Cement paste model

The moisture content of samples, in equilibrium, in the ESEM is varied at constant temperature (10°C), while lowering the pressure in the ESEM chamber from 9 torr to 2 torr. At the same time, relative humidity varies from 100% to 20%. The created chamber climate induces evaporation of the unbound (free) water in the cement paste (CP) samples without or with an embedded aggregate (to model simple concrete). It has been observed that curing conditions, sample age, water/cement (w/c) ratio, the presence of an aggregate, as well as the value of the RH, gives rise to different drying behaviour of the... 

The content of non-evaporable water, relative to that in a fully hydrated paste of the same cement, was used as a measure of the degree of hydration. Portland cement paste takes up additional water during wet curing, so that its total water content in a saturated, surface dry condition exceeds the initial w/c ratio. Evidence from water vapour sorption isotherms indicated that the properties of the hydration product that were treated by the model were substantially independent of w/c and degree of hydration, and only slightly dependent on the characteristics of the individual cement. The hydration product was thus considered to have a fixed content of non-evaporable water and a fixed volume fraction, around 0.28, of gel pores. 

Brunauer and co-workers (B55,BI08) considered that the gel particles of the Powers-Brownyard model consisted of either two or three layers of C S-H, which could roll into fibres. D-drying caused irreversible loss of interlayer water, and the specific surface area could be calculated from water vapour sorption isotherms, which gave values in the region of 200m g for cement paste. Sorption isotherms using N2 give lower values of the specific surface area this was attributed to failure of this sorbate to enter all the pore spaces. 

Fig. 8.4 Feldman-Sereda model of the structure of the C-S-H gel of Portland cement paste, showing C-S-H layers (lines), interlayer water molecules (crosses) and adsorbed water molecules (circles). After Ramachandran el al. (R32).

A number of systems besides porous rocks have been investigated. For instance, the two-site fast-exchange model has been apphed successfully to measure the pore space of cement pastes during hydration." Results show that two distinct pore volume components are present. " ... 

In the experiments carried out so far, it was shown that the longitudinal change behaviour of mortars is negatively influenced by polymer modification. In order to draw conclusions what kind of effect this has as far as cohesion in the PCCs is concerned, model tests were carried out on stone samples which were coated on both sides with modified cement paste. In preliminary works before, the development of the dynamic E-modulus as well as of the tensile strength of the cement stones had been investigated. 

The application of classic double layer model to the cement pastes is questionable because the surface in this case is not in thermodynamic equilibrium. The surface of cement grains reacts continuously with water and, as a result, the releasing of different ions into the liquid phase occurs and the surface charge varies all the time. Therefore opposite to the classic double layer its irmer part changes continuously. For this reason appeared the concept to replace the classic potential by the dynamic potential, which is changing continuously dining the hydration process [26]. However, the potential of hydrating cement is often measured and an example of these measurements results is shown in Fig. 5.18 [27]. 

Several stmctural models have been proposed for cement paste [1, 16]. The model proposed by Legrand[16] seems to be more rehable than the other ones. The irregular shape of cement grains is assumed and the flocculated stmcture is formed as a... 

The particular concepts of stmcture models of cement paste are based mainly upon the speciQc surface area measurements and the sorption properties of this material, as well as on the smdies of porosity and pore size distribution. 

The specific surface is measured based on the adsorption isotherm. As it is known, the expeiimental data are used for modeling of cement paste. 

There are two schools first of Bmnauer and the second one of Feldman and Sereda which, on the basis of different hypotheses, hy to explain the differences of cement paste porosity, measured with HjO sorption or other gases, principally N2 [35, 43, 48, 54]. These Itypotheses are connected rigorously with the proposed stmctural models for C-S-H phase. 

Fig. 2.3. Upper. Pattern obtained by cross-section of a hardened cement paste. Pores and pieces of matter of many different sizes can be seen, and no characteristic length scale is apparent. Lower. A model capturing the essential property of the real material, viz., it is constructed according to the same logic of repetition by dilation, whatever the observational scale...

The influence of freezing water in capillary pores is a very complex phenomenon and various processes accompany simple hydraulic pressure of ice on hardened cement paste, but the model proposed by Powers was universally accepted as explaining the destructive influence on concrete elements with acceptable agreement with experimental results. That model is considered as a basis for standardized requirements for the design of structures. Furthermore, there is strong experimental evidence that the distribution of the voids is a key factor governing the frost resistance of concrete. This statement is supported by experience and investigations in several countries where the temperature in winter falls below zero. An example of such... 

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. 

The selection of an appropriate model is made according to its ability to represent experimental results and also its simplicity. The properties of cement paste are not the only factors determining the behaviour of cement-based composites, concretes, mortars, fibre concretes, etc. Other components of the fresh mix, their volume fractions and properties are also important. That is why, for practical considerations, it is not necessary to use very complicated models for the cement paste. 

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

The C-S-H phase is the main binding agent in portland cement pastes. The exact structure of C-S-H is not easily determined. Considering the several possibilities by which the atoms and ions are bonded to each other in this phase, a model may be constructed. Figure 6 shows a number of possible ways in which siloxane groups, water molecules, and calcium ions may contribute to bonds across surfaces or in the interlayer position of poorly crystallized C-S-H material.1 1 In this structure, vacant comers of silica tetrahedra will be associated with cations, such as Ca". ... 

In order to prediet the performance of concrete, it is important to have a model of cement paste that incorporates its important properties and... 

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