Belite hydration

Belite hydration occurs analogously but this is significantly shifted in time after 28 days the degree of hydration is approximately 25 %, this corresponds to the hydration degree of alite after 1-1.5 day reaction with water. 

Rapid cooling of the clinker is preferred for many reasons, notably to prevent the reversion of alite to belite and lime in the 1100 1250 °C regime and also the crystallization of periclase (MgO) at temperatures just below 1450 °C. The magnesium content of the cement should not exceed about 5% MgO equivalent because most of the Mg will be in the form of periclase, which has the NaCl structure, and this hydrates slowly to Mg(OH)2 (brucite), which has the Cdl2 layer structure (Section 4.6). Incorporation of further water between the OH- layers in the Mg(OH)2 causes an expansion that can break up the cement. Accordingly, only limestone of low Mg content can be used in cement making dolomite, for example, cannot be used. Excessive amounts of alkali metal ions, sulfates (whether from components of the cement or from percolating solutions), and indeed of free lime itself should also be avoided for similar reasons. 

Unless otherwise stated, this chapter relates to ordinary Portland cements hydrated in pastes at 15-25°C and w/c ratios of 0.45-0.65. XRD powder studies on such pastes have been reported by many investigators (e.g. C38,M67). The rates of disappearance of the phases present in the unreacted cement are considered more fully in Section 7.2.1. Gypsum and other calcium sulphate phases are no longer detectable after, at most, 24 h, and tbe clinker phases are consumed at differing rates, alite and aluminate phase reacting more quickly than belite and ferrite. The ratio of belite to alite thus increases steadily, and after about 90 days at most, little or no alite or aluminate phase is normally detectable. 

Transmission electron microscopy of ion-thinned sections provides data at higher resolution than can be obtained with polished sections. Rodger and Groves (R24) described regions which had probably formed in situ from the ferrite phase, and which consisted of C-S-H, a hydrotalcite-type phase and a poorly crystalline phase containing iron that could have been the precursor of a hydrogarnet. The particles of this last constituent were almost spherical and some 200 nm in diameter. The same investigation also showed that much of the product formed in situ from alite or belite was essentially pure calcium silicate hydrate. 

For pastes of typical ordinary Portland cements cured for 3-12 months, the CH content found by thermal methods or QXDA is typically 15-25%, referred to the ignited weight (P29,R14,M37,H37,T17,D12). Pressler et al. (P29) found that for pastes of various ages of ordinary (US Type I) Portland cements, it was linearly related to the content of non-evaporable water, but that for cements high in belite (US Type IV), it tended to a maximum while the latter continued to increase. This is readily explained, since belite yields only a little CH on hydration. The author has noticed similar behaviour even with modern cements high in alite, and that the CH content can possibly even decrease slightly after 28-91 days (T5). il... 

The rates of reaction of the clinker phases are greatly influenced by the RH of the atmosphere in which curing occurs. For a typical Portland cement paste of w/c ratio 0.59 cured at 20°C and 100% RH, Patel el al. (P28) found the fractions of the alite, belite, aluminate and ferrite phases hydrated after 90 days to be respectively 0.94, 0.85, 1.00 and 0.51. If the RH was lowered to 80%, the corresponding values were 0.77, 0.19, 0.83 and 0.32. The hydration rate of the belite thus appears to be especially sensitive to RH. On the basis of earlier data from the literature, Parrott and Killoh (P30) concluded that the effect of RH on the hydration rate (da/d/) of each of the phases could be represented by a factor (RH — 0.55)/0.45. ... 

Temperature has a large effect, especially in the earlier stages of hydration for example, from QXDA, Copeland and Kantro (C39) found that in a Portland cement paste of w/c = 0.57, the fraction of the alite hydrated at 2 days was 0.28 at 5°C, 0.63 at 25°C and 0.81 at 50°C. Apparent energies of activation calculated from such data were 41 kJ moC at a = 0.6 and 26 kJ mol " at a = 0.7 for belite, a value of 56 kJ mol at a = 0.4 was obtained. The decrease in the apparent energy of activation in the case of alite was attributed to a gradual change in rate control from a chemical process to diffusion. 

Parrott and co-workers (P30,P32,P35,P33) described a more sophisticated method for modelling the hydration process. The fraction of the total water porosity that was below 4nm was calculated by multiplying the volume fraction of C-S- H by an appropriate factor, which depended on whether the C-S-H was formed from alite or belite, the temperature and the amount of space available. The constants assumed were based on experimental data obtained using a procedure based on methanol sorption (Section 8.3.4). The effect of drying was allowed for (P35) by introducing a factor of 0.7 - -1.2(RH — 0.5) for 0.5 < RH < 1, or of 0.7 for RH 0.5. These refinements allow some deviation from the Powers-Brownyard postulate of a fixed volume ratio of gel porosity to product. Typical results for the volume fractions of pores larger than 4 nm in mature pastes of a cement with an alite content of 56% were approximately 0.26, 0.16 and 0.07 for w/c ratios of 0.65, 0.50 and 0.35, respectively (P32). For the two higher w/c ratios, these results are near the capillary porosities of Powers and Brownyard, but for w/c 0.35 the latter value is zero. 

The hydrated material has been analysed by X-ray microanalysis and analytical electron microscopy. In a 3-day old paste, that formed in situ from alite or belite did not differ significantly in composition from the corresponding product in pure Portland cement pastes (H4). but at later ages Ca/ Si is lower and Al/Ca higher (R25,R26,T44,U 17,U 18,R42). Ca/Si is typically about 1.55, but the value decreases with age and ratio of pfa to clinker. Uchikawa (U20,U17) reported a value of 1.01 for a 4-year-old paste with 40% replacement of cement by pfa. Several of the studies (R25,T44,U20,U 17) showed that the C-S-H was higher in alkalis if pfa was present, but one cannot tell to what extent potassium or sodium apparently present in the C-S-H has been deposited from the pore solution on drying. For material close to the pfa particles in a 10-year-old mortar. Sato and Furuhashi (S92) found a Ca/Si ratio of 1.1-1.2. 

In Ciment Fondu, the ferrite phase seems to play no significant part in early hydration at 20 C, but at 30-38 C over 80% was found to have reacted by 2 months (C47). The melilite and pleochroite seem to be unreactive. When belite is present, silicate ions can be detected in the solution within a few minutes, but then disappear it seems that precipitation occurs and further dissolution is inhibited. Among the minor oxide components. TiO, and MgO mainly occur in the unreactive phases. Na,0 and K,0 scarcely affect the solution equilibria at early ages, as their concentrations are very low (M88). 

Belite reacts with water at a slower rate than alite but the end product is the same. It takes about 2 days for the hardening process to get started and about a year to be completed. The mechanical strength of fully hydrated belite is similar to that of hydrated alite. 

In the hydration reaction alite absorbs about 40% by weight of water, of which 24% is chemically bound, and releases 500 J/g. For belite, 21% by weight of water is chemically absorbed, only 250 J of heat per gram are released, and less than half the amount of slaked lime is formed compared with the reaction of alite with water. Hydration of the aluminate phase is the reaction, which consumes most water, up to twice its own weight of water can absorbed in the final product, and releases most heat, 900J/gram. 

The detailed structures of alite and belite are known. Both consist of isolated SiO, telrahedra, the so-called Q species. Following hydration to form Si-OH groups, there is a condensation reaction that results in pairs of SiO tetrahedra becoming joined as dimers, the so-called Q species [57]. Condensation reactions continue and these cause short-chain silicate species to form, with Q end groups and mid-chain units (ie, SiO tetrahedra joined at two comers to other SiO tetrahedra [62]). In addition, some of the mid-chain units contain aluminium rather than silicon, and so are CF(1 Al) species. 

The setting reactions of the varions tricalcium silicate cements are very similar, as described in Chapter 8. They also resemble the setting of Portland cement. The initial setting involves the hydration of the alite (Ca SiO ) and belite (p-Ca SiO ) phases to form a poorly crystalline gel phase consisting of calcinm hydroxide in calcium silicate hydrate (approximate formula CajSi O ) [102], After the initial hardening, further condensation reactions occur which improve the strength and give rise to short silicate chains within the structure [103],... 

Effects of common minor and trace elements derived from recycling waste materials in fuels and as raw materials for clinker production, as well as cement hydration, are summarized by Uchikawa and Hanehara (1997). Crystal size and optical property variations in clinker phases (alite, belite, aluminates, and ferrite), and their hydraulic reactivities, are shown to be related to concentrations of sulphm, magnesium, phosphorous, fluorine, chlorine, chromium, manganese, zinc, and many other elements. The cement industry is based in crystal chemistry. 

Ikabata, T. Honda, M. and Yoshida, H., "The Effect of Solid Solution of Minor Components in Alite and Belite to the Hydration Properties of Cement," Reviews, 42nd General Meeting of the Cement Association of Japan, 1988, pp. 26-29. 

Owing to the lower reactivity of belite, the overall rate of hydration, and along with it the strength development up to about 90-180 days, is slowed down with increasing belite and decreasing alite contents in the cement (Bei and Ludwig, 1990). At the same time the final strength of a Portland cement with an elevated C2S content may exceed that of an ordinary Portland cement, because more C-S-H and less portlandite is formed in the hydration of dicalcium than of tricalcium silicate. 

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