What is “Greening?”
Hardened concrete containing slag
cement may show mottled green or bluegreen
areas on the surface in the first few
days after placement. This temporary
condition is commonly called "greening."
What Causes Greening?
The blue-green color is attributed to a
complex reaction of sulfide sulphur in
slag cement with other compounds in
portland cement. 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
occurs in only a small percentage of concrete
made with slag cement. However,
the interior of the concrete may remain
blue-green indefinitely.
Greening and Concrete Performance
Greening does not affect concrete
performance. The desired hardened concrete
properties, such as strength, lower
permeability and durability are not compromised
if greening occurs.
How Long Does Greening Last?
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. If greening does occur, it usually
appears within a week of concrete placement
and typically disappears within a
week, after oxidation starts.
Surface greening diminishes as oxidation
progresses and does not need to be
treated. Typically, surfaces showing
greening will mature to a uniform light
gray or white appearance. Some factors
that may cause slower oxidation rates
include: extended wet curing, wet
weather, cold weather, shade, continuously
moist, dark environments, sealed
surfaces, and/or a steel-troweled, burnished,
or otherwise densified surface. It
is uncommon for greening to persist
after a week of exposure to dry air and
sunlight.
Slag cement is not recommended for
swimming pool applications because
pools are continuously wet which
inhibits oxidation.