Fresh cement paste

The effect that air-entraining agents have on the rheology of fresh cement pastes can be considered from the point of view of changes due to the admixture itself, and those due to the presence of entrained air. 

To an extent that increases with the w/c ratio, fresh cement pastes exhibit the phenomenon of bleeding, i.e. settlement of the solid particles. The interparticle attractions are sufficiently strong that particles of all sizes settle at the same rate, typically about 2pms . Settlement also tends to increase the w/c ratio at the top and to decrease it at the bottom of the sample. It decreases with increased fineness or increased early hydration rate of the cement. In a concrete, it can produce layers of water beneath aggregate particles or reinforcing bars. 

Table 3 shows the test plan of wood-chip concrete. Fresh cement paste for injection was mixed by the double mixing method. The primary mixing water cement ratio was 0.23 and the chemical admixture was added to the primary water. 

The suitable fresh cement paste for injection into the void of wood-chip can be made by double mixing with the special admixture. The amount of primary mixing water is 0.23 in water-cement ratio. 

E.Tazawa and T.Kasai, Double mixing effect of fresh cement paste Proceedings of JICE No. 396/V-9,1988.8... 

Non-reactive constituents of inorganic binders do not react chemically to any significant extent in the course of hydration. They may, however, alter the rheology of the fresh cement paste and/or some properties of the hardened material by their physico-mechanical action. 

A polymeric dispersion may be interblended with the fresh cement paste to yield a... 

Consistency—degree of thickness of a fresh cement paste, mortar mix or concrete mix. 

The material s structure changes with time, mainly because during the hydration and hardening processes the fresh cement paste is becoming a hard and brittle material. This strong time-dependency is perhaps one of the characteristic features of concrete-like composites and their structures. 

The material behaviour in the fresh stage may be represented by different rheological models in which shear stress and displacements are related. Two of these models are described below. According to Newton s model, shear stress t is related to coefficient of viscosity tj and to the rate of shear 7 by the following equation t = rjy. This is the model of an ideal fluid and even fresh cement paste behaves in a different way than that due to many factors for example, attraction between cement particles in a relatively dense suspension. 

Figure 12.2 Two simple rheological models used for fresh cement paste ...

Uchikawa, H., Hanehara, S., Shirasaka, T., and Sawaki, D., Effect of Admixture on Hydration of Cement, Adsorptive Behavior of Admixture and Fluidity and Setting of Fresh Cement Paste, Cem. Concr. Res., 22 1115-1129(1992)... 

Solution of fresh cement paste in acid at the time t = 0 days... 

Coating a thim layer of fresh cement paste with the same W/C ration as that in the bulk paste on the smooth steel bar ... 

Uchikawa, H., D. Sawaki and S. Hanehara (1995). Influence of kind and added timing of organic admixture on the composition, structure and property of fresh cement paste . Cement and Concrete Research 25(2) 353-364. 

Cement samples are often mixed with distilled water here the fresh cement paste initially exhibits one single and long relaxation component. This rapidly evolves towards shorter relaxation times (as cement grains dissolve into water) and into different relaxation components (as cement reactions create hydrates). Numerous experiments on hydrated white cements have led to the classification of hydrogen protons in two main categories ... 

Zingg, A., L. Holzei A. Kaech, E. Winnefeld, J. Pakusch, S. Becker and L. Gauckler 2008. The Microstructure of Dispersed and Non-Dispersed Fresh Cement Pastes - New Insight by Cryo-Microscopy . Cement and Concrete Research 38 (4) 522-529. 

Solvent exchange with isopropanol has been proven to be the most suitable way to stop hydration without inducing significant changes on the pore structure. As mentioned above, for fresh cement pastes, cooled (5°C) isopropanol is recommended to preserve ettringite microstructure (Mantellato et al. 2012). 

Mantellato, S., M. Palacios and R. J. Flatt (2012). Reliable Specific Surface Measurement of Fresh Cement Pastes . In 14. GDCh-Tagung Bauchemie. Diibendorf, Switzerland. 

---