Colored and Decorative Concretes and Concrete Staining

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Once, when architects and designers mentioned concrete, people rarely got excited. It usually meant a dull, grey, and visually unappealing. Recently though, thanks to new technological advances, concrete can be cast in a variety of finishes and colors, offering architects and the construction industry a material that is both eminently practical and aesthetically pleasing.

Colored Concretes

Since modern pigmented concrete products were introduced in the early 1950s, colored concrete has rapidly increased in popularity. In many towns and villages, it is now common for planning authorities to specify the use of colored building materials to help retain its traditional character. However, technology has advanced further, enabling artistic images to be incorporated in the very fabric of a building.

Colored concretes have been used to dramatic effect in new structural developments such as the Trafford Centre in Manchester, UK; the Basilica of Yamoussoukro on the Ivory Coast; and the European Parliament building. Dramatic effects are not only achieved in large structures though - colored concrete is just as effectively used in architectural pavements, paving stones and internal flooring.

Cementitious Floor Screeds

Cementitious floor screeds, first developed in Scandinavia, have become an integral part of the UK flooring and construction industry, but unlike the traditional resin floorings, cementitious systems cannot be pigmented to produce a vast variety of colors. However, research by Armorex Ltd in Suffolk has focused on producing bright, intensively colored screeds in which the color is consistent throughout the materials. 

Cemlevel is a revolutionary colored cementitious self-smoothing compound, containing metallic non-oxidizing aggregate 5-10mm thick. Cemlevel produces a durable surface with an aesthetically pleasing colored finish and has been implemented successfully in both industrial and commercial applications.

Methods for Adding Color to Concrete

Color can be added to concrete in two basic ways - using colored cement or by adding a coloring agent into the concrete or mortar during mixing. The color is determined by the pigments selected, which are generally divided into two categories - natural and synthetic.

Natural Pigments

Natural pigments are made from mined ores such as carbon and manganese (blacks), chromium (yellows, reds, browns, greens), and cobalt spinels (blues).

Synthetic Pigments

Synthetic pigments meanwhile are made from iron salts, by-products of the steel industry or from scrap steel dissolved in acid and precipitated. The main colors are red, yellow and black.

Permanent Coloring

While the careful selection of pigments (type and amount) helps to achieve the desired color, exposing colored aggregates incorporated in the mix gives a more permanent coloring system with both pigment and complementary aggregate color. This minimizes the effects of ultraviolet lights and other stains like efflorescence.

Stains on Concrete

Stains, particularly white stains, have a greater level of significance when they appear on colored concrete compared with standard material.

Efflorescence

Efflorescence is the most common cause of color fading in this type of concrete. It takes the form of white deposits commonly observed on the surfaces of concrete, mortar, and brickwork of both buildings and block paving.

Lime weeping and lime bloom are the most commonly observed forms of efflorescence – and crystallization of soluble salts is another. The difference between the two forms is in their physical appearance. Lime weeping usually forms first as blemishes of hazy white layers, which become visible as the concrete dries out to produce thicker white crusts. Lime bloom migrates to the surface as the material dries. At the surface, it reacts with CO2 which creates a surface deposit of calcium carbonate.

Composition of Efflorescence and Its Evolution

In November 20012, Professor John Bensted, Cement Technologist and visiting Professor at Birbeck College, London, presented at the SCI meeting on the ‘Chemistry of Efflorescence’, which detailed the chemical composition of efflorescence. Bensted stated that ‘The white deposits are composed of calcium carbonate, normally in the form of calcite, and arise by the effects of aeration (hydration plus carbonation). These effects promote the transport of ions like Ca2+ and OH- in solution through the structure to the external surface, where evaporation of water occurs and calcium hydroxide Ca(OH)2 is deposited. This quickly carbonates by reaction with moist air containing carbon dioxide CO2 to form calcite, which remains on the surface, giving the surface of colored concrete an unsightly appearance,’.

The Effect of Efflorescence

The effect efflorescence has on the visual appearance of many concrete structures has been a concern for a long time. While almost unnoticeable on white concrete, as the depth of colors increases, the effects of efflorescence on colored structures are unacceptable.

Reducing the Effects of Efflorescence

There is no known cure for both primary efflorescence (occurs during the first few weeks as the concrete cures and dries out) and secondary efflorescence (occurs during subsequent wetting and drying cycles). However, some manufacturers of decorative concrete products have found that using metakaolin, sometimes in combination with other measures such as lower water/cement ratios, blended cements, and increased impermeability, can reduce the effects of efflorescence.

Screen-Printed or Photo-Engraved Concrete

Despite the problems associated with colored concretes, the versatility of the material means that the architects’ favorite medium remains unchanged. Architects in Germany, Switzerland, and France became interested in decorative processes such as ‘screen-printed’ and ‘photo-engraved’ concrete. These were developed by French-based company Pieri, using the Serilith system. A photograph or design is screen-printed, using a retarder, onto a polystyrene sheet as a layer of tiny dots. The photo-sensitive sheet is then placed into a mold ‘face-up’ and the concrete is poured on top. Approximately two days later, the mold is stripped and the unit pressure-washed. By selecting colored aggregates and sands, a good reproduction of the photograph or design is obtained.

This technique has been used to photoengrave animals onto the walls of the Hunting Association building in France, a wall of trees on the Stockholm Metro and repetitive photographic images covering the entire exterior of the Eberswalde Technical School Library in Germany. The technique is not restricted to internal and external walls, it can also be used to create photographs or designs onto ceilings or floors.

Summary

Previously - particularly in Europe - houses had to be made from white concrete and it was forbidden to alter the surfaces. From the turn of the century, concrete’s versatility has been realized and as architects and the construction industry discover what can be achieved with this material, concrete is becoming more visually interesting as a building material and is now seen as more ‘designer’ than ‘dull’.

This article was updated on 7th February, 2020.

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