WHITE CEMENT 2

WHITE CEMENT

Special cements differ from conventional Portland cements in their chemical and phase composition as well as in their properties. Their properties can be achieved by using a modified raw material mix, a grinding admixture or by adjusting the grinding fineness of the cement.

Special cements include a number of cements whose production volumes are considerably lower compared to those of conventional cements (max. 5 to 10 %). However, they constitute a significant part of the assortment supplementing the conventional cement.

These materials include cements with high early strengths, sulphate-resistant Portland cements, road cements, white cement, and colored cements, expanding cements, Portland cement with a low heat of hydration, Portland cement with a higher MgO content, insulating Portland cement, plasticized Portland cement, strontium and fungistatic cements.

The properties of white cement

The whiteness is understandably one of the most important properties of white cement. This must be considered already in the selection of the raw materials proper.

White cement is made from raw materials with a low content of coloring elements such as Fe, Mn, Cr, and Ti.

Use is made of high-grade limestone or chalk containing less than 0.15 wt.% Fe2O3 and less than 0.015 wt.% MnO, and of white clay, kaolin or by-products of its processing, and other materials which should not contain more than 1 wt.% FeO and 0.8 wt.% TiO2.

Gypsum, active and inert mineral additives are introduced in grounding stage and must also exhibit whiteness corresponding to cement of the given class.

The raw materials, the intermediate product and the final product must also be protected from contamination at all stages of the technology.

White cement may be manufactured with standardized surface-active plasticizers or hydrophobic additives (in amounts of up to 0.5 wt.%) not impairing the whiteness of the product. According to the whiteness degree, white cement is divided into three groups of the reflection coefficient: I. min. 80 %, II. min. 75 % and III. min. 68 %.

The whiteness degree is determined according to the reflection coefficient in percent of the absolute scale. White cement is manufactured in strength classes of 325, 400 and 475.

The typical silicate module of a white cement clinker is 4 to 6 (in contrast to 2.3 – 3 of grey Portland cement), and its aluminate module is 15-20 (that of grey Portland cement being 1.5 – 2.5).

MS = SiO2/(Al2O3 + Fe2O3)

MA = Al2O3/Fe2O3

In view of its low iron content the raw material melts at higher temperatures, at 1420 – 1450 C (conventional Portland cement at about 1250oC) with consequently high temperatures in the sintering zone (up to 1600 C).