Roadscience: The Chemistry of Concrete Admixtures.
by Tom Kuennen, Contributing Editor
of Road Building
ortland cement concrete (PCC) is the most widely
used structural material in transportation infrastructure, and it’s widely used for pavements as well.
But the control of freshly mixed PCC properties is essential
to ensure workability during placement, and compressive
strength and long-term durability.
Not unlike modiﬁers in bituminous pavements (see
Asphalt a la Carte: Modifiers Control Mix Performance, April 2012,
pgs. 16-27), mineral and chemical concrete admixtures
work both physically and chemically to improve the durability and quality of portland cement concrete, boost or
retard set time, increase resistance to frost, sulfate attack and
alkali-silica reactivity, and improve placement.
Adding chemical admixtures to PCC before or during
mixing can manage its rate of early hydration, and regulate
its ﬂuid (rheological) properties. The most common chemical admixtures include air-entraining agents, water reducers, superplasticizers (high range water reducers), retarders,
10 May 2012 Better Roads
and accelerators. Concrete admixtures are classiﬁed as either
mineral or chemical.
Cement and Concrete
Hydraulic portland cement is the product of the calcining,
or pyroprocessing, of limestone (a calcium compound),
aluminosilicates (clays and/or sand), and iron oxide at
phenomenal temperatures, in the range of 2,700 and 3,000
deg. F. The raw materials are crushed, screened and deposited in an inclined, refractory tile-lined cement kiln, which
rotates while the materials undergo calcination.
During calcining, the raw materials tumble in the rotary
kiln, moving downward as they are exposed to heat. The
chemically combined water and carbon dioxide from the
raw materials is driven off, leaving behind new compounds
such as tricalcium silicate, dicalcium silicate, tricalcium
aluminate and tetracalcium aluminoferrite, according to the
Portland Cement Association.
These compounds are contained in the resulting fused
lumps – the size of a ﬁst or smaller – called clinker. This
clinker tumbles out the lower end, is cooled and is either
shipped (exported) elsewhere for grinding into cement, or
ground at the plant in a ball mill, which results in an hydraulic portland cement that is so ﬁne it will pass through a
sieve that will hold water.
Fresh concrete, of course, is the mix of aggregates (sand
and gravel or crushed stone), water and portland cement.
Cement makes up from 10 to 15 percent of the concrete
mix, by volume, although in recent decades the cement industry has encouraged speciﬁcation of equally performing
cement containing up to 15 percent limestone ﬁnes.
The addition of water initiates hydration of the cement,
which binds the sand and aggregates into a hardened product resembling stone. Compounds like calcium hydroxide
and calcium silicate hydrate form within the paste. The
strength of concrete is measured by its resistance to compressive forces after a month of curing (28-day compressive
strength). But curing actually continues for decades, albeit
at a much slower pace. Theoretically concrete never really
How Concrete Cures
The curing or hardening of PCC is a physio-chemical process that begins at the molecular level. The scale at which
this curing takes place is the realm of the atom and mol-