Superplasticizing admixtures

A study of the hydration of cement and cement compounds in the presence of superplasticizers is useful for theoretical and practical considerations. Many t5 es of thermal techniques including DTA, DSC, TG, DTG, Conduction Calorimetry, and EGA have been used for such studies. They have yielded important results that could be correlated with physical and mechanical characteristics of cement systems. Investigations have included the measurementofheat of hydration, the mechanism of reactions, strength development, microstmcture, permeability, durability aspects, compatibility problems between cement and superplasticizers, the prediction of some properties, material characterization and selection, mathematical modeling of hydration, development of test methods, and cement-superplasticizer interactions. 

Collepardi, et dl.P applied DTA and XRD to the study of the hydration of C3A + Ca(OH)2mixtures containing 0.6% SNF. There were no substantial changes in the rate of hydration of this mixture in the presence of SNF. 

The adsorption and dispersion effects of superplasticizers on the aluminate phase have been studied.Olt l Adsorption of SMF on C3A occurs as soon as the solution comes into contact with the solid. The rate and ihe amount of adsorption on C3A far exceeds that on the C3S phase or cement (Fig. Even the hexagonal phase adsorbs large amounts of SMF. 

Adsorption is irreversible, indicating that a chemical interaction occurs between the hydrating C3A and SMF. Thus, the retardation of C3A may be explained by adsorption of SMF on the hydrating C3A surface. Similar 

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Anon. (1976). Superplasticizing Admixtures in Concrete, Cement and Concrete Association, 28. 

Despite wide publication of the adverse effects that retempering causes, job site addition of extra water to compensate for slump loss is still a common practice, particularly in hot-weather conditions. Under these conditions, the use of both conventional and superplasticizing admixtures helps to minimize the amount of water required for re-tempering so that the loss of strength is minimized [46, 47], This is shown in Table 7.14. 

HRWRA=high range water-reducing admixture (superplasticizer), admixture. AEA=Air-entraining Admixture. WRA=water-reducing ... 

CEMENT ADMIXTURES ASSOCIATION/CEMENT AND CONCRETE ASSOCIATION. Superplasticizing admixtures in concrete. Report of Joint CAA/C CA Working Party. Cement and Concrete Association, London, 1976. pp. 32. Publication 45.030. 

Water reducers and superplasticizers. Admixtures aimed at increasing the workability of concrete are chemical substances that improve the mobihty of the particles in the fresh concrete mix by increasing the effectiveness of the mixing water. They contain surface-active agents that are adsorbed on the surface of solid particles and modify their electrical surface charge, so that they do not tend to flocculate, and thus are more dispersed, and the water can better lubricate the mix. 

In practice, since the cement content cannot be increased too much, the water content has to be reduced in order to achieve low w/c ratios as a consequence, the workability of concrete decreases. Below a certain w/c ratio, which depends on the technique used for compaction, a proper consolidation of fresh concrete cannot be achieved and thus the strength of hardened concrete decreases (as shown by the dashed lines in Figure 12.2). The use of a superplasticizing admixture is therefore necessary to obtain a suitable workability in particular for a low water content. 

An optimization of mortar mixture proportions was carried out on mortars prepared with 10% waste particles, by adding a limestone powder as filler, at a dosage of 20% by weight of cement, replacing quartz sand, in order to improve the mortar mechanical performance. Limestone powder was added with and without an acrylic-based superplasticizing admixture, at a dosage of 1.0% by weight of cement. 

On the other hand, the addition of limestone powder to the mixture produced higher thermal conductivity as well as higher compressive and flexural strength. In particular, in the presence of a superplasticizing admixture and 10% PU particles, similar thermal and mechanical performance of the reference mortar could be achieved. 

Dhir, R.K., Zhu, W.Z., and McCarthy, M.J. (1998) Use of Portland PFA cement in combination with superplasticizing admixtures. Cement and Concrete Research 28,... 

Massazza, F., Costa, U., and Coibella, E., Influence of Beta Naphthalene Sulfonate Formaldehyde Condensate Superplasticizing Admixture on C3A Hydration in Reaction of Aluminates During Setting of Cements, CEMBUREAU Rept., p. 3 (1977)... 

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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