Ferrite phase hydration

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. ... 

The ferrite phase hydration results in the formation of the same products as in the case of C3A, however, around the relics of ferrite grains the phase rich in iron is observed. Some authors suggested the presence of Fe(OH)3 but this was not proved experimentally. 

Hydration of fly ash cement differs from pure cement in terms of the hydration rates of the clinker phases, amount of calcium hydroxide formed, composition of the clinker hydration products, and additional hydration products from the reaction of the fly ash.I l Lower amounts of lime are formed in the presence of fly ash because ofthe pozzolanic reaction between the fly ash and lime formed in cement hydration. Fly ash generally retards the reaction of alite in the early stages and accelerates the middle stage reaction. The accelerated reaction is attributed to the existence of nucleation sites on fly ash particles. The aluminate and ferrite phases hydrate more rapidly in the presence of fly ash, and also there is a significant difference in the hydration rate of the belite phase up to 28 days. Table 1 gives the relative hydration rates of cement compounds in the presence of fly ash as derived from conduction calorimetry. [" 1 It can be seen that the earlier rates of hydration are generally retarded, and the later stage hydration rates are accelerated. 

The hydration of the ferrite phase (C AF) is of greatest interest in mixtures containing lime and other cement compounds because of the strong tendency to form soHd solutions. When the sulfate in solution is very low, soHd solutions are formed between the cubic C AH and analogous iron hydrate C FHg. In the presence of water and siUca, soHd solutions such as C3 ASH4-C3FSH4 may be formed (33). Table 7 Hsts some of the important phases formed in the hydration of mixtures of pure compounds. 

Table 6.7 Standard enthalpies of formation (A// kj mol" ) for some compounds relevant to hydration of the aluminate and ferrite phases (25 C except where otherwise stated)...

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. 

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). 

This is the consequence of low mobility of Fe ions which do not migrate but are located in hydrates formed in situ from the ferrite phase [27]. Apart from the iron ions in octahedral sites there are the AF+, Tf + and ions the latter ones are concentrated inbrownmillerite in clinker. The magnesium ions show also a low mobility in pore solution they do not produce bracite but hydrotalcite [Mg Fe, 2(011)2] (C03)o, 25(OH)o5 [27]. The formation of hydrotalcite in situ after brownmillerite is promoted by the highest concentration of magnesium in this phase. 

The role of the ferrite phase, generally identified as brownmillerite, should be mentioned too. In the case of sulphate attack this phase can be the source of almninate ions [237] moreover the ferrite ions can form the analogue of ettringite or to substitute the aluminate ions in all calcium aluminate phases [222]. The latter case is undoubtedly the most common one in the Portland cement paste. However, the reaction of sulphate ions with ferrites is slower. There is a view that the F/Al ratio in the hydrated phases is lower than in brownmillerite hence, some amount of iron(in) hydroxide is always present [222] (see also Sect. 4.1.1.). This hydroxide occurs in the gel-like form and therefore the diffusion of ions through the gel layer is slowed down. Therefore, the corrosion process is hindered. The other phases containing the Fe ions can be produced too, it is discussed in Chap. 3. 

Finally, it is known that the increase of ferrite phases content in clinker is advantageous. They are formed at low temperatures and produce the melt, which promotes the calcium oxide reaction with silica. The ferrite phases react quickly with water, giving with lime the hydrated componnds, analogous to the aluminate hydrates. 

The products formed in the hydration of the ferrite phase are similar to those formed in the hydration of C3A the Fe replaces to a limited degree in the crystalline lattice. The A/F ratio in the hydrates formed is usnally higher than that in the original calcium aluminate ferrite, and the fraction of iron that has not been incorporated into any of the hydrate phases remains in the hardened cement paste in the form of amorphous iron oxide, hydroxide or another trrtspecified iron-containing phase. 

In the hydration of high-iron Portland cement the calcium alrrminate ferrite phase reacts with the calcium sirlfate present to yield the AFt (ettringite) phase. As the... 

The C2(A,F) phase also hydrates very rapidly, yielding the AFt phase as a product of reaction. A large fraction of the ferrite phase is consumed within the first day of hydration (Kasselouri et al, 1995). 

Friedel s salt (C3A.CaCl2. IOH2O) or—more likely—a product in which part of the CaCl2 is substituted by Ca(OH>2 or CaCOj. This phase is formed in the hydration of calcium aluminochloride and also the ferrite phases. In the latter case Al may be partly substituted by Fe. ... 

The hydration of the ferrite phase is much slower than that of CA and and this... 

Out of the two calcium ferrite phases, C2F hydrates appreciably faster than CF. In paste hydration about 65% of the former and 10% of the latter l drate within 28 days at ambient temperature. Initially, low-basicity hydroferrites are formed as products of hydration, which contribute to a moderate strength of the hardened paste. However, after a few days of hydration these convert into cubic C3FHg and amorphous FefOHlg, and this conversion is associated with an almost complete loss of bonding properties. 

---