Cement superplasticizers

Zeecon in millimeters per 50 kg cement. Superplasticizers in percent of liquid by weight of cement. 

With some cement-superplasticizer combinations, various problems have been reported such as low fluidification effect, rapid slump loss, severe segregation, extended set retardation and loss of entrained air. These are briefly reviewed below. 

HIGH-PERFORMANCE CONCRETE (CEMENT-SUPERPLASTICIZER COMPATIBILITY)... 

In spite of the mat r years experiences in use of superplasticizers in modem concrete technology there are many unresolved questions. The most important is the conformity of the system cement-superplasticizer, determined as compatibility [385-393]. It is known that at any given admixture the concrete mixture, produced from any given cement, reveals required properties (good, stable woric-ability), while the results are not satisfactory for another cement. A situation which happened in Toulouse in 1989, where the loss of woikability followed 5 min after concrete mixture producing and another superplasticizer was necessary, would be a good example. 

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. 

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

In order to understand the way in which superplasticizers affect the properties of concrete, studies have often been made on cement pastes in view of the convenience of such investigations. 

Superplasticizers reduce both the yield value and plastic viscosity [24] of cement pastes. At higher dosages (for example 0.8% for SNF) [5, 6], the yield value approaches zero and the system becomes essentially Newtonian. 

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],... 

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). 

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 ... 

Cement has a positive zeta potential which is diminished and eventually becomes negative on the addition of a superplasticizer [31]. 

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],... 

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. 

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 ... 

When superplasticizers are used to reduce the water-cement ratio, normally any increase in air content will be minimal, especially in high-cement-content mixes. When it is a requirement to intentionally air-entrain such mixes, the dosage required to obtain a given air content is often considerably increased, presumably because of the reduced aqueous medium in the concrete. 

Superplasticizers cause dramatic increases in workability as measured by slump or flow table spread, or alternatively allow very large decreases in water-cement ratios to be made while maintaining workability. Figure... 

There is some indication that the higher-C3A-content cements lead to more rapid slump loss of superplasticized... 

Fig. 2.18 Slump loss at 21°C of superplasticized concretes with OPC and CAE or SNF ploymer-based admixtures. The figures on the slump-loss curves indicate the percentage of the superplasticizer active ingredient by mass of cement.

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 ... 

Fig. 2.19 Strength development of high-strength flowing concrete containing melamine-based superplasticizer compared to concrete made with 400 kg of normal Portland cement per m in the stiff to low workability ranges (25-100 mm slump). 

Fig. 2.20 The relationship between water-cement ratio and compressive strength of concretes containing superplasticizers.

Despite the fact that the minimum spacing factor is exceeded, the concrete freeze-thaw resistance of the concrete does not appear to be adversely affected [71]. Table 2.7 clearly shows how the presence of an SNF superplasticizer increases the spacing factor of the air void system at each of the three water-cement levels evaluated [71]. 

Rixom, M.R. (1998) International Workshop on Supplementary Cementing Materials, Superplasticizers, and Other Chemical Admixtures, CANMET/ACI, Toronto, Canada. 

---