White cements

Use. Titanium dioxide is mainly used in the production of paints and lacquers (55—60%), plastics (15—20%), and paper ( 15%). Other apphcations include the pigmentation of printing inks, mbber, textiles (qv), leather, synthetic fibers, ceramics, white cement, and cosmetics. 

Special purpose and blended Portland cements are manufactured essentially by the same processes as ordinary Portland cements but have specific compositional and process differences. White cements are made from raw materials of very low iron content. This type is often difficult to bum because almost the entire Hquid phase must be furnished by calcium aluminates. As a consequence of the generally lower total Hquid-phase content, high burning-zone temperatures may be necessary. Past cooling and occasionally oil sprays are needed to maintain both quaHty and color. 

Specialty cements. For special architectural appHcations, white Pordand cement with a very low iron oxide content can be produced. Colored cements are usually prepared by intergrinding 5—10% of pigment with white cement. 

Cement. Portland cement, a mixture of calcium siUcate and calcium aluminate minerals, is produduced by the calcination of argillaceous limestone or mixtures of limestone and clay (see Cement). Although other clays can be used, ka olin is preferred because of its alumina and siUca content and low level of impurities. It is especially desirable in the manufacture of white cement and other types requiring careful control of chemical composition. Air-floated ka olin, because of its low cost, is usually used. 

Other areas of application for TiOz pigments include the enamel and ceramic industries, the manufacture of white cement, and the coloring of rubber and linoleum. 

White cements can be made without discoloration problems caused by transition metal cations. 

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). 

AR. For lower ARs, the interstitial material consists essentially of ferrite of higher iron contents, and in white cements it consists of aluminate very low in iron, possibly together with glass. In each case, small amounts of silicate phases are probably also present. 

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. 

Calcium aluminate cements have also been termed aluminous or high-alumina cements, but the former term is less specific and the latter is more appropriately used for the white cements high in AljO, mentioned above. The cements described here are those manufactured in France and the UK. Those made elsewhere are similar in essential respects. 

In commercial calcium aiuminate cements, the only phases that hydrate signilicantly at early ages are normally CA and, if present. C,A-. Some sintered cements made at relatively low temperatures also contain free lime or CH. which are very active in the hydration process. The reaction of CAj present in many white cements high in AI2O3, is very slow (B117), possibly because of gel formation. 

White cement is a Portland cement manufactured from particularly pure raw materials with very low iron oxide contents. 

White cement Sol-gel dip-coating Removal of 3-nitrobenzenesulfonic acid [511]... 

White cement Samples of white cement having a thickness of 2 mm were placed on supports having a discoid form with a diameter of 3.2 cm and a thickness of 7 mm, containing 5% of T1O2 (Degussa P25) Removal of NOx and BTXE (Benzene, Toluene, Ethylbenzene and Xylene) [526, 528]... 

L. Cassar, C. Pepe, G. Tognon, G.L. Guerrini, R. Amadelli, White cement for architectural concrete, possessing photocatalytic properties , 11 International Congress on the Chemistry of Cement, Durban, South Africa, 11-16 May 2003, Volume 4, pp. 12. 

G.L. Guerrini, F. Corazza, White cement and photocatalysis part 1 fundamental . First Arab International Conference and Exibition on The Uses of White Cement, Cairo, Egypt, 28-30 Aprile 2008. 

The efflorescence on the surface of concrete have no practical importance from the durability point of view however, they affect badly the appearance of external decorative fagades of concrete buildings and other structures. They are particularly noticeable when the colored or white cement plasters are employed. From the aesthetic reasons the remedies for minimizing this phenomenon are needed. In the case of concrete pavers, very often colored, the efflorescence are particnlarly undesired. The producers of pavers are famiUar with this problem and look for the new remedial treatments to avoid it. 

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]. 

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