Ferrite phase in Portland cement

Dibutylstibinic acid, 3 73 Dibutyltin diacetate, 24 823 Dibutyltin dilaurate, 24 823 Dicadmium triantimonide, 4 510 Dicalcium alumino monosilicate, phase in Portland cement clinker, 5 472t Dicalcium ferrite, phase in Portland cement clinker, 5 472t Dicalcium hexaborate pentahydrate,... 

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... 

Schwarz, W., Novel Cement Matrices by Accelerated Hydration of the Ferrite Phase in Portland Cement Via Chemical Activation Kinetics and Cementitious Properties, Adv. Cem. Based Mater., Vol. 2, 1995, pp. 189-200. 

There may be a chelating effect whereby TEA reacts with the ferrite phase of Portland cement [10], as illustrated in Fig. 5.2. 

In contrast with liquid phases with a high Si02 content, the lime-rich alumino-ferritic liquid in Portland cement clinker undergoes complete crystallization even when cooled rapidly. 

Composition of ferrite phase in clinker depends in great degree on the A/F ratio and also on the burning conditions. The compositions ate ranging from CgAp2 through C AF to CgAjF. Many authors found that in Portland cement clinkers of common composition ferrite phase has the composition close to brownmillerite [171, 180]. Ferrite phase has always the orthorhombic synunetiy. 

The grey colour of Portland cements is due to the presence of the names ferrite phases in absence of elements other than calcium, aluminium, iron and oxygen, calcium... 

The ferrite phase constitutes about 8-13% of an average portland cement. In portland cement the ferrite phase may have a variable composition that can be expressed as C2(A Fi ) where 0[Pg.46]

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.

The ferrite phase makes up 5 15% of normal Portland cement clinkers. It is tetracalcium aluminoferrile (CajAIFeOj) substantially modified in composition by variation in Al/Fe ratio and incorporation of foreign ions. The rale at which it reacts with water appears to be somewhat variable, perhaps due to differences in composition or other characteristics, but in general is high initially and intermediate between those of alite and belite at later ages. 

In many clinkers, the ferrite phase is closely mixed with aluminate due to a similarity in cell parameters, oriented intergrowth can occur (MIS). The close admixture often renders X-ray microanalysis difficult or unreliable. For ordinary Portland cement clinkers, the compositions found in dilferent laboratories are nevertheless remarkably consistent. Table 1.2 includes an average value based on the results of investigations using X-ray microanalysis (H8,K1,B2,U1,H3,B4) or chemical analysis of separated material (Yl). Table 1.3 includes suggested site occupancies corresponding to these data. 

Portland cement clinker emerges from a dry process kiln as rounded pellets, or from a wet process kiln as irregularly shaped lumps, in either case typically of 3-20 mm dimensions. Typical clinkers are greenish black, the colour being due to ferrite phase that contains Mg (Ml8) in the absence of Mg , they are buff. Reducing conditions in the kiln typically produce clinkers that are yellowish brown, especially in the centres of the lumps, where less reoxidation has occurred. Light colours can also arise from underburning. 

Fig. 4.3 Backscattered electron images of polished sections of (A) a Portland cement clinker and (B) grains of a Portland cement in a fresh paste. In both sections, alite is the predominant clinker phase. In (A), the relatively large, darker areas are of belite, and the interstitial material consists of dendritic ferrite (light) in a matrix of aluminate (dark) cracks and pores (black) are also visible. In (B), the belite forms well-defined regions, which are rounded, striated and darker than the alite the interstitial material, present, for example, in a vertical band left of centre within the larger grain, consists mainly of ferrite (light) and aluminate (dark). Scrivener and Pratt (S28).

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. 

In an ordinary Portland cement, only some two-thirds of the Fe occurs in the ferrite, the rest being contained largely in the alite and aluminate (Table 4.3). On hydration, the Fe " in these other phases probably does not enter a hydrogarnet, but goes into AFm phases or layers formed in situ. This would account for the observation by analytical electron microscopy that small amounts of Fe are present in the AFm phases. 

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. ... 

Regulated-set cement and jet cement are modified Portland cements in which the normal aluminate phase is replaced by CuA CaF, through the use of a raw mix containing CaF,. Uchikawa and Tsukiyama (U2I) gave chemical (Table 10.4) and phase compositions of two jet cements. Botli contained approximately 60% of alite, 20% of C, i A CaF,. 1% of belite and 5% of ferrite. Admixtures are required to control the rate of reaction of the C, 1 A CaFj and the nature of the products. One of the cements included a proprietary retarder based on citric acid, togetlier with 2"/o of CaC O, . The other contained 2.5% of hemihydrale. In each case, Na,S04 (T o) and anhydrite were also present. The specific surface areas were around 550 m kg . 

Portland cement clinker is composed of fora principal phases alite, which is close to tricalcium orthosilicate Ca3[Si04]0, belite close to dicalcium orthosilicate Ca2[SiOJ, tricalcium aluminate Ca3[Al20g] and ferrite C2(A, F). Chemical composition of these phases is complicated by solid solutions, which have the decisive influence on their reactivity with water. For the elements from which chrrker phases are composed isomorphism is typical and very developed phenomenon. The highest concentration of isomorphic elements is encountered in tricalcium aluminate (about 12-13%), and then in alumino-ferrites (about 10-11%), belite (about 6%), and the lowest in alite (about 4 %). 

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. 

In ordinary Portland clinker (corresponding to ASTM type I Portland cement) tricalcimn silicate is the most abundant phase, present in amormts between about 50% and 70%. Dicalcium silicate usually constitutes 15-30% of the clinker. Typical amounts of tricalcium aluminate are 5-10%, and of the ferrite phase 5-15%. In special Portland clinkers the individual clinker pliases may be present in increased or reduced amounts, or may be absent entirely. Free lime, free MgO and alkali sulfates are minor constituents of Portland clinker that are present only in small amounts or may be absent entirely. Calcium sulfate is only rarely a constituent of Portland clinker, but is intergroimd in limited amounts with it, to obtain Portland cement. In Table 2.1 different types of Portland cement are compared. 

As the typical gray color of ordinary Portland cement is due to the presence of the ferrite phase, low-iron clinkers may be used in the production of white Portland cement, if the Fe203 content is kept suffieiently low, and if appropriate measures are taken to eliminate or reduce the effect of residrral iron on the color of the resultant clinker. 

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... 

Belite (or belitic) cements are produced by grinding belitic clinkers with limited amounts of calcium sulfate (gypsum or anhydrite). Such clinkers contain belite (dicalcium silicate) as their sole or main calcium silicate phase. In addition, they contain tricalcium aluminate and the ferritic phase. Alite (tricalcium silicate) may also be present in some belitic clinkers, but only in very limited amounts. They differ both from ordinary Portland clinker and from Portland clinker with an elevated C2S content (see section 2.4) by having a lower CaO content, which results in a lime saturation factor of not more than LSF=80. 

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