Researchers from Xi’an Jiaotong-Liverpool University found that green pigment causes structural issues in concrete. The green pigment typically used has impurities that created porous, poor quality concrete.

Pigmented architectural concrete is currently in high demand for use as a visually appealing alternative to traditional grey concrete. While pretty, the team’s findings point to a need for improvements in the stability of green concrete.

Chemicals are used to create the pigments in these concretes. There is an impurity in green pigment, called muscovite, that is weakening the material. When hydrated with cement, this chemical creates significant quantities of a component that causes excessive porousness, which reduced the strength and longevity of the concrete.Source: Unsplash

Cement samples with three pigments -- blue cobaltous aluminate, green chromium oxide and red iron oxide --. added at 1, 5 and 10% levels. Both Portland and Portland composite cement types were tested to understand how the admixtures and by-products affect the properties of concrete.

Past studies have found that there is no adverse effect of using a green pigment made of chromium (III) oxide, which has no muscovite. The team hoped that this material would eliminate the porosity issues of the other pigments.

The experiment was conducted in four parts. First, the team tested the water absorption of the cement mixtures to evaluate the change in physical properties. Hydration properties of the cement were analyzed, and the kinetics of hydration were gathered through watching the evolution of the temperature of each mix. Finally, the team examined the flexural and compressive strength of the cement to understand how the pigments were affecting mechanical strength.

Results showed that the morphology of hydration products and kinetics are related to the samples’ compressive strength. The poor performance of the green pigment stood out. Red and blue had minimal effects. Blue and red pigments could be used with Portland and Portland composite cement without weakening the concrete strength.

A paper on this research was published in Case Studies in Construction Materials.