Frequently Asked Questions
Cement & Concrete
Technology Home > FAQs > SCMs and
Blended Cements
Q.What are supplementary cementitious materials
(SCMs) and how do they differ from blended cements?
A.More than half of ready-mixed concrete contains fly ash, ground
granulated blast furnace slag, silica fume, metakaolin, or other
pozzolanic materials. These materials are collectively referred
to as supplementary cementitious materials (SCMs).
SCMs can be included in concrete, either as an ingredient added
at batching, or as a component of a blended cement, or both (Figure
1). SCMs can be added during batching along with portland cement.
SCMs can also be added to concretes made with blended cements. SCMs
added directly to concrete are governed by ASTM C 618 (fly ash and
natural pozzolans), C 989 (slag), or C 1240 (silica fume), while
blended cements are governed by ASTM C 595 or C 1157.
Since the benefits of SCMs arise from their physical and chemical
characteristics, it might be assumed that similar performance in
concrete is achieved, for example, by adding a fly ash at a ready-mixed
concrete batch plant or through use of a blended cement made with
fly ash. Although good concrete performance can be achieved through
both techniques, blended cements provide an advantage in that they
can be produced with the same quality control techniques as portland
cements, including control of fineness and optimization of sulfate
content. Sulfate optimization can be particularly important for
some fly ashes with high aluminate contents. Although a rare occurrence,
some fly ashes can throw off the sulfate balance in fresh concrete,
leading to problems with workability and setting. The quality control
of blended cements takes one variable out of the concrete batching
process.
More information on SCMs, click
here.
Additional references:
Supplementary
Cementing Materials For Use in Concrete CD
Fly Ash, Slag,
Silica Fume, and Natural Pozzolans
Design and Control
of Concrete Mixtures
Supplementary Cementing
Materials for Use in Blended Cements
Benefits of Ternary Mixtures
|