Fresh pastes

Commercial cowpea flour available in Nigeria has not been well received by consumers because of its poor water absorption and because akara prepared from the flour is heavy, lacks crispness, and lacks the flavor typical of products made from fresh paste (8). 

The bubbles are less than 0.25 mm diameter and probably do not exist in the fresh paste with diameters less than 10 pm because the high pressure present in such small bubbles would cause the air to be dissolved. 

Fig. 4.3 Backscattered electron images of polished sections of (A) a Portland cement clinker and (B) grains of a Portland cement in a fresh paste. In both sections, alite is the predominant clinker phase. In (A), the relatively large, darker areas are of belite, and the interstitial material consists of dendritic ferrite (light) in a matrix of aluminate (dark) cracks and pores (black) are also visible. In (B), the belite forms well-defined regions, which are rounded, striated and darker than the alite the interstitial material, present, for example, in a vertical band left of centre within the larger grain, consists mainly of ferrite (light) and aluminate (dark). Scrivener and Pratt (S28).

The total volume of paste per unit mass of cement is V + lwlc)ID, where is the specific volume of the cement and the density of the pore solution in the fresh paste. Assuming a value of... 

When cooked, fresh pastes are light to dark buff in appearance, depending upon the degree of conversion. As the pastes cool to room temperature they turn opaque, becoming nearly white. Low conversion white dextrins have little viscosity stability, and when cooled, thicken or set-back to form soft gels. By contrast, high conversion white dextrins cook-up to form pastes that congeal less, are more fluid, and exhibit better viscosity stability. 

Specimen Preparation. The specimens were prepared with the dosage of SBR dispersion or powder of 10% and water/cement-ratio of 0.40. The fresh paste was mixed round according to ISO 9597 1989. The specimens were demolded after being cured at 20°C and 90%RH for 24 hours, and subsequently were cured for 2 days (immersed in 20°C water), 6 days (3 days immersed in 20°C water followed by 3 days at 20°C and 50%RH) or 27 days (6 days immersed in 20°C water followed by 21 days at 20°C and 50%RH). 

As it has been mentioned, the hydration of CjA has a decisive impact on the rheological properties of fresh paste. The high rate of reaction with water leads to the saturation of solution with aluminate and calcium ions and as a consequence to the crystallization of C AHj. This corresponds to the quick stiffening of paste, determined as flash set. All the substances modifying the rate of CjA reaction with water by adsorption on the surface of this phase or by the change of the ions concentration in the liquid phase will have a great impact on the rheological properties of paste. 

If ground together with Portland clinker, limestone will predominate in the fine fractions, as it is significantly softer than the clinker. In a fresh paste made from such a cement, the fine limestone particles will fill the spaces between the coarser particles of clinker, and consequently the amount of water needed to attain a given consistency will decline, up to an optimum limestone addition. This amount of limestone may vary in different cements, up to as much as 25 wt%. 

Non-reactive constituents of inorganic cements do not react chemically in the course of hydration to any significant extent, even in the presence of calcium or alkaU hydroxides. They may, however, alter the rheology of the fresh paste and some properties of the hardened material mainly by their physico-mechanical action. 

The Portland clinker used should contain a high amount of tricaldum silicate, preferably more than 45%. This is necessary as the hydration of this phase produces the calcium hydroxide needed for a pozzolanic reaction of the ash. The hydration of the clinker minerals is mainly responsible for the setting and initial strength development of the cement, as the reaction rate of the fly ash is rather slow. The lydration of the ash contributes to strength only at longer hydration times, but also affects other properties of the hardened material. The calcium sulfate added in the form of gypsum or anhydrite serves to control the setting of the fresh paste in a similar way as in plain Portland cement. 

Banfill, P.F.G., and Gill, S.M. (1993) Superplasticizers for ciment fondu effect on rheological properties of fresh paste and mortar. Advances in Cement Research 5,131-138. 

Uchikawa H, Hanehara S, Sawaki D. The role of steric repulsive force in the dispersion of cement particles in fresh paste prepared with organic admixture. Cement Concrete Res 1997 27 37-50. 

In principle there are two different ways of blending the individual components when producing a fresh paste, mortar or concrete, as it is possible to (1) make a premix of the dry materials, which is later mixed with water and eventually aggregates to obtain the paste or the mortar, or (2) prepare directly the paste or the mortar from the individual ingredients. 

The second case - mixing the individual components with water - requires a thorough mixing process of the fresh paste or mortar see Section 1.6.1 for details. 

Figure 1.9 (a-d) Examples of powder mixers. The mixer in b can also be used to prepare fresh pastes and mortars. (Courtesy of J. Rossen, Ecole Polytechnique Federale de Lausanne, Switzerland [a] R. Snellings, Flemish Institute of Technological Research, Mol, Belgium [b] and C. Muller, Saint-Gobain Weber AG, Winterthur [c, d].)... 

Very fine particles such as microsilica are difficult to incorporate both in dry mixes and in fresh pastes, mortars or concretes. Further information and suggestions on how to treat such material are given in Section 1.6.1. 

Weigh the fresh paste inserted into the NMR tube. The amount of hydrogen within the sample can thus be calculated with the knowledge of the u)lc of the material. Note that this estimation of the hydrogen content will remain valid only if the sample is kept tightly sealed from early after mixing. 

---