May 28 2019 0Comment

Greening Of Slag Cement Explained

Greening Of Slag Cement Explained

What Is Slag Cement?

Slag cement is produced when granulated blast furnace slag (GGBFS) is grounded to an appropriate fineness and is used for replacing a portion of Portland cement. Slag cement is an Industrial by-product which is recovered from an iron blast furnace. Molten slag which is abstracted from the iron blast furnace is speedily chilled, after which glassy granules are produced that yields the required reactive characteristics to acquire the right fine consistency for the cement. Once the slag is cooled and grounded into usable fineness, it is correctly stored and shipped to supplying companies. Slag cement is usually found in soil cement, masonry, precast concrete, ready-mix concrete, and high-temperature resistant building materials.

Slag Cement

What Is Greening?

Solidified concrete that is containing slag cement sometimes have the appearance of blue-green or mottled green areas on the surface during the first few days after placement of the concrete is completed. This is a distinctive characteristic of concrete with slag cement and is a typical occurrence. This temporary phase is referred to as “greening.” Greening is an indication of the concrete being cured very well in its early days and is excellent for strength development purposes. The blue color will generally start fading, and the concrete will become the usual color after being exposed to air for three to five days.

What Is Causing The Greening?

The reason for the green/blue coloring appearing is the minimum amount of FeS and Mns are materialized during the GGBS hydration procedure; therefore, the hydrate has a blue color. After being exposed to air for a few days, the chemical composition will change to FeS04 and MnSO4 resulting in the blue color disappearing completely.

This is basically a complex process which happens when sulfide sulfur in the slag cement is reacting with other compounds in Portland cement. The extent and degree of the blue or green coloration is dependent on the percentage of slag used, the rate of oxidation happening, curing circumstances as well as the porosity of the concrete surfaces. Surface greening is only occurring in a tiny percentage of the concrete which is made with slag cement. However, the inside of the concrete will remain a blue-green color indefinitely.

Does Greening Have An Effect On Concrete Performance?

Greening does not compromise concrete performance which includes strength, durability and lower permeability.

How Long Does It Last?

Most concrete which is made with slag becomes light grey or white after a few hours after which the concrete surface is exposed to direct sunlight and air. If greening is occurring, it generally appears after a week of concrete placement and disappears within a few days after oxidation begins. Surfaces, where greening is showing, will settle and will become light grey or white in color. Factors that may result in slower oxidation are:

Effect On Concrete Performance


  • Prolonged wet curing
  • Wet weather conditions
  • Cold weather
  • Shade
  • Sealed surfaces
  • Densified surfaces
  • Dark Environments

Generally, greening will not persist for more than a week after being exposed to dry air and sunlight.


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