White cement clinker

Tentative composition for aluminate phase in white cement clinkers. 

Table 1.2 includes an average composition for belites in ordinary production clinkers, based on the results of many studies by X-ray microanalysis (T2,K1.G3,B2,G4,S1,H3,H4) or chemical analysis of separated material (Yl). As with alite (Section 1.2.3), most of the results from different laboratories are in relatively close agreement. Belites in white cement clinkers have also been analysed (B3). Pure C2S, for comparison, contains 34.9% of Si02 and 65.1% of CaO. 

Because of the close admixture with other phases, which is often on a scale of lOpm or less. X-ray microanalysis of the aluminate phase in clinkers is frequently difficult or unreliable. Data have been reported for cubic, orthorhombic, pseudotetragonal or unspecified forms of the aluminate phases in ordinary clinkers (R1,K1,B2,H3) and for aluminate phase (G3,G4,S1,B3) and glass (B3) in white cement clinkers. Tables 1.2 and 1.3 include, respectively, average compositions based on these somewhat scanty data, and suggested site occupancies based on them. The values in both tables take into account both the experimental data and the requirement of reasonable site occupancies. 

Fig. 3.4 DTA curve for a white cement clinker raw mix, obtained in air at 20°Cmin . From Chromy (C9).

The white cement clinker should be produced from the pure raw materials, first of all with very low content of Fc203 and MUjOj, which are the most commonly occurring coloring oxides. 

Blanco-Varela, M.T. (1997) Cap2 and CaSO in white cement clinker production. Advances in Cement Research 9,105-113. 

Yo, S., and Guan, H. (1991) Effect of magnesium oxide in the production of white cement clinker (in Chinese). Wuhan Gongye Daxue Xuebao 13,10-14 [ref. CA 118/153076]. 

Liquid or pulverized solid fuels are blown into the kiln through a nozzle with primary air. Additional secondary air is drawn into the kiln through the clinker cooler. The flame in the rotary kiln must meet several requirements. The clinker must be correctly burned, so as to minimize its content of free lime, with the least expenditure of fuel. The ash from a solid fuel must be uniformly absorbed by the clinker. For normal Portland cements, the conditions must be sufficiently oxidizing that the iron is present as Fe however, for white cements, mildly reducing conditions may be preferable. Proper flame control also extends the life of the refractory lining of the kiln. Computer-aided or fully automated control of kiln operating conditions is increasingly used. 

For normal cements, the effects of reducing conditions are all undesirable, but for white cements, which contain very little iron, reducing conditions are preferred because they yield a whiter product. Locher (LI2) concluded that the bad effects of reducing conditions were avoided so long as the clinker left the kiln at a temperature of at least 1250 C and thereafter cooled rapidly in air however, reoxidation of Fe in alite below 1300°C has been observed to cause the formation of exsolution lamellae consisting of CjF and belite (Lll). 

The results will be less accurate for slowly cooled clinkers, as the compositions of the ferrite and possibly also the aluminate phases may differ significantly from those assumed here. At present, there are not enough data to deal with this problem. The method is not applicable without major modification to clinkers made under reducing conditions. It is doubtful whether the procedure is applicable to white cements, both for this reason and because they may contain glass. 

Concentration of these ions in pore solution of concrete plays a decisive role in the formation of efflorescence. Pore solution, in the case of ordinary cements, is always satinated with calcium hydroxide— the basic somce of efflorescence. The concentration of potassium and sodium is important too the potassium content is always several times higher. These both components occm in pore solution in the form of hydroxides, as a result of reaction with calcimn ions. Particularly sodium is a very mobile ion and in the case of concrete produced from alkali activated slag cement, with sodimn compound addition and without Portland cement clinker, Na+ ions migrate to the smface and the white sodimn carbonate effloreseenee is formed. 

The lack of iron and high sihca ratio causes the sintering of clinker difficult and the mineralizers must be apphed. The fluorite is commonly used, but the more effective fluorosilicates and aluminates, for example Na3AlFg can also be applied. The CaCl2 appears in some reports, because there is an opinion of iron volatilization possibility, in the form of FeCl3 [35]. In spite of the mineralizers addition, the free CaO content in white cements is generally higher, on the level from 2 to 3 %. 

The colored cements can be produced from the white clinker. The technology consists in mixing white cement with mineral dyes, most often the metal oxides, added from 1 to 3 % by mass. There are some reports deahng with alternative colored cements technology, consisting on the production of colored chnkers [37]. 

Isometric, orthorhombic, tetragonal, and monoclinic forms of tricalcium aluminate (CajAl O ) with a melting point of approximately 1542°C are termed CjA in the cement industry. Tricalcium aluminate normally consists of uniform, small, xenomorphous to rectangular crystals (1 to 60 pm) in low-alkali or alkali-free clinker. CjA may comprise as much as 18 percent in ordinary clinker. Crystals show poor cleavage parallel to (001), conchoidal fracture, and a hardness of 6. Tricalcium aluminate is normally isotropic in cross-polarized light, colorless (in white cement) to tan and brown in transmitted light in ordinary portland cement, with an index of refraction of 1.710. It reflects gray on unetched polished surfaces and etches blue to... 

Portland cement, the binder most commonly used in civil engineering applications, normally has a gray color, which is caused by the presence of trivalent iron in its structure. Portland clinkers made from iron-free raw meals are white, and are used in the production of white cement (see section 2.7). These are also used mainly for decorative purposes. 

Fig. 10 Portland cement clinker micrograph obtained with reflected light elite dark grey, mostly with straight boundaries belite light grey, curved boundaries ferrite white matrix aluminate dark inclusions in white matrix...

Except in the manufacture of white cement, water is used only for after-cooling the clinker. The recuperation of heat must not be affected by it. The cooler should be so amply designed that it will be necessary to have recourse to water cooling only... 

Figure 4.18 Progressive refinement cycles in the data analysis of an XRD scan of a hydrated cement (a) fitting result when adding only the clinker phases refined in the original anhydrous cement (b) addition of the crystalline cement hydrate phase to the refinement (c) fitting result including a C-S-H peaks phase extracted from a 7-year-hydrated white cement. (From Snellings, R., A. Salze and K. Scrivener. Cement and Concrete Research 64 89-98, 2014b.)...

Ciment Fondu is normally made by complete fusion of limestone and bauxite at 1450-1600 C. In order to produce a cement with the desired rapid-hardening properties, both raw materials must be low in SiO,. The molten clinker is tapped off continuously from the furnace, solidifies and is typically crushed and ground to a fineness of about. 00 m- kg . Some iron is reduced to Fe . The colour of cements produced from bauxite can vary from yellow brown to black, but is commonly greyish black. White calcium aluminate cements are usually made by sintering calcined alumina with quicklime (calcium oxide) or high-purity limestone. 

Chromy, S., "Mechanism of White Clinker Formation," Sixth International Congress on the Chemistry of Cement, Supplemental paper, III-8, Moscow, 1974, pp. 2-18. 

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. 

Cooling of the clinker with water, rather than in air, has been foimd to be particularly effective (Sharma et al, 1992). Under these conditions the average size of periclase crystals that crystallize from the melt in the comse of coohng is reduced, white their number is increased. In the hydration of such a cement the stresses caused by the conversion of periclase to brucite are more evenly distributed within the hardened paste, and the formation of cracks due to the presence of large MgO crystals may be prevented or reduced. Some reduction of the size of the formed periclase crystals may also be achieved by finer grinding of the raw meal. 

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