Towards uncolored environmentally-friendly concrete

In the quest for environmentally friendly materials, some companies, like ECOCEM, are using concrete, mortar and other materials based on slag. These display a blue/Green colour when they get wet. When the concrete is exposed to air, the blue/Green colour turns to grey (in the case of cement concrete) or white (slag concrete). However, if the concrete is let to set for a long time in a formwork or cured under water, it keeps the colour for up to its entire service life. This specific colour is concerning in applications where visual aesthetics are important. The origin of this blue/Green tinge remains a mistery, even if it has been attributed to the presence of poysulfur species in the slag, but without any scientific evidence.

In order to find out the origin of this coloration, scientists went to ID21  and used K-edge X-ray absorption near edge structure (XANES) spectroscopy to follow the evolution of the speciation of sulfur as the slag is hydrated.

The team used three methods to actívate the slag: termal, Portland cement and sodium silicate. First, they compared each method and then they compared the XANES results on blue and white samples and on zones that were not coloured.

They found that, independently of the activating method, the blue colour is related to the presence of a pre-peak in the corresponding XANES spectrum. This doesn’t happen anywhere else in the samples. They linked this pre-peak to the presence of the polysulfur radical anion S3. This blue chromophore is also at the origin of the deep blue of the stone lapis lazuli or the ultramarine pigment (derived from the Lazurite mineral). 

Professor Gao, from Insa Toulouse, says that "This research did not exclude the possibility of other elements influencing the coloration of slag-based concretes, in particular the presence of transition metals, but it showed clearly that the trisulfur radical anion is present and may play a major role."

Laurent Frouint, ECOCEM materials R&D director, explains that "This study helps us to discover the reason of blue color formation to their clients. And it also helps us indirectly to find solutions to eliminate the blue/green color". 

Top image: Portland cement (left) and ECOCEM cement (right)