Hydration cement pastes

Fig. 1.22 The level of various anionic species present in the aqueous phase of hydrating cement pastes.

The differences in chemical composition are accompanied by differences in the morphology of the tobermorite gel. Spicular or cigar-shaped rolled sheets are formed in the normal plain hydrated cement paste, whilst in the presence of calcium chloride, thin crumpled sheets or foils are formed. It has been suggested [16] that either the high lime content or adsorbed chloride prevents rolling of the sheets. 

Because of the complexity of hydrated cement pastes and the variety of possibilities for binding, a study of the binding of metal and metalloid ions to specific cement minerals is advantageous. The binding to single components of the hydrated cement paste can be compared to the teachability of an ion bound in a hydrated cement paste and deductions made as to the dominant mechanism that limits the solubility in the porewater. 

Whatever the fine structure and reactions associated with hydrating cement paste, the permeability of the hardened matrix will depend on the sizes of interconnected capillary openings remaining after hydration. 

Water retained after D-drying, known as non-evaporable water, has often been wrongly identified with chemically bound water. It excludes much of the interlayer water in C-S-H, AFm and hydrotalcite-type phases and much of the water contained in the crystal structure of AFt phases. It is often used as a measure of the fraction of the cement that has reacted, but can only be approximate in this respect, because the clinker phases react at different rates and yield products containing different amounts of non-evaporable water. Fully hydrated cement pastes typically contain about 23% of non-evaporable water, referred to the ignited weight. Copeland et al. (C38) determined the non-evaporable water contents of a series of mature cement pastes and carried out regression analyses on the cement composition. For pastes of w/c ratio 0.8 and aged 6.5 years, they obtained the approximate expression ... 

Na MAS NMR has provided useful information about the interaction between NaCl and the calcium silicate hydrate phases typically occurring in hydrated cement pastes (Viallis et al. 1999). The results suggest that the Na is absorbed together with its hydration sphere on the surface of dry calcium silicate hydrate, whereas in the hydrated material, the cations are located in a diffuse ion swarm on the calcium silicate surface. 

Concrete is a composite material made of aggregates and the reaction product of the cement and the mixing water, i. e. the porous cement paste. The structure and composition of the cement paste determines the durability and the longterm performance of concrete. Concrete is normally reinforced with steel bars. The protection that concrete provides to the embedded steel and, more in general, its ability to withstand various types of degradation, also depends on its structure. This chapter illustrates the properties of the most utiHsed cements and the microstructure of hydrated cement pastes. Properties of concrete and its manufacturing are discussed in Chapter 12. 

Figure 1.2 Dimensional range of solids and pores in hydrated cement paste [3]...

Figure 1.4 Example of microstructure of hydrated cement paste (scanning electron microscope)...

Figure 1.5 Influence of the water/cement ratio (o) and curing (fa) on the distribution of pore size in hydrated cement pastes [3]...

The decrease in capillary porosity increases the mechanical strength of cement paste and reduces the permeability of the hydrated cement paste (Figure 1.6). A distinction should be made between capillary pores of larger dimensions (e. g. >50 nm), or macropores, and pores of smaller dimensions, or micropores [3]. The reduction in porosity resulting of both the macro- and the micro-pores plays an essential role in increasing mechanical strength. 

A certain amount of water is contained in the pores of the hydrated cement paste. The actual quantity of water in the pores of concrete, i. e. the moisture content, depends on the humidity of the surrounding environment Several ions produced by the hydration of cement are dissolved in the pore hquid, so that in reality it is a quite concentrated aqueous solution. 

The chemical composition of the solution in pores of hydrated cement paste depends on the composition of the concrete, mainly on the type of cement, but also on the exposure conditions, e. g. it changes due to carbonation or penetration of salts. 

Water may be present in the hydrated cement paste in many forms that may be classified on the basis of the degree of difficulty with which it can be removed [16]. 

In moist environments, carbon dioxide present in the air forms an add aqueous solution that can react with the hydrated cement paste and tends to neutralize the alkalinity of concrete (this process is known as carbonation). Also other acid gases present in the atmosphere, such as SO2, can neutralize the concrete s alkalinity, but their effect is normally limited to the surface of concrete. 

T. A. Bier, J. Kropp, H. K. Hilsdorf, Carbonation and realkalinization of concrete and hydrated cement paste , Proc. of T Int. Rilem Congress From Materials Science to Con-... 

L. Bertolini, C. L. Page, W. Y. Shu, Effects of electrochemical chloride extraction on chemical and mechanical properties of hydrated cement paste . Advances in Cement Research, 1996, 8, 93-100. 

Fig. 5.18 potential of hydrating cement paste as a function of time and initial pH of the liquid phase, (according to [27])... 

At low fly ash additions— that is, below 15%—the extent of carbonation in mature fly ash concrete tends to be equal to or lower than that in similar concrete mixes with no ash, in spite of the lower calcium hydroxide content of the formed hydrated cement paste (Buttler et al., 1983 Hobbs, 1988 Goni et al, 1997). This is due mainly to the reduced permeability of the paste to CO2. However, at higher fly ash contents the resistance to carbonation is significantly reduced (Goni et al, 1997). 

Coleman, N.J., and Page, C.L. (1997) Aspects of pore solution chemistry of hydrated cement pastes containing metakaolin. Cement and Concrete Research 27,147-154. Cong, X. et al. (1992) Role of silica fume in compressive strength of cement paste, mortar and concrete. ACl Materials Journal 89,375-379. 

Mitchell, Hinczak, 1., and Day, R.A. (1998) Interaction of silica fume with calcium hydroxide solutions and hydrated cement pastes. Cement and Concrete Research 28, 1571-1584. 

Figure 23.3 Effect of temperature on free CaO/Ca(OH content of hydrated cement pastes made with different cements. QPC, ordinaty Portland cement BFSC, blast fiimace slag cement with 50% of granulated blast furnace slag PFA, fly ash cement with 25% of pulverized fly ash Trass, trass cement with 25% of trass SF, Portland cement with 10% of added sihca fume.

Ztirz, A., Odler, I., and Abdul-Maula, S. (1986) Thermal decomposition of hydrated cement pastes, in Proceedings 8th ICCC, Rio de Janeiro, Vol. 5, pp. 176-180. 

Calcium silicate hydrate (C-S-H) is the main constituent of hydrated cement paste and determines its cohesive properties.However these are being replaced by secondary cementitious materials (SCMs). These SCMs are generally alumina-rich and as a consequence some aluminum is... 

In the presence of portlandite (CH), the tetracalcium aluminum monosulfate hydrate (C ASHjj) is converted to ettringite when the hydrated cement paste comes into contact with free sulfuric acid from the wet oxidation of sulfides ... 

The content of SF is usually between 7% and 15% of cement mass as an addition or replacement and in most cases 8% is considered as optimum. A higher amount may adversely decrease the hydrated cement paste alkalinity below normal value of pH = 12.5 which is considered as necessary as protection against corrosion of steel reinforcement. This limitation does not concern special concretes like Reactive Powder Concrete (RPC) where even 30% of SF is used, e.g. in DUCTAL (Jovanovic et al. 2002), cf. Section 13.4.6). Indications for the use of SF in concrete may be found in the ACI 234R-06, ASTM C1240-05 and EN 13263 2005 recommendations. 

Bentz, D. P. (2006) Influence of alkalis on porosity percolation in hydrating cement pastes, Cement and Concrete Composites, 28(5) 427-31. 

Concrete is well known to be made up of two primary components stone and sand aggregates surrounded by a hydrated cement paste matrix. It is the latter which acts as the glue that hinds the aggregates together. It is also the hydrated cement paste that is the dominant factor when it comes to permeahihty, since the aggregates typically used in concrete tend to be far less permeable than the surroimding matrix. 

Examining the hydrated cement paste matrix reveals that there are two primary huilding blocks that make up its microstructure calci-um-siUcate-hydrate (C-S-H) and calcium hydroxide. The C-S-H takes the form of very small crystals packed closely together to form a very dense structure. The calcium hydroxide, on the other hand, forms much larger, layered, plate-like crystals. These crystals do not pack weU and tend to exhibit weakness between layers due to poor bonding. Ultimately, it is the calcium hydroxide that represents the weak link in both strength and permeability of the hydrated cement paste. 

The mechanical behavior of concrete should be viewed from the point of view of a composite material. A composite material is a three dimensional combination of at least two chemically and mechanically distinct materials with a definite interface separating the components. This multiphase material will have different properties from the original components. Concrete qualifies as such a multiphase material. Concrete is composed of hydrated cement paste (C-S-H, CH, aluminate, and ferrite-based compounds) and imhydrated cement, containing a network of a mixture of different materials. In dealing with cement paste behavior, basically it is considered that the paste consists of C-S-H and CH with a capillary system. The model of concrete is simplified by treating it as a matrix containing aggregate embedded in a matrix of cement paste. This model provides information on the mechanical properties of concrete. 

Electrochemical methods appear to have distinct advantages in the study of cement hydration. Methods involving potential measurement (including pH, zeta potential, and selected ion potential), conductivity measurement, and a.c. impedance measurement provide useful information related to both ion concentration of pore solution and microstructural change in hydrating cement paste. The early hydration and setting behavior of OPC-CAC and OPC-Hydrated-CAC paste systems can be determined using these techniques. 

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