White Cement _report
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Transcript of White Cement _report
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Introduction
Portland cement is most often a gray or slightly brownish color due to the presence of
coloring oxides in the clinker. For certain applications, usually aesthetic ones, it may be
desirable to use a lighter or white Portland cement. While white cement is seen as a luxury
product, there are a number of practical applications. Superficially, white cement can be used on
its own for surface treatments, such as in a dry shake on flatwork, with or without a pigment.
Decoratively, white cement concrete can be used to make terrazzo, architectural fixtures, and
sculptures or exterior cladding. In this way, decorative concrete can be cast with the flexibility
of concrete, letting the imagination dictate the size and shape, while at the same time mimicking
more convincingly the appearance of natural stone. Practically, white cement concrete can be
used to increase safety or energy efficiency. Because it is highly reflective, it can either
highlight median barriers or increase the light in large industrial building. Alternatively, the
reflectance can be used to maintain the same level of light in a room with fewer light fixtures or
can be used to reduce the costs of heating/cooling. Examples of these uses can be seen below in
Figure 11.
(a) (b)
Figure 1: Practical and decorative uses of white cement and white cement concrete (a)
a median barrier, (b) a highly reflective floor1.
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Finally, white cement concrete can be used structurally, in the same manner as gray cement
concrete, with the added drama of color. Unlike the dry shake or exterior cladding, the color is
integral with the structure and less maintenance is required of the surface if there is chipping or
cracking exposing the interior concrete. A case study of structural white cement concrete, the
new Jubilee Cathedral (Dio Padre Misericordioso) in Rome, is discussed at the end of this report.
Particularly interesting is the attention given by Italcementi Gruppo, the precast concrete supplier
on this project, to aesthetic durability.
This report will address the following questions common in the discussion of white cement
concrete:
Where is white cement being produced? Where and how is it being used? In what ways do gray Portland cement and white Portland cement differ with respect to
chemical composition and manufacture?
How does this cement type influence mix designs, aggregate choice, and admixtureusage?
How is the construction process influenced by the use of white cement concrete? Are there differences in mechanical properties?
Figure 1: (c) a balustrade1.
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What are the long-term concerns that must be addressed with white cement concrete?
Geography
Though produced and available in all around the globe (North America, Europe, Africa,
Middle East, Asia), the world-wide market is dominated by the Aalborg White, part of Aalborg
Portland A/S which is the only cement producer in Denmark, whose world-wide production is
estimated at 1.5 million metric tons2.
As a luxury product, white cement concrete is most often seen in high profile buildings,
such as cathedrals and major hotels. Here, the white cement concrete is one of the architects
most novel tools and has been increasing in popularity. For such projects, it is necessary to use a
major producer of white cement to ensure that the large amounts of concrete have uniform color.
However, for the low-profile, decorative or practical applications mentioned earlier, local white
cement manufacturers, selling by the 20-50kg bag, are adequate and convenient.
White Portland Cement
Remove all the coloring components in Portland cement to make white Portland cement is
much easer said than done. The raw materials that are used must be chosen carefully and
screened to minimize the presence of coloring oxides, and the production process must be
monitored to maintain the lightness and whiteness.
Raw Materials
Because the gray color of concrete can be attributed to its iron content, it is expected that
the first step taken to make a lighter and whiter cement would be to keep the iron content in the
raw materials as low as possible. Lightness can be measured by reflectance, and compared to a
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very light reference standard (MgO powder is considered to have a reflectance of 100%).
Furthermore, other coloring oxides include manganese, chromium and magnesium, and though
the lightness of the resulting cement may be the same, there may be some hint of color that is
undesirable in the white cement3, 4
. Therefore, it is recommended to keep these coloring oxides
below the following values in the raw mix:
Table 1: Recommended maximum values of coloring oxides in raw materials in the production
of white cement clinker3.
Compound Recommendedlimit
Color influence on clinker
Fe2O3 < 0.4 % gray
Mn2O3 < 0.02% grayish-violet,
reddish-violet
Cr2O3 < 0.01 % greenish
MgO < 3% greenish-brown
To keep these values below these recommended values, the raw materials used to make
the clinker should have low amounts of these oxides. Examples of these are china clay
(kaolinite) and white chalk or limestone. However, if they do not contain enough free silica,
ground white sand is added to the mix5. To further lower the content of these coloring oxides in
the raw materials, it can be beneficial to remove the fine particles, in which the undesirable
(coloring) oxides are more particularly concentrated, by preliminary screening before the
material is fed to the crusher3.
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Production / Manufacture
As a result of keeping the iron levels as well as the levels of other coloring oxides low,
white cement manufacture requires higher clinkering temperatures than (gray) cement
manufacture does in order to achieve more or less complete combining of lime in the burning
process4. This is one reason why white cement manufacture is more expensive than gray
cement manufacture.
Keeping the raw materials constant, the resulting cement clinker can still have varying
lightness and color. Therefore, though much effort may be taken to ensure that the raw materials
have the appropriate composition, further care must be taken during the production process,
when other sources of coloring oxides are present and may adversely affect the lightness or
whiteness of the cement. These sources include the grinding media and mill lining materials and
the fuel used for burning the clinker. It is recommended that ceramic grinding media and mill
liners be used instead of metallic ones, because iron and chromium may be mixed in as it is
abraded from those surfaces. This is particularly crucial if there is a high amount of free quartz
in the raw meal. In addition, oil fuel is often used in place of pulverized coal in the burning
process to avoid contamination by coal ash3.
Titanium dioxide is sometimes added to cement for a brightening effect, but tests have
shown that it may not have any actual affect on the brightness of the cement. However, as will
discussed in the case study, titanium dioxide can be added to maintain the whiteness of the
cement over time.
Regulations and Standards of Practice
ASTM has no specification explicitly for white cement, though producers of white
cement will conform to the requirements in ASTM C 150 for cement Types I, II, III and V or the
corresponding European or country standards. Because there are such low amounts of C4AF,
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there are usually higher amounts of C3S and C3A, which makes Types I and III more common.
However, ASTM C 150 does not address the issue of the cements lightness or color6.
As mentioned before, it is useful to quantify the cements lightness using its reflectance
compared to a reference standard, which is the industry norm for rating the whiteness of a
producers cement. Ordinary commercial grades of white cement (have) a whiteness of 80 84
per cent; superior grades of world market quality have a whiteness of 84 88 per cent.
Whiteness values above 88 per cent are attainable only by using very special raw materials3.
Chemical Composition of Clinker
The following tables list the ASTM requirements for various types of cement and typical
chemical compositions of white cement available. Figure 2 is the product information given
from Aalborg White, which can certainly be classified as world market quality. It should be
noted that the lightness (brightness of colour) of the clinker is enhanced by grinding to a high
degree of fineness, so white Portland cements are generally finer than gray ones4.
Table 2: Comparison of gray and white cement compositions5
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Table 3: ASTM cement composition requirements7
Figure 2: Commercially available white cements, from Aalborg White8.
C3A levels are often very high for white cements, making it easier for cement
manufacturers to conform to Types I and III, though Types II and V are also available. This
cement available from Aalborg White conforms to all of those types. Because the raw materials
are so carefully chosen, white cement usually has a low alkali content, making alkali-silica
reaction less of a concern1.
Concrete Mix Design
As mentioned before, white cement can be used alone for surface treatments of gray
cement concrete. In white cement concrete, the aggregates added are usually colored.
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Therefore, to maintain uniform color in white cement concrete, the mix design will be changed.
It is generally richer in cement and has higher fine sand content6. Generally, it is favorable to
have a continuously graded aggregate, but surface effects can be made when gap grading
aggregates.
Because aggregates impart a pigmenting effect on the final concrete, aggregate choice is
often dictated by color. This is particularly true of the fine aggregates, and it is usually
recommended that light colored sand be used. If a very white concrete is desired, white coarse
aggregates, marble or limestone, are used. However, the white cement can allow dramatic color
play to be made with distinctly colored aggregates. This is done particularly for terrazzo. The
following figure shows what surface textures can be achieved with white cement and various
aggregates.
Figure 3. Various effects achieved with white cement concrete and white or colored aggregate8.
Mineral admixtures may still be added to (or used as replacement for) white cement in
concrete, but these should be as close to white as possible. For very white concrete, common
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pozzolans (fly ash) are not used. Instead, there are choices between white silica fume,
metakaolin, and slag1.
Chemical admixtures should also be colorless, and their use be evaluated on a test
specimen before actual usage. Interestingly, the testing of compatibility of white cement with
various superplasticizers has shown that white cement concrete will exhibit higher fluidity with
the same dosage of superplasticizer than gray cement concrete. This has been attributed to the
uneven adsorption of superplasticizer on different cement particles. It adsorbs more readily on
C3A and C4AF than on C3S and C2S and the more even the adsorption of (superplasticizer) on
cement mineral, the higher the fluidity of the paste will be. White cement will have a more
even adsorption because of its low C3A + C4AF and low alkali content. Thus, it exhibits higher
fluidity even though it is most often finer than gray cement. The results vary in degree for
various types of superplasticizer, but all those tested showed this same trend9.
(a)
(b) (c)
Figure 4: Effect of cement type
on fluidity, with varying
dosages of superplasticizer (a)
ordinary, (b) low alkali, (c)white cements
9.
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Construction
Even with the best white cement and mix design, however, one can still obtain variable
qualities of final product because of the construction methods used. With white cement concrete,
especially when using pigments, one must be more vigilant about keeping equipment clean,
maintaining quality of forms and finishing equipment, and using experienced finishers with the
right technique. Maintaining uniformity throughout the project cannot be stressed enough.
Batching of ingredients should be measured accurately and consistently1. Otherwise,
there will be local variations that will adversely affect the overall appearance of the structure. A
low water-cement ratio paste is almost always darker than a high water-cement ratio paste made
with the same cement, which can be seen in the figure below. Therefore, if there are variations
in water content from batch to batch, there will most likely be blocking of colors or shades.
Figure 5: Color variations with different water-cement ratios6.
The mixing procedure of white cement concrete is done in a similar manner to gray
cement concrete, though the mixing time is usually extended to ensure proper dispersion of
cement and pigment (if any) throughout the mixture. This is also necessary not only for color,
but also because concretes having higher cement content and more finely ground cement, which
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white cement concrete has, take longer to blend6. As with the measurements in batching, the
mixing times should be kept constant from batch to batch.
The usual placing techniques are still used with white cement concrete, but there are a
few points that can affect the appearance of white cement concrete in particular. Firstly, if there
is splattering of concrete onto the forms, there will be defects on the surface of the concrete.
This can be prevented by having a moderate slump and using an elephant trunk to guide the
concrete into place.
Formwork can also influence the color and surface condition of the completed concrete.
As with the metallic grinding media in the production of the clinker, the material of the
formwork can stain the concrete. For this reason, steel forms that impart coloring oxides are
either avoided or treated with rust-inhibiting compounds. Plastic forms, coated or uncoated,
have been used, but they may not be as durable unless a release agent is used. Untreated wood
can (also) discolor white or colored concrete1. Furthermore, the untreated wooden formwork
can absorb water from the surface of the concrete, resulting in local variations of water-cement
ratios that will, as mentioned before, different regions of lighter and darker concrete. Lacquering
can alleviate this as well as extend the life of the form. With all types of formwork, there is the
possibility of leakage that will also compromise local color and overall appearance. Tight forms
that do not leak will take care of this.
For white cement concrete flatwork, the finishing crew should be experienced so that
burning or blackening of the surface does not occur. This can happen when a metal trowel is
abraded by stiff concrete, but the primary reason is that the concrete is overfinished to the point
where the water-cement ratio is drastically reduced. Curing of white cement concrete should be
given as much or more attention as gray cement concrete. Plastic sheets should be avoided
because they can produce a mottled effect with incomplete contact to the surface of the
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concrete. Non-staining waterproof paper with a layer of dry sand have worked well. Curing
compounds are also safe to use without much risk of discoloration1.
Mechanical properties
The mechanical properties of white cement concrete are generally assumed to be the
same as those of gray cement concrete. However, because of the cement composition, there are
things to note about the strength evolution. Looking again at the chemistry of white cement,
there is generally more C3A and C3S than with gray cement. It is also ground finer. Because of
these reasons, the set time for white cement is usually shorter and the early strength usually
higher. The following figure compares gray and white cement concretes of different nominal
strengths at different ages. The early strength is much more pronounced in the concretes of
higher nominal strength. Note that in this experiment, the white cement used was coarser than
the gray cement. Therefore, it would be expected that for most white cements, the difference
would be even more marked10
.
Figure 6: Variations of cylinder compressive strength of concrete with age10
.
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Case Study
The Jubilee Cathedral, as seen in Figures 7 and 8, is a prime example of why and how
white cement concrete is used. It is located in the suburbs of Rome, where there are dozens of
cathedrals from hundreds of years of Roman Catholic Church history. This one, made to mark
the third millennium, still, as the architect Richard Meier puts it,
upholds the citys rich architectural tradition . . . (but) was always intended to be a
work of contemporary architecture, meaningful for our time and one that is marked by
openness. Transparency and light cascade down from the skylit roof, literally invading
the interior of the church.People in the atrium are enveloped with mystical light.
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Figure 7: exterior of the Jubilee Cathedral, Tor Tre Teste, near Rome11
.
The precast, post-tensioned white cement concrete panels are indeed contemporary while
still giving the air of marble that so characterizes the Roman Catholic Cathedrals of the past.
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durability. In what they have patented as Bianco TX Millenium, Italcementi have added
titanium dioxide to the cement to maintain its whiteness over time. The photocatalytic particles
contained in the white cement allow it, once it has hardened in the form of paste, mortar, or
concrete, to oxidize the organic and inorganic air pollutants in the presence of air and light.
This attention to aesthetic durability is warranted, especially when one considers the effect of the
urban pollution on such famous structures in nearby Greece (i.e. the Acropolis)12
.
Conclusion
White cement technology has been used for primarily the luxury cement market and was
developed not out of need, but out of creativity. The structures and various other applications
have exploited the flexibility in form despite its increased cost, both of money and of attention.