Supplementary Cementitious Materials
Modern concrete often includes the use of supplementary cementitious materials
(SCM).These materials are often co-products of other processes or natural materials.
Some require further processing before they are suitable for concrete. Some of these
materials have cementitious properties when used alone; others, called pozzolans, do not.
Composition and Consistency
Slag cement and fly ash are the two most
common SCMs used in concrete. Most
concrete produced today includes one or
both of these materials. For this reason
their properties are frequently compared
to each other by mix designers seeking to
optimize concrete mixtures.
While chemical similarities exist, these
materials have different effects on concrete.
These differences are based in part
on the proportion of oxides in each material
(Table 1).
The ternary diagram, shown in Figure 1
shows that slag cement is more closely
related to portland cement than fly ash.
This is one reason why slag cement can
be used in much larger amounts. Both are
used as a replacement for a portion of the
portland cement. Slag cement replaces as
much as 50 percent in normal concrete
(and up to 80 percent in special applications
such as mass concrete). Fly ash is
usually limited to 20 or 30 percent.
Slag cement is the co-product of a controlled
process, iron production, which
results in a very uniform composition
from source to source. Fly ash is a
byproduct of electric power generation
that varies from source to source.
Concrete Properties
Slag cement is a more uniform product
than fly ash. As a result, concrete made
with slag cement will generally have more
uniform properties than concrete made
with fly ash.
Plastic Properties
Water Reduction. The use of either material
should result in a reduction of the
required water content to reach a given
consistency. This effect with slag cement
is due to its influence on paste characteristics
and absorption. With fly ash, this is
primarily due to the particle shape and
size distribution. This allows for small
reductions in water reducing admixtures.
Air Entrainment. Air contents can vary
depending on any number of factors.
Carbon content variability in fly ash is one
of the major causes of fluctuating air contents.
Slag cement does not contain carbon
and does not cause instability in the
entrained air content.
Time of Set. Time of initial set is influenced
by the use of slag cement and fly
ash. Concrete made with slag cement can
have faster set times than concrete made
with fly ash (Figure 2).
Pumpability and Finishability.
Pumpability with slag cement and fly ash is generally improved largely due to the addition of fines to the matrix. Finishability is also improved.
Hardened Properties
Strength. At 28 days, both slag cement
and Class C fly ash will achieve higher
strength than Class F fly ash and straight
portland cement in concrete mixtures.
Permeability. At normally specified
replacement levels, concrete made with
slag cement will have lower permeability
than concrete made with Class F or C fly
ash when tested according to ASTM 1206
(rapid chloride permeability test).
Sulfate Attack and Alkali-Silica
Reaction (ASR). Slag cement and Class F
fly ash will both provide protection against
sulfate attack and ASR. Class C fly ash
may not provide this protection.
Color. Slag cement will provide lighter
concrete with higher reflectivity than plain
portland cement concrete. Class C and
Class F fly ash will produce, respectively,
a buff or darker gray concrete.