages this increase rarely exceeds two hours for materials that are not intentionally formulated to retard. 

Concretes containing a cellulose ether AWA will have an extended set time. Those containing an acrylic AWA do not have extended set time except when combined with superplasticizers (Fig. 6.5). 

Admixtures are substances that are added during the mixing process in small quantities related to the mass of cement, in order to improve the properties of fresh or hardened concrete [7,8]. The most utilised admixtures are water reducers and superplasticizers that may be added to improve the workability of concrete or reduce the amount of mix water accelerators that are used to increase the rate of development of early strength of concrete set-retarders that reduce the setting time of concrete air-entraining agents that increase the freeze-thaw resistance of concrete (Section 3.1). Recently, corrosion inhibitors have been developed in order to increase the corrosion protection of embedded steel these will be treated in Chapter 13. 

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. 

Setting is not observed even after 54 h when the addition of superplasticizer is repeated for the third time [3 71 ]. At the second and third lignosirlphonate addition the viscosity is decreasing too, but the stiffening occurs practically irmnediately. Thus the method of repeated adrnixmre addition should be verified ejqrerimentally before its application in common practice [371], The mechanism of this phenomenon is not explained, particnlarly that the melamine resins do not retard setting. 

The use of set retarders is recommended in order to restrict the quick stiffening in the case when superplasticizer is added several times [377], The special compositions of superplastieizers are examined too [361]. 

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. 

Figure 13.4. Compressive strength of concrete with and without superplasticizer (0.25 wt% of sodium salt of P-naphthol sulfonate formaldehyde condensate) vs. curing time. [Data from El-Hosiny F I Gad E A M, J. Appl Polym. Set., 56, No.2, 11th April 1995, p. 153-9.]...

The physico-chemical phenomena associated with the setting of cement is complex. The time of setting is defined in ASTM in terms of the penetration of a needle to a specific depth. Ramachandranl l examined by DSC several cement samples containing water reducers, retarders, and superplasticizers that had just set. In all the mixes, an endothermal peak typical of Ca(OH)2 was detected, suggesting that at the time of set at least some CjS must hydrate. It was also observed that the superplasticized cement paste had hydrated to a lesser extent than the reference cement paste at the time of setting. This indicates that physical forces are also involved in the setting phenomenon. 

Superplasticizing admixtures are used widely in high performance concrete production. Not only do they influence the rheological parameters, but also the setting characteristics. These effects depend on the type and dosage of the admixture. In Fig. 17, the role ofthree types of superplasticizers, viz., 0.5% Ca-SNF, commercial SMF, or Na-SNF on the hydration of slag cements is examined. Addition of the superplasticizer results in the retardation in terms of the time of appearance of the exothermal peak and also a decrease of the peak intensity. Na-SNF retards most of the superplasticizers studied. 

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