Chemical Engineering September 2015 - 24

Concrete basics
C
oncrete exhibits a host of qualities that explain why it is
the world's most used human-made material. Here are
some basic facts and definitions surrounding concrete:
Concrete components: Concrete is a composite material
that typically consists of gravel and crushed rock - known as
coarse aggregate - sand, cement, water and air. Although
there are many types of concrete, each resulting from a different
blend of materials, a typical concrete makeup consists
of the following (Figure 1): coarse aggregate (gravel, rock,
crushed recycled concrete) - 41%; fine aggregate (sand) - 26%; cement - 11%; air (often intentionally entrained to provide resistance to
freeze-thaw cycles that can lead to breakdown of material over time) - 6%; water (essential for hydration of cement) - 16%; additives (can
control concrete properties, such as flowability, setting time and others - 1%.
Cement raw materials: Ordinary portland cement was named after the Isle of Portland (off the British coast) by its inventor, stonemason Joseph
Aspdin in 1824. Modern portland cement is the most common type for general use, and is a component of concrete, stucco and mortar.
Portland cement is made from a proportioned blend of the following raw materials: CaCO3 from limestone, chalk, shells, shale or calcareous
rock; SiO2 (silica), usually from sand, clay, old bottles or argillaceous rock; Al2O3 (alumina), usually from bauxite, recycled aluminum or clay;
Fe2O3, (iron oxide) from clay, iron ore, scrap iron or flyash; CaSO4 · 2H2O (gypsum), usually found with limestone.
Cement manufacturing process: The solid raw materials (excluding gypsum) are ground and fed into large steel rotary kilns lined with special
firebrick. The long, cylindrical kilns are angled downward and rotated to allow the raw materials to move through the kiln. Rotary kilns are
equipped with heaters at the lower end that heat the material to 2,700°F. As a result of heating, the raw materials undergo a dehydration stage,
where free water is driven off, followed by a calcining stage in which bound water and CO2 are driven off. As the heat increases, a final sintering
process occurs to form calcium silicates. The grayish, anhydrous pellets resulting from the process are known as clinker and are cooled upon
exiting the kiln. The clinker (see table) is pulverized into the fine powder that is commonly called cement. Gypsum is added to the ground clinker
to help control the time the cement takes to set.
Energy consumption has fallen drastically in cement manufacturing in recent decades. A state-of-the-art dry kiln equipped with a pre-calciner
consumes 50% less energy than the long wet kilns that dominated cement manufacture in the past, Lafarge's Buffenbarger says. Further, more
alternative fuels are being used to heat cement kilns, including biomass and waste materials, she adds.
Hydration chemistry: When water is added to a cement paste or a concrete blend with cement and aggregate materials, the cement minerals
dissolve, forming an ionic solution. When the ionic species become supersaturated, a precipitation process occurs, where new solid species
form. This dissolution-precipitation process allows the complex hydration reactions to occur and cause the cement to harden. Cement does not
harden by drying; it sets due to the chemical hydration reactions. Hydration involves the formation of chemical bonds between water and the
major compounds in cement. The main reaction products include the following: Calcium silicate hydrate (a major source of concrete strength)
and calcium hydroxide (formed by alite hydration).
Cement Compound
Tricalcium silicate (alite)
Dicalcium silicate (belite)
Tricalcium aluminate
Tetracalcium aluminoferrite
Weight
Percentage
50 %
25 %
10 %
10 %
Gypsum (added to ground clinker) 5 %
Chemical Formula
Ca3SiO5 or 3CaO·SiO2
Ca2SiO4 or 2CaO·SiO2
Ca3Al2O6 or 3CaO ·Al2O3
Ca4Al2Fe2O10 or 4CaO·Al2O3·Fe2O3
CaSO4.2H2O
additive promotes improved flowability
of concrete while preventing the
segregation of gravel from the wet
concrete slurry. " The rocks don't sink
to the bottom, " says Grace's Myers,
and the concrete will self-level without
segregation, saving jobsite labor. "
Another approach is to introduce
substances that catalyze the specific
hydration reactions that impart
strength to the hardened concrete.
An example comes from Grace
Construction Products, which has
introduced a set of patented alkanolamine
catalysts added to cement
to accelerate specific chemical
reactions that help develop strength
in concrete, Grace's Myers says. The
catalyst forms a reversible complex
with iron, transporting the insoluble
iron from the cement grain surface,
where it impedes reaction of the
strength-giving silicate phases. Catalyst
dosages as low as 200 ppm in
the cement can generate 10-20%
strength improvements, Myers says.
Grace is also test-marketing verifi, a
product that allows a concrete truck's
mixer to act as a rheometer. Using
24
wireless communication and specially
designed algorithms, the verifi system
sets up a feedback loop that controls
the addition of water and additives to
the concrete mixture based on the
realtime rheological profile of the concrete
in the transport truck, ensuring
that the concrete arrives at a jobsite
with the desired properties.
Molecular analysis
The Massachusetts Institute of Technology
(MIT; Cambridge, Mass.;
www.mit.edu) Concrete Sustainability
Hub (CSHub) is a team of interdisciplinary
researchers that has been
investigating questions about concrete
infrastructure science, engineering
and economics for the past
six years. Among the CSHub's most
recent work is what MIT calls the
" most detailed molecular analysis to
date of the complex structure of concrete. "
The new analysis suggests
that reducing the ratio of calcium to
silicate in cement would not only cut
CO2 emissions, but would actually
produce better, stronger concrete.
The findings are described in the
journal Nature Communications by
MIT senior research scientist Roland
Pellenq and others.
In conventional cements, Pellenq
explains, the calcium-to-silica ratio
ranges anywhere from about 1.2 to
2.2, with 1.7 accepted as the standard.
But the resulting molecular
structures have never been compared
in detail. Pellenq and his colleagues
built a database of all these
chemical formulations, finding that
the optimum mixture was not the
one typically used today, but rather a
ratio of about 1.5.
As the ratio varies, he says, the
molecular structure of the hardened
material progresses from a tightly
ordered crystalline structure to a
disordered glassy structure. They
found the ratio of 1.5 parts calcium
for every one part silica allows the
material to achieve " two times the
resistance of normal cement, in mechanical
resistance to fracture, with
some molecular-scale design. " n
Scott Jenkins
For more on concrete technology, see the online
version of this article at www.chemengonline.com.
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