Hydration, of cement

In terms of reinforcement, hydrocalumite/polymer nanocomposites may also be considered as promising in the field of cement-related materials. The so-called macro-defect-free (MDF) cements are based on the filHng by polymer of the macroscopic voids responsible for the breakdown of cement. The hydrocalumite phase is formed during the hydration of cement and is known to readily incorporate polymers between its layers [71-75]. 

Fig. 5.9 The degree of hydration of cement pastes in the presence of calcium chloride in comparison to a plain paste measured by X-ray analysis (Young). 

Offsetting of the decreased rate of hydration of cement at low temperatures. 

The banning of calcium chloride during the last decade provided the impetus for the development of alternative materials which accelerated the hydration of cement without the potential for corrosion. A number of inorganic and organic compounds including aluminates, sulfates, formates, thiosulfates, nitrates, silicates, alkali hydroxides, carbonates, nitrites and calcium salts have been evaluated. Commercialization and field experience, however, is limited to only a few of these materials. 

Chemical interference with hydration reactions and or physical interaction with the hydration products may occur, resulting in the alteration of the rate of hydration of cement constituents, or in the composition and morphology of the hydrated products formed. 

Studies using ion-thinned sections, wet cells and backscattered electron images of polished sections show that a space develops between the shell and the anhydrous material (S40,S41,S68) (Fig. 7.6c). In this respect, the hydration of cement differs from that of C3S, in which the C-S-H grows directly over the C3S surfaces, without any detectable separation (S41). By 12 h, the spaces are up to 0.5 pm wide. They are likely to be filled with a highly concentrated or colloidal solution, and the shells are evidently sufficiently porous at this stage that ions can readily migrate through them (S68). The existence of spaces shows that reaction proceeds by dissolution and precipitation further evidence for this is provided by the fact that the C-S-H also deposits on the surfaces of pfa particles, if these are mixed with the cement (D28). Some other relatively unreactive or inert admixtures behave in the same way. 

Waters with a low content of salts. The most important process is leaching of soluble constituents of material. As far as concrete is concerned, particularly the release of Ca(OH)2 during the hydration of cement should be borne in mind. 

After the mix has set, the hydration of the less reactive fractions of the lime continues (as does the slower hydration of cement). This results in a progressive stiffening of the mix as well as a progressive rise in temperature. The latter increases the water vapour pressure within the hydrogen bubbles and if that pressure exceeds a critical level, it causes cracking within the cake, and loss of strength. 

The hydration of cement involves a number of exothermic reactions which liberate a great deal of heat and, when building substantial concrete structures, this must be removed to prevent cracking or other deterioration of the structure. The evolution of heat takes place over a period, and the rate of heat evolution is as important as the total amount of heat... 

During the hydration of cement paste, the gross volume of the mixture practically does not change, so that the initial volume, equal to the sum of the volumes of mixed water (V,) and cement (VJ is equal to the volume of the hardening product. As indicated in Figure 1.3 from Neville and Brooks [5], this consists in the sum of the volume of cement that has not yet reacted (Vuc)> hydrated cement (Vp + Vg ), the capillary pores that are filled by water (V ) or by air (VgJ. The volume of the products of hydration can be assumed to be roughly double that of the... 

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. 

Hydration of cement produces a solution that consists mainly of NaOH and KOH. Depending on the composition of the cement, the pH of the pore solution may be between 13 and 14. When concrete undergoes carbonation (Chapter 5) the pH of the pore solution drops to values approaching neutrality (pH 9) as a consequence of a drastic reduction in the concentration of hydroxyl ions. Penetration of salts from the environment may also lead to a remarkable change in the composition of the pore solution. 

During hydration of cement a highly alkaline pore solution (pH between 13 and 13.8), principally of sodium and potassium hydroxides, is obtained (Section 2.1.1). In this environment the thermodynamically stable compounds of iron are iron oxides and oxyhydroxides. Thus, on ordinary reinforcing steel embedded in alkaline concrete a thin protective oxide film (the passive film) is formed spontaneously [1-3]. This passive film is only a few nanometres thick and is composed of more or less hydrated iron oxides with varying degree of Fe and Fe [4j. The protective action of the passive film is immune to mechanical damage of the steel surface. It can, however, be destroyed by carbonation of concrete or by the presence of chloride ions, the reinforcing steel is then depassivated [5j. 

When the coloring of latex-modified mortar and concrete is required, alkali-resistant, weatherproof pigments are used. Furthermore, it is important that the pigments do not obstruct the stability of polymer latexes and the hydration of cements. Alkali-resistant glass, steel, polyamide, polypropylene, polyvinyl alcohol (poval), aramid and carbon fibers are employed as mixable reinforcements. Reinforcing bars for ordinary cement concrete are also used for the reinforcement of the latex-modified concrete. 

Figures 4.8l °l and 4.91 show the setting behavior of the latex-modified mortars and concretes respectively. The setting is delayed with an increase in the polymer-cement ratio. The slower setting does not cause inconvenience in practical applications. NR-modified mortar causes the most delay in setting. Usually, the reasons for the setting delay are that the surfactants such as alkylbenzene sulfonates and caseinates contained in latexes inhibit the hydration of cement.I J Rheological studies on PVAC-modified concrete by Zivical l revealed that the hydration of cement is inhibited by the adsorption of the surfactants on the binder surface.

The kinetic hydration of cement is widely studied in the literature and some of these papers reported the chemical, physical and mechanical behaviour [2-5], Simplified models were used by Knudsen[6], Basma et al [7], Schindler and Folliard [8], Bentz [9]. The majority of these models are empirical, based on experimental observations of macroscopic phenomena, and they take into account the effects of curing temperature, water-cement ratio, fineness, particle size distribution and chemical composition of cement [5]. 

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