WHITE CEMENT - Ceramics-silikaty

1 INTRODUCTIONS pecial cements differ from conventional Portlandcements in their chemical and phase composition aswell as in their properties. Their properties can beachieved by using a modified raw material mix, agrinding admixture or by adjusting the grinding finenessof the CEMENT . Special cements include a number ofcements whose production volumes are considerablylower compared to those of conventional cements ( to 10 %). However, they constitute a significant partof the assortment supplementing the conventionalcement. These materials include cements with highearly strengths, sulphate-resistant Portland cements,road cements, WHITE CEMENT and coloured cements,expanding cements, Portland CEMENT with a low heat ofhydration, Portland CEMENT with a higher MgO content,insulating Portland CEMENT , plasticized Portlandcement, strontium and fungistatic present review has the aim to summarize theaspects concerning WHITE CEMENT as no article devotedto this subject has as yet appeared in the professionalliterature, not even in the form of review papers read atinternational congresses on the chemistry of CEMENT [1 - 4].

2 Exceptions are represented by some publicationsin which a separate chapter has been devoted to thesubject of WHITE CEMENT , as for example in the book byRojak and Rojak [5] or that by Butt, Sychev andTimashev [6]. Most extensive information can begained from the patent literature dealing particularlywith the issue of cooling WHITE CEMENT properties of WHITE cementThe whiteness is understandably one of the mostimportant properties of WHITE CEMENT . This must betaken into account already in the selection of the rawmaterials proper. WHITE CEMENT is made from rawmaterials with a low content of colouring elements suchas Fe, Mn, Cr and Ti.

3 Use is made of high-gradelimestone or chalk containing less than Fe2O3and less than MnO, and of WHITE clay,kaolin or by-products of its processing, and othermaterials which should not contain more than 1 and TiO2[5, 7, 8]. Gypsum, active andinert mineral additives are introduced in groundingstage and must also exhibit whiteness corresponding tocement of the given class. The raw materials, theintermediate product and the final product must also beprotected from contamination at all stages of thetechnology. The initial composition of the raw materialsor their properties are dealt with by [9 - 16].

4 WHITE CEMENT may be manufactured withstandardized surface-active plasticizers or hydrophobicadditives (in amounts of up to ) not impairingthe whiteness of the product. According to thewhiteness degree, WHITE CEMENT is divided into threegroups of the reflection coefficient: I. min. 80 %, 75 % and III. min. 68 %. The whiteness degree isdetermined according to the reflection coefficient inpercent of the absolute scale. WHITE CEMENT ismanufactured in strength classes of 325, 400 and typical silicate module of a WHITE cementclinker is 4 to 6 (in contrast to - 3 of grey Portlandcement), and its aluminate module is 15-20 (that of greyPortland CEMENT being - ).

5 MS= SiO2/(Al2O3+ Fe2O3)MA= Al2O3/Fe2O3In view of its low iron content the raw materialmelts at higher temperatures, at 1420 - 1450 C(conventional Portland CEMENT at about 1250oC) withconsequently high temperatures in the sintering zone(up to 1600 C) [8,17].The problems involvedin the manufacture of WHITE cementThe content of Fe2O3in WHITE CEMENT may varyover the range of to If the limit isexceeded, the iron oxides are responsible for a greenishshade of the clinker which would be undesirable forexample for architectural applications of WHITE CEMENT . The following reducing reactions of colouring ionstake place during manufacture of WHITE CEMENT clinkerin a reducing atmosphere:Fe3O4+ CO 3 FeO + CO2Fe2O3+ CO 2 FeO + CO2 MnO2 + CO MnO + CO22 CrO3+ 2CO Cr2O3+ 2CO2+ O2 [18].

6 Review paper158 Ceramics Silik ty 45(4) 158-163 (2001) WHITE CEMENT - PROPERTIES, MANUFACTURE, PROSPECTSKARTE INA MORESOV , FRANTI EK KV RADepartment of Glass and Ceramics, Institute of Chemical Technology, Technick 5, 166 28 Prague 6, Czech RepublicE-mail: January 6, 2000; accepted March 12, : WHITE CEMENT , Review, PropertiesThe colouring effect of Fe, Cr, and Mn cationsdepends on their valency in the crystalline phases incement. Oxidized forms of free oxides of Fe3+, Mn3+and Mn4+absorb more light than reduced forms Fe2+andMn2+. The higher the temperature and/or theconcentration of the reducing agent such as CO, thewhiter the CEMENT appears to human eye.

7 This isimportant particularly in the cooling of WHITE cementclinker. If it is cooled slowly from the initialtemperature in the kiln (1200 - 1100 C or lower) in thepresence of oxygen, the colouring oxides of higher-valent metals will be reformed. Contrarily, if the coolingis quick, the high-temperature reduced crystalline formsare "frozen-up" and the light absorption will be of the methods of bleaching WHITE cementTo increase the whiteness, the clinker is fired underthe conditions mentioned above and the bleaching isaffected by rapid quenching with water. The clinker at1250 - 1350 C is withdrawn from the kiln through aspecial outlet into a water bath and hence passes into adrying chamber where it is dried at temperatures notexceeding 300 C.

8 According to Grachyan, Gayzhotov,Zubyechin and others [5] the bleaching by quenchingproceeds in order to fix the phase composition of theclinker formed at high temperatures and exhibiting ahigh reflection degree. On slow cooling, the iron oxidesare bound to calcium aluminoferrites C6A2F, whereas onfast quenching they are fixed to a smaller number ofparticles of type C6A2F aluminoferrites while separatingwhite calcium aluminates. The whiteness of cementincreases with increasing content of tricalcium silicate(C3S) and aluminate, and decreases with increasingcontent of dicalcium silicate (C2S). In 1937, Cherepovskii and Aleshinova [9]submitted a method of bleaching in oxygen-freeatmosphere at 1100 - 1200 C, where at a lowconcentration of CO2, dissociation to Fe3O4 takes placeas a result of different partial pressures of the clinker obtained in this way acquires a higherwhiteness degree because Fe3O4suppresses thecolouring ability of iron oxides.

9 Implementation of themethod requires introduction of special bleachingcoolers inhibiting penetration of oxygen from theambient atmosphere. The gas employed for bleaching(containing O2< , CO2 >5 ), prepared byheating producer gas in special kilns, is then passed tothe hermetically sealed bleaching cooler. Clinker fromthe kiln passes into this bleaching cooler and is cooleddown to 200 C while not coming into contact with theambient atmosphere, with demonstrated by Zubyechin s experiment [5],positive results were obtained by firing the clinker in anoxygen-free atmosphere and subsequently bleaching itin a water bath.

10 Grachyan [5] has devised an efficienttwo-stage bleaching method. The clinker dischargedfrom the sintering zone is cooled for 1-2 minutes in aconverted gas and then enters a water bath. Theconverted gas is obtained by reacting natural gas withwater vapour at 900 - 1000oC according to the equationCH4+ H2O CO + 3H2. In statu nascendi, carbonmonoxide and hydrogen exhibit high reactivity andhave strong reducing effects on iron and manganeseoxides. The scientists also succeeded in achieving ahigher whiteness degree by bleaching in water with anaddition of magnesium, as well as in solutions of diluteacids hydrochloric, sulphuric and others.