Effect of Cement Characteristics on Concrete
You may often wonder why concrete behaves the way it does: why
does it give off heat when setting, why does concrete gain strength
differently with different cements, why does it shrink, why are
some concretes more resistant to deterioration? Most of the properties
of fresh and hardened concrete are affected to some extent by the
cement properties. Therefore, an understanding of cement characteristics
can provide insight to the many material-related questions that
arise in concrete construction.
In the manufacture of cement, limestone, sand, clay, and iron ore
are blended, ground, and heated to 1400°C – 1550°C
(2550°F - 2825°F) in a rotating kiln. The resulting material,
called clinker, is cooled, pulverized, and mixed with gypsum to
create what is known as portland cement, a hydraulic material primarily
made up of calcium silicates.
Cements differ from plant to plant due to changes in raw material
properties, kiln temperatures, and fineness upon grinding. These
changes can significantly affect concrete properties when different
cements are used in concrete.
Besides the main constituents used in the manufacture of portland
cement, minor components are naturally present. Important minor
components include the alkalies, sodium and potassium. Although
present only in small amounts, the alkalies can affect the setting
time, strength development, reactivity with fly ash or slag, and
the durability of concrete. Other minor components – oxides
other than the main oxides comprising the four clinker phases C3S,
C2S, C3A, and C4AF –are derived from raw materials, fuel,
refractory material and wear parts from manufacture equipment, and
generally do not affect the portland cement.
Hydraulic cements set and harden, not by drying, but through a
chemical reaction between the cement grains and water. During this
process (called hydration), the calcium silicates from the portland
cement form calcium hydroxide and a gel-like calcium silicate hydrate
(C-S-H). The rate of this reaction is dependent on many factors
including the type and proportion of portland cement components
(C3S, C2S, C3A, and C4AF),
the fineness and particle size distribution of the cement grains,
and the placing and curing conditions of the concrete. Understanding
the hydration process and the creation of C-S-H are critical for
understanding the engineering properties of concrete: setting, strength
gain, and durability.
Tracing some of the fresh and hardened concrete properties back
to the influences of the cement can often answer both fundamental
questions and more complex problems in concrete construction.
Admixtures are chemicals added to concrete in small quantities for
a specific function (for example water-reducing, set-retarding,
or set-accelerating). These chemicals affect the hydration and/or
are adsorbed by the cement particles. Certain combinations of chemicals
and cement properties may adversely affect the setting behavior
and be deemed incompatible. Examining the chemistry of the hydration
reactions and the components of the cement can give clues to the
source of the incompatibility.
The ultimate compressive strength and rate of strength
development of concrete is strongly influenced by the chemical reactivity
of the portland cement. Varying hydration rates of the different
cement compounds can help explain how the relative proportions of
these compounds affect the rate of strength gain. For instance,
the C2S reacts slowly and contributes to long-term strength
gain. C3S, on the other hand, has a much faster hydration
rate, and contributes to higher early-strength gain. Thus, cement
with a higher proportion of C3S – as is the case
with most of today’s cements – will tend to have a higher
early strength, and allow for early form removal or post-tensioning.
With a complex interdependence of cement and concrete properties,
it is important to evaluate them systematically to achieve the greatest
understanding. PCA’s Effect
of Cement Characteristics on Concrete Properties,
EB226, provides a thorough compilation of the range of properties
and their relative effects. Below is a table excerpted from this
publication that relates changes in cement to changes in concrete.
The complex interactivity of all the properties of cement means
that it is not possible to quantify the effects discussed. The intention
of this table is to provide general guidance and trends rather than