Q: We are using a concrete mix with 60% slag in architectural concrete and are having significant issues with the color of the concrete. What can we do to avoid this?

A:We asked Tony Fiorato, former CEO of CTLGroup and currently executive director of the Slag Cement Association, about this. Here’s his response:

Hardened concrete containing slag cement can show mottled green or blue-green areas on the surface in the first few days after placement. This temporary condition commonly is called “greening.” Surface greening occurs in only a small percentage of concrete made with slag cement and does not affect concrete performance. Hardened concrete properties, such as strength, low permeability, and durability, are not compromised by greening.

The blue-green color is attributed to a complex reaction of sulfide sulphur in slag cement with other compounds in portland cement. In most concrete made with slag cement, the surface becomes light gray or white within hours after the concrete surface has been exposed to direct sunlight and air. In fact, slag cement often is used because it enhances the whiteness and brightness of concrete. The interior of the concrete, however, can remain blue-green indefinitely.

Greening on the Canadian Chancery in Washington, D.C., disappeared with exposure to sunlight and dry air.
Lafarge and Slag Cement Association Greening on the Canadian Chancery in Washington, D.C., disappeared with exposure to sunlight and dry air.

If greening does occur, it usually appears within a week of concrete placement and typically disappears within a week after oxidation starts. The degree and extent of the coloration depends on the rate of oxidation, the percentage of slag used, curing conditions, and the porosity of the concrete surfaces. Surface greening diminishes as oxidation progresses and normally does not need to be treated. It is uncommon for greening to persist after a week of exposure to dry air and sunlight, although in extreme cases it can take a month or more. Slag cement is not recommended for applications, such as swimming pools, where concrete will remain continuously wet, because this inhibits oxidation. Factors that may impact greening include a low w/cm ratio that results in low permeability concrete, extended wet curing, shady exposure, and a steel trowelled densified surface.

Although it is preferable to wait for natural oxidation to dissipate the greening, there are cases where the process has been accelerated through surface application of hydrogen peroxide (3% to 10%), bleach (sodium hypochlorite, 3% to 6%), or diammonium citrate (about 2 pounds per gallon) solutions. Generally these are brushed onto the concrete surface, which is then rinsed and allowed to dry. This should be tested first in small areas that are out of view. Manufacturers’ recommended safety and environmental precautions should be followed with any treatment used.

Thanks to Henry Prenger at Lafarge and Peter Bohme at Holcim for technical information.