White blended cements

White blended cements It is used to produce white clinker and white supplementary materials such as high-purity metakaolin. 

In more complex mixes the errors and detection limits are expected to be larger. A particular difficulty in decomposing hydrated cements is acquiring a suitable model for the C-S-H phase. Figure 4.15 shows a white port-land cement hydrated for 7 years. C-S-H is the main amorphous phase in this sample and a C-S-H peak model, thus, can be fitted and used to decompose similar hydrating cements. The situation becomes more complex for C-S-H formed in blended cements, which may show a slightly different peak profile and therefore quantification errors may increase. 

The deconvolution approach for anhydrous portland cement can be easily extended to hydrated portland cements and portland cement-SCM blends. For the latter systems, deconvolved subspectra for the SCMs, e.g. slags, metakaolin and natural pozzolans, can be established and incorporated in the optimisation routine. As an example. Figure 6.28 illustrates the deconvolution of the Si MAS NMR spectrum for a white portland cement hydrated for 2 weeks. The model used for this deconvolution includes subspectra of belite and Mm alite as well as single peaks for the Q Q (lAl) and resonances of the C-S-H phase. For the C-S-H phase. 

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. 

Two methods are available for the preparation of the powder (Smith, 1969). In one, zinc oxide is ignited at 900 to 1000 °C for 12 to 24 hours until activity is reduced to the desired level. This oxide powder is yellow, presumably because zinc is in excess of that required for stoichiometry. Alternatively, a blend of zinc oxide and magnesium oxide in the ratio of 9 1 is heated for 8 to 12 hours to form a sintered mass. This mass is ground and reheated for another 8 to 12 hours. The powder is white. Altogether the powder is similar to that used in zinc phosphate cements. 

Magnesium oxide is always blended with the zinc oxide prior to ignition. Magnesium oxide promotes densification of the zinc oxide, preserves its whiteness and renders the sintered powder easier to pulverize (Crowell, 1929). The sintered mixed oxide has been shown to contain zinc oxide and a solid solution of zinc oxide in magnesium oxide (Zhuravlev, Volfson Sheveleva, 1950). Specific surface area is reduced compared with that of pure zinc oxide and cements prepared from the mixed oxides are stronger (Crowell, 1929 Zhuravlev, Volfson Sheveleva, 1950). 

A colored cement may also be produced by blending an ordinary or white cement with appropriate amoimts—up to 10%—of a suitable pigment. The pigment ... 

Natural cement colors tend to be distinct from portland cement colors, typically ranging from yellow to oUve green to brown at full strength, and from warm buffs, to grays and off-whites when blended with hme. The colors can be difficult to match accurately by simply adding iron oxide colorants to other binders. 

The samples for the measurements of physical-mechanical parameters represented different types of Roman cement mortars, such as casts, renders with high and low ratios of cementiaggregate, and finally a blend of Roman cement with Kme. The latter, a rather common procedure to produce soft renders at their times, can be easily identified with the naked eye by its white lumps of badly dispersed lime and was confirmed through microscopic investigations. 

Figure 8.53 (a) EDS analyses from SEM and STEM compared for a white cement sample hydrated for 5 years (b) same comparison as (a) for a blend with 40% slag, hydrated for 5 years. 

Figure 9.13 shows typical MIP results for pastes composed of white cement, grey cement and a blend of calcium aluminate cement with calcium sulfate. 

---