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      White cement
      1 Introduction
        - White cement is a variety of portland cement that is made from carefully selected raw materials so that they contain 
          virtually no iron, manganese or chromium, or other materials that give it color.
        - White cement is seen as a luxury product.
        - It is used primarily for aesthetical reasons due to its whiteness.
        - White cement:
        - Applications can be:
          * Architectural fixtures
          * Terrazzo
          * Sculptures
          * Exterior cladding
          * Concrete elements
        - Some pictures of application:
      2 Raw materials
        - Limestone and kaolin are the main components to produce the clinker of white cement.
       2.1 Limestone:
        - Limestone is the main component of white cement (75-85%), a common type of white sedimentary rock of high
          chemical purity (98%), composed mainly of calcite (CaCO3 - 90%) and dolomite (Ca, MgCO3). 
        - When calcined, it gives rise to lime (calcium oxide, CaO). 
        - Other components present in its composition are iron oxide, fossils and other minerals. 
        - These components are necessary for the formation of the clinker in the later stages.
        - Limestone main derivatives are lime, calcium carbonate and cement.
       2.2 Kaolin:
        - Kaolin, or Chinese clay, is a very pure (low iron content) clay type, soft and white which retains its white color
          during cooking.
        - It is the second most important component in white cement (10-25%) and provides the necessary silica.
        - Its main constituent is the kaolinite mineral, a hydrated aluminum silicate, formed by the decomposition of
          other aluminum silicates, especially feldspar.
        - Chimical form: Al2Si2O5(OH)4
        - Presently, it is extracted in many areas all over the world.
       2.3 Production of clinker:
        - The process is similar to the grey cement.
        - But white cement manufacture requires higher clinkering temperatures than gray cement in order to achieve
          complete combining of lime in the burning process.
        - This is one reason why white cement manufacturing is more expensive than ordinary cement.
       2.4 Chimical and mineralogical components:
        - Below, a comparison between grey and white cement:
        - Concerning the color, coloring oxides make normal Portland cement gray or slightly brown, white cement has raw
          materials that reduce these.
        - Recommended maximum values of coloring oxides in raw materials in the production of white cement clinker are
          the following:
      3 Whiteness
        - Some theoretical reminders about color and whiteness:
      3.1 Definition of the color:
        - The color can be defined by the Hunter L,a,b scales.
        - This method is more visually uniform than the XYZ color scale developed before.
        - The Hunter L,a,b scales  utilize three values to create a three-axis graph on which the values can be plotted and color 
        - The three values utilized by Hunter are:
          * L = luminosity (the degree of lightness from black to white)
          * ± a = the degree of redness or greenness on a scale from red to green
          * ± b = the degree of yellowness to blueness on a scale from yellow to blue
        - These scales are based on the perception of color as a series of three oppositions:
          * black vs White
          * red vs green
          * yellow vs blue
        - These scales are utilized to provide an accurate and consistent description of color.
        - (From the text of PrintWiki – the Free Encyclopedia of Print)
      3.2 Representation:
        - Theses scales are represented below:
        - Whiteness is measured by comparing reflectance with a reference standard.
        - And is expressed in % where pure white is 100%.
      3.3 Classification:
        - Whiteness levels are the following:
        - Example of white cement clinker:
      4 White cement grinding process
        - Grinding circuits are similar to ordinary portland cement.
        - Ball mills in two chambers are normally used.
        - Due to the high Blaine often required, a closed circuit is necessary.
        - Components of the fresh feed are clinker and gypsum.
      4.1 Clinker:
        - Clinker of white cement is normally very soft.
        - On the Slegten scale, a normal grey clinker has a hardness around 120.
        - A normal white cement clinker has a hardness of 50-60.
      4.2 Gypsum:
        - As the grey cement, the main function of its addition to clinker is to regulate the hydration and cement setting.
        - The gypsum used in the manufacture of the white cement needs to be very pure.
      4.3 Cement properties:
        - The most important characteristic of the white cement is its whiteness.
        - Consequently, the Blaine fineness target is always higher than for the grey cement.
        - Blaine fineness figures:
        - Mostly, the mechanical properties are assumed to be the same for white and grey Portland cement.
        - But because white cement is ground finer, the set time is usually shorter and the early strength usually higher.
      5 Ball mill configuration
        - The ratio Length / Diameter is between 3 and 3,5.
        - The speed of rotation is around 75% of the critical speed.
      5.1 First chamber:
        - Due the clinker characteristics, the first chamber must be shorter than for grey cement.
        - The length of the first chamber is calculated in function of the specific energy required:
        - Example of calculation:
        - Balls of 90mm and 80mm diameter are not necessary and are not used.
        - The ball charge consists of balls of diameter 70, 60 and 50mm.
        - The lining is always a lifting lining (type STEP).
        - One must be especially vigilant on the quality of the balls (and linings) in such a way they do not pollute cement
          by excessive wear.
      5.2 Second chamber:
        - Balls of 60mm and 50mm diameter are not necessary and are not used.
        - The ball charge consists of balls of diameter (40), 30, 25, 20 and 17mm.
        - The lining is often a classifying lining.
        - The wear in the second chamber is less important but balls of good quality are necessary to prevent the pollution
          of the cement.
      5.3 Alternative ball charge:
        - In order to avoid the pollution of the cement, ceramic balls can be used.
        - But due to the lower bulk density of ceramic balls in comparison with casted balls, there will be a drastic drop
          of production.
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