Clinker white

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. 

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. 

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

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). 

There should be little free lime. What there is should occur as rounded grains, typically 10-20 pm in size, and associated with alite and interstitial material. Lime appears cream in sections etched with HF vapour. Its presence may be confirmed by a microchemical test using White s reagent (5 g of phenol in 5 ml of nitrobenzene + 2 drops of water) long, birefringent needles of calcium phenate are formed. The test also responds to CH. Alkali sulphates occur in the clinker pore structure they are etched black with HF vapour, and inhibit the etching of silicate phases with which they are in contact. 

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. 

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. 

Orthorhombic or pseudotiigonal phase in clinker rich in alkalis In white clinkers In sulphate resistance clinker... 

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. 

Simultaneously it displace Fe from the silicate phases increasing its content in interstitial matter [31], Kawashima et al. [33] found that in the industrial clinkers, produced in the conditions far from the equilibrium, the solid solutions of MgO and Fc203 in silicates lower the whiteness. 

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. 

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... 

Disappearance of ferrite phase, iron transformed to metallic state, clinker color changes to white, alite decomposition structures vanish Extreme reducing conditions (Woermann, 1960)... 

If the encapsulation contains clinker fragments, a common practice for the writer, the sawcut surface companion to the thin section is ground and polished for use in reflected light in the normal manner for clinker examination. Thus, one can also observe the polished section characteristics of some of the raw feed particles. For example, pyrite (FeS ), a common constituent in limestones and a major source of sulfur in some plants, is easily detected and identified by its pale yellowish white color in reflected light. Metallic... 

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

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