What is Greening in Concrete During Curing?
When using slag cement in concrete, a temporary surface discoloration known as “greening” may
occur. This phenomenon appears as mottled green or blue-green areas on the concrete surface
within the first few days after placement. Although visually distinct, greening is a temporary and
surface-level condition.
What Causes Greening?
Greening is attributed to the reaction of sulfide sulfur in slag cement with compounds in Portland
cement. The extent and intensity of the coloration are influenced by factors such as the rate of
oxidation, the proportion of slag cement used, curing conditions, and the porosity of the concrete
surface. While surface greening occurs in a small percentage of slag cement concrete, it is typically
only visible on the exterior, while the interior of the concrete may remain blue-green indefinitely.
Does Greening Affect Performance?
Importantly, greening does not impact the performance of the concrete. Key properties such as
compressive strength, low permeability, and durability remain unaffected. Therefore, greening is
purely a cosmetic issue that poses no concern for the structural integrity or longevity of the
concrete.
How Long Does Greening Last?
In most cases, greening starts to fade within hours of exposure to sunlight and air, with concrete
surfaces maturing to a uniform light gray or white appearance within a week. Factors that may prolong the presence of greening:
- wet
- curing
- cold weather
- shade
- sealed or highly densified surfaces
Greening is a temporary condition. It’s important to note that slag cement is not recommended for swimming pool applications, as continuous exposure to water can prevent the oxidation process necessary for greening to disappear.
For any project where aesthetics or performance is a concern, you should consult with a slag
cement professional for expert guidance and tailored advice to ensure the best outcomes.
