Calcium aluminate ferrite

The ferrite phase [calcium aluminate ferrite, 2Ca0(Al203, Fc203), abbreviation CjfA, F)]... 

The composition of calcium aluminate ferrite (the ferrite phase) in Portland cements may vary between about C2(Aq Fq 3) and C2(A3, Fg and thus it may be best expressed by the formula C2(A, F), even though the formula C4AF is also used occasionally. On the nanometer scale the stmcture of this phase consists of Ca ions, each of them surroimded by seven oxygen neighbors, combined with AP" and Fe ions distributed between octahedral and tetrahedral sites. In industrial clinkers the ferrite phase may also contain significant quantities of foreign ions the Fe is partly substituted by Mg and equal... 

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. 

C2S also gets under way. Tricaldum aluminate— and to a lesser extent calcium aluminate ferrite— lydrates, and yields additional amounts of AFt until the whole amount of calcium sulfate present in the original cement has been consumed. 

The ferrite phase (calcium aluminate ferrite C2(A,F)) is formed in the presence of Fc203 in the raw meal. The ferrite phase present in sulfoaluminate cements possesses a higher reactivity than in ordinary Portland cement, presumably because of its formation at a lower temperature (Sharp et al., 1999). It contributes to both the short-term strength and the ultimate strength of the cement. 

Calcium aluminate cement with a lower or intermediate AI2O3 content cannot be produced in rotary or shaft kilns of the type common in the manufacture of Portland clinker. The temperature range between incipient melting and complete fusion of the raw mixes is too narrow to permit successful clinkerization, in which a melt and solid phases must coexist. Moreover, the viscosity of calcium aluminate (-ferrite) melts is significantly lower than that of the calcium silicate-aluminate-ferrite melts formed when Portland clinker is produced. 

As noted in Section 3.3.1, the decarbonation of calcite is greatly favoured by intimate mixing with quartz or clay minerals. Under these conditions, much CO2 is lost before any free lime can be detected. The formation of C2S as an early product is well established, but the situation with the calcium aluminate phases is more complex, no one phase being dominant as an initial product (W11). The aluminate or aluminosilicate phases most often reported as early or intermediate products in laboratory experiments with pure chemicals or raw meals have been CA, CjjA, and, less frequently, gehlenite (W12,D6-D8,W13,L8,L9,I6,C9,C10,R10). CjA can form by 850°C (L9,C10) but seems usually to be a later product. Ferrite phase forms readily, and is initially of low Al/Fe ratio (D7,R10). 

The calcium aluminate still present reacts with the calcium hydroxide formed to tetracalcium aluminate hydrate. Calcium aluminum ferrite forms the analogous hydration product, which reacts more slowly. Part of the aluminum is replaced by iron ... 

The pellets are then fed onto the sinter strand and are heated by burning blast furnace gas. This ignites the coke and raises the temperature of the solids to about 1280 °C. The limestone is calcined and reacts with silica and alumina in the ore to produce a molten calcium silicate-aluminate-ferrite system, which bonds the iron ore particles and produces strong pellets. 

In the process formation two stages can be distinguish at the range of lower temperature to about 1300 °C, when the reactions proceed, as a rule, in the sohd state, in the presence of very low quantities of the liquid phase, and at higher temperature, at which it is already about 25 % of the melt. It is assumed that the dicalcium sihcate and calcium aluminate and ferrite are formed principally as the result of reactions in the sohd state. However, tricalcium silicate is formed by ciystalhzation from the liquid phase. 

The reactivity of ferrites toward water changes consecutively C2F CgAFj CgA2p [150]. The calcium aluminate and ferrite hydrates form the unlimited solid solutions. However, in hydrates generally is higher A/F ratio than in the anhydrous phase therefore some amount of Fe(OH)3 is formed [149], which can be described schematically ... 

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. 

Ettringite cements contain an aluminate donor such as monocalcium almninate, tricalcium aluminate, tetracalcium trialmninate sulfate, or tetracalcium aluminate ferrite, together with calcium sulfate dihydrate and in some instances also calcium hydroxide. In the hydration of such mixes ettringite is formed as the main or sole reaction product. If allowed to react umestricted, the hydrating paste exhibits a significant expansion however, hardened pastes with strengths comparable to those of other cements may be produced if the hydration is allowed to take place under mechanically restricted conditions. This may occur in completely closed steel molds, by which measure undesired expansion of the paste m be effectively prevented (Odler and Yan, 1994). 

In addition to polyvinyl alcohol/acetate and polyacrylamide, some other polymers have also been employed as the orgarric constituents of MDF cement, including polypropylene glycol (Hsu and Juaang, 1992) and hydroxypropyl-methyl cellulose (Drabik et al., 1992, 1998). As well as Portland and calcium aluminate cements, sulfoaluminate-ferrite-belite cement (in combination with hydroxypropyl-methyl cellulose) has also been employed as constituent of an MDF material (Drabik ei a/., 1992, 1997, 1998). 

Krivoborodov, Yu.R., and Samchenko, S.V. (1992) Sulfate-bearing solid solutions of calcium aluminates and ferrites, in Proceedings 9th ICCC, New Delhi, Vol. 3, pp. 209-215. 

The rate of cooling also affects the state of crystallization, the reactivity of the clinker phases and the texture of the clinker itself. For instance, rapid cooling will produce fine closely-intergrown tricalcium aluminate (C3A) and calcium alumino-ferrite [C2(A,F)] crystals, which react slowly with water. 

Ammonia synthesis catalysts of increased low-temperature activity, prepared from iron and potassium oxides, cobalt ferrite, and calcium aluminate. V. S. Komarov, P. D. Rabina, and L. M. Dmitrenko. SU 598632 (1978). 

Figure 4.1 XRD scans (IO°-45° 20 range) of typical industrially produced cements white Portland cement (WPC), plain portland cement (PC), calcium aluminate cement (CAC) and calcium sulfoaluminate cement (CSA). The diffraction peaks of the main phases are indicated alite (CjS M3), belite (P-CjS), aluminate (CjA), ferrite (C4AF), calcium aluminate (CA), ye elimite (Yee), anhydrite (Anh), gypsum (Gyp), gehlenite (Geh), mayenite (May) and magnetite (Mag).

Figure 4.3 XRD scans showing the effect of selective dissolution treatments for a port-land cement. Top residues of SAM middle residues of KOSH treatment bottom original untreated portland cement. The KOSH treatment dissolves the aluminate, ferrite, sulfate and most minor phases, leaving only the calcium silicate phases. In contrast, the SAM treatment dissolves the calcium silicate phases and thus concentrates the aluminate, ferrite and minor phases in the residue. In this particular example, the SAM treatment led to the clear identification of goergeyite (K2Ca5(S04)6 H20) as a minor phase in the cement.

Other reactions taking place throughout the hardening period are substitution and addition reactions (29). Ferrite and sulfoferrite analogues of calcium monosulfoaluminate and ettringite form soHd solutions in which iron oxide substitutes continuously for the alumina. Reactions with the calcium sihcate hydrate result in the formation of additional substituted C—S—H gel at the expense of the crystalline aluminate, sulfate, and ferrite hydrate phases. 

In the cement industry, the term hydration is used to describe a range of reactions between cement and water to produce a hardened product. A cement clinker particle is a multiphase solid having massive calcium silicate grains (50-100 pm) in a matrix of interstitial aluminate and ferrite. This is described as analogous to a distorted clay sequence, which traps regions of porosity-pore size distribution from nanometer to micrometer. 

Table 2.3 lists some phases containing MgO that are in varying degrees relevant to cement chemistry. It is not a complete list of phases with essential MgO in the CaO-MgO-AljOj-SiOj system. As seen in Chapter 1, some MgO is also taken up by all four of the major clinker phases, typical contents being 0.5-2.0% for alite, 0.5% for belite, 1.4% for the aluminate phase, and 3.0% for the ferrite phase. Magnesium oxide (periclase), like calcium oxide, has the sodium chloride structure it is cubic, with a = 0.4213 nm, space group Fm3m, Z = 4, = 3581 kgm (S5) and refrac-... 

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. 

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