Hydration of the calcium silicate phases

Portland cement reacts with jarosite in the presence of water to form various Ca-Al-Fe silicate-sulphate-hydrate phases (Figure 2) and Ca-Al-Fe oxide ( Ca4Al2Fe08.5 or Ca2(Al,Fe)205). Among the various cement reaction products, the Ca-Al-Fe silicate-sulphate-hydrate and Ca-Al-Fe silicate-hydrate phases appear to be the most common. Table II shows the average electron microprobe-determined compositions of the calcium silicate phase in the Portland cement and of the Ca-Al-Fe silicate-sulphate-hydrate cement reaction products. As the compositions of these reaction products vary widely, the values in Table II are only indicative. 

Table II - Average Electron Microprobe-Determined Compositions of the Calcium Silicate Phase in the Portland Cement and the Ca-Al-Fe Silicate-Sulphate-Hydrate Cement Reaction Products (wt%) ...

In mixes with Portland cement as the main constituent, the setting is due to a rapid formation of ettringite and the hydration of calcium aluminate cement. The hydration of the calcium silicates has little influence on the setting process, but contributes to the subsequent strength development (Gu et al., 1994). In addition to the phases formed in the hydration of pure Portland and calcium aluminate cements, stratlingite (gehlenite hydrate, C2SAHg) may also be formed in the hydration process. 

The reasons for the fast setting of OPC+CAC blends are not fully understood. It is assumed that in blends high in Portland cement the rapid setting is due to the formation of ettringite in a reaction between the CA phase of the calcium aluminate cement, calcium sulfate present in the Portland cement, and calcium hydroxide formed in the hydration of the tricalcium silicate phase present in Portland clinker ... 

Studies of the kinetics of the C3S hydration in the absence and presence of accelerators show that the extent or degree of hydration of the silicate phase in the presence of calcium chloride is considerably increased, right up to at least 28 days, whether measured by the quantity of lime produced [6] (Fig. 5.8), X-ray analysis [15] (Fig. 5.9), or the amount of non-evaporable water [16] (Fig. 5.10). Figure 5.8 also shows that a small amount of TEA retards the hydration of the C S phase for a considerable time, and the trend... 

This chemical equation is not balanced because, in practice, x varies between the approximate limits of 1.8-2.2, y varies around a mean value of 1.0 and aq means that water is also combined in the material in an indeterminate amount. The idealised composition of the calcium silicate hydrate phase is Ca2Si04 aq. The reaction is rapid and continues for up to approximately 20 days. Considerable heat is evolved - in the order of 500 J per gram of powder -and care must be taken to remove this heat when forming large masses of concrete into structures. The reaction gives a high-strength product. 

The conditions of the calcium silicate hydrated phases formation given in Sect. 4.2.2 are of significant importance from the practical point of view. Depending on the temperature, time of theimal treatment, w/c ratio and the composition of initial mixture there are various intermediate phases produced. The reactivity of many phases, inert in relation to water, is markedly increased in the paste hydro-thermal treatment [216]. 

The hypothesis of the mechanism of healing is that the free calcium oxide in the cement and the calcium hydroxide liberated by the hydration of the tricalcium silicate of the cement, is carbonated by the carbon dioxide in the surrounding air and water. The carbon dioxide reacts with a solution of calcium hydroxide on the surfaces of the cracks. As the concentration of calcium hydroxide is reduced at the surfaces, more of it migrates from the interior of the material. Likewise, as the concentration of the soluble carbonates is reduced in the cracks, more diffuses in from the water phase. Calcium carbonate crystals precipitate and grow out from the surfaces of the cracks. The rate of diffusion of the carbonates is much greater into the cracks than into the solid, relatively nonporous cement paste. Therefore, the calcium carbonate crystals accumulate in the cracks... 

The C3A phase is subjected to competitive reaction between water and gypsum to form a small amount of the initial C3A hydrates covered by a protective coating of ettringite, and also a proportion of the C3S hydrates to form a calcium silicate hydrate of low calcium content in the region of 100 nm thick. 

In the presence of calcium lignosulfonate [58], the calcium silicate hydrate gel from the C3S and C2S phases tends to have a greater proportion of the crumpled foil morphology type than the corresponding system without the admixture. This observation tends to be made only at high concentrations of... 

The experimental considerations applying to calcium silicate pastes (Sections 5.1 and 5.2) are equally relevant to cement pastes. Of the methods so far used in attempts to determine the degrees of reaction of the individual clinker phases as a function of time, QXDA (C39,D12,T34,P28) has proved much the most satisfactory. Procedures are essentially as for the analysis of a clinker or unreacted cement (Section 4.3.2), but it is necessary to take account of overlaps with peaks from the hydration products, and especially, with the C-S-H band at 0.27-0.31 nm. The water content of the sample must be known, so that the results can be referred to the weight of anhydrous material. If a sample of the unhydrated cement is available, and its quantitative phase composition has been determined, it may be used as the reference standard for the individual clinker phases in the paste. 

Portland cement and high-alumina cements contain, in addition to calcium silicate phases, calcium monoaluminate, CaAl204 (or CA in cement chemist s shorthand, where C = CaO and A = AI2O3). The Al NMR spectra of this compound, in which the Al is exclusively in tetrahedral coordination, and a number of other calcium alu-minates have been determined (Muller et al. 1986), and more recently, using satellite transition spectroscopy (SATRAS) which has allowed the multiple tetrahedral sites in the various calcium aluminates to be distinguished (Skibsted et al. 1993). The NMR parameters for the synthetic aluminates and a number of their hydration products are shown in Table 5.4. 

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. 

Figure 19. Cement hydration process. Calcium silicate hydrates to form C-S-H, a quasi-amorphous gel of composition close to C3S2H3. The excessive rate of hydration of the aluminate phase is controlled by gypsum through the formation of a calcium trisidfoaluminate hydrate, ettringite. (Reproduced with permission from reference 5. Copyright 1991 Elsevier.)...

First Step. When polymer latexes are mixed with fresh cement mortar or concrete, the polymer partides are uniformly dispersed in the cement paste phase. In this polymer-cement paste, flie cement gel is gradually formed by the cement hydration and the water phase is saturated with calcium hydroxide formed during the hydration, whereas the polymer partides dqrosit partially on the surfaces of the cement-gel-unhydrated-cement partide mixtures. It is likely that the calcium hydroxide in the water phase reacts witit a silica surface of the aggre tes to form a calcium silicate layer.I It is confirmed that tire formation of the calcium hydroxide and ettringite in the contad zone between tire cement hydrates and aggregates is attributed to the bond between them.I lPl... 

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