Cement paste in concrete

Figure 7.2 Schematic image of the transition zone and bulk cement paste in concrete, after Mehta (1986).

Figure 13.10 Relative volume of pores larger than 0.5 pm in the interface, in bulk cement paste in concrete and in neat cement paste at age of 180 days, after Scrivener et al. (1988).

The performance of concrete depends on the quality of the ingredients, their proportions, placement, and exposure conditions. For example, the quality of the raw materials used for the manufacture of clinker, the calcining conditions, the fineness and particle size of the cement, the relative proportions of the cement phases, and the amount of mixing water influence the physico-chemical behavior of the hardened cement paste in concrete. In addition, the cementtype, nature of fine and coarse aggregates, water, temperature of mixing, admixture, and the environment will determine the physical, chemical, and durability aspects of concrete. Thermal analysis techniques are widely applied to investigate the physico-chemical behaviors of cement compounds, cement, and concrete subjected to various conditions. 

C. W. Lent2, in Special Keport90, Structure of Portland Cement Paste and Concrete Highway Research Board, NRC-NAS, Washington, D.C., 1966. 

The graphs given in Fig. 1.38 show the logarithmic relationship between the water-cement ratio and the permeability coefficient of hardened cement paste. Thus concrete with a paste water-cement ratio of 0.4 will be almost impermeable. Water-reducing agents can be used to reduce the water- cement ratio, so ensuring that the permeability is kept to a minimum. 

Volume deformations are largely a function of the nature and quantity of the cement paste in the concrete and it has been shown [113] that studies on... 

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. 

A typical distribution of pore radii in the hardened cement paste of concrete was shown in Fig. 1.40 which indicated that the majority of pores lie in the region of 0.05 and 1.0 pm diameter and it is through these pores that water passes by applied pressure or capillary rise, as shown in Fig. 4.5(a). 

The type of data produced by H O and N adsorption is relevant to gel pores having radii up to about 50 A,2but larger pores and capillaries exist in the hardened cement paste and probably are more significant in determining the porosity or permeability of the hardened paste in concrete to gases and liquids. 

Unless a wet cell or cryo stage is used, the fine microstructure is much altered by dehydration in the instrument (J10,S41). However, localized drying occurs in any paste even before it is placed in a high vacuum, as soon as the RH falls below saturation. The water is lost initially from the wider pores, which are probably represented disproportionately on fracture surfaces. The state of the cement paste in a practical concrete may thus vary on both a macro and a micro scale between dry and saturated. 

Rheological studies on cement pastes and concrete have been reviewed in a book (T42) and several shorter articles (H40,L40,S76). The results in the literature show wide variations, many of which reflect the large effects of seemingly minor differences in experimental technique. Since cement pastes do not show Newtonian behaviour, methods giving only a single parameter are inadequate. Viscometers in which the material is studied in shear between a shallow cone and a plate, or between concentric cylinders, have generally been used. 

Fig. 12.3 SEM (backscattcred electron image) of a concrete showing gel (dark patches) formed by ASR, present in the cement paste in places far removed from the reactive aggregate. Bright regions are of unhydrated clinker. Courtesy WHD Microanalysis Consultants Ltd, Ipswich, UK (H64).

B142 Budnikov, P. P. and Erschicr, E. Ya., in Symposium on Structure of Portlaiul Cement Paste and Concrete (Sp. Rpl 90), p. 431, Highway Research Board. Washington (1966). 

The type of cement also influences the carbonation rate. In fact, concrete s capacity to fix CO2 is proportional to the alkahnity in its cement paste. In Portland cement, about 64% of the mass of the original cement is composed of CaO (mainly converted to solid portlandite) and about 0.5-1.5% of Na20 and K2O (mainly in solution as NaOH and KOH). If we only consider calcium oxide, the quantity of... 

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