Definitions
Learn the difference between cement and concrete, and understand
the different types of materials that can be used to partially replace
cement in a concrete mix.
Concrete
Concrete is basically a mixture of two components:
aggregates and paste. The paste is usually composed of Portland
cement and water, and it binds together the fine and coarse aggregates.
Supplementary cementing materials may also be included in the paste.
A typical mix is about 10 to 15 % cement, 60 to 75 % sand/aggregate,
10 to 20 % water and 5 to 8 % air. When freshly mixed, it is plastic
and malleable, allowing it to be poured into place and finished.
Then, through a chemical reaction called hydration, the mixture
hardens and gains strength to form the concrete we see in buildings,
sidewalks, bridges and other structures. Concrete is the most commonly
used construction material in the world.
Image courtesy
of the Portland
Cement Association
EcoSmart™ Concrete
What makes EcoSmart concrete different from conventional concrete
is that it uses an optimum percentage of supplementary cementing
materials to replace cement in the mix. Depending on the application
and the SCM used up to 80 % of cement can be replaced with supplementary
cementing materials. These materials are industrial by-products,
so EcoSmart concrete is generally cheaper and can lower construction
costs. In laboratory tests and field applications, EcoSmart concrete
often outperforms conventional concrete in strength development
and durability. It also offers significant environmental benefits,
since each tonne of cement replaced by a supplementary cementing
material reduces CO2 emissions by approximately one tonne.
Portland Cement
Portland cement is the key component of concrete and is produced
by intergrinding clinker and gypsum into a fine grey powder. Clinker
is a granular product produced by intergrinding raw materials such
as limestone, shale, clay and sand in predetermined proportions,
and heating the ground materials at very high temperatures (>1500
°C) in rotating kilns. Gypsum (a mineral) is added to regulate
the setting time of the cement after the clinker is cooled, prior
to conversion into cement. Producing one tonne of cement emits approximately
one tonne of CO2 gas due to the calcination of the raw materials
and the combustion of fuels.
Supplementary Cementing Materials
Supplementary cementing materials (SCMs) (i.e. pozzolans and cementitious
hydraulic slags) are used to partially replace cement in concrete.
They are often added to concrete to make the mixtures more economical,
reduce permeability, increase strength, or influence other properties.
Typical examples include natural pozzolans (like volcanic ash),
fly ash, ground granulated blast furnace slag, rice husk ash, and
silica fume. Pozzolans react chemically with calcium hydroxide (Ca(OH)2)
from the hydration of Portland cement to form calcium silicate hydrates
or CSH. CSH is the strong binder that hardens in concrete. SCM +
Ca(OH)2 = CSH
The glassy particles in pozzolans react slowly with the Calcium
Hydroxide salts to form CSH, so the strength and impermeability
characteristics generally take longer to develop than they do in
conventional concrete.
Fly Ash
One of the most commonly used pozzolans in concrete is fly ash,
a by-product from coal-fired power plants. Using fly ash in concrete
generally decreases permeability, improves sulphate resistance and
other durability aspects of concrete, and allows lower water content
in the mixture. Using fly ash improves the plasticity and workability
of fresh concrete, and produces a warmer coloured concrete. The
annual production of fly ash in the US and Canada is 60 million
tonnes per year, and there will be an estimated 600 million tonnes
produced worldwide by the end of this year. Currently, about 80
% of the fly ash produced ends up in landfills. In North America,
fly ash is typically used to replace an average of 8 % of the cement
in concrete, while in many European countries, the replacement rate
is greater than 25%.
Ground Blast Furnace
Slag
Consisting of silicates, aluminosilicates of calcium, and other
compounds, blast-furnace slag is the by-product of molten iron production
in a blast furnace. The slag is then rapidly chilled to assure a
high percentage of glass, and then ground to produce a fine powder
for use as a supplementary cementing material in concrete. The ground
granulated blast furnace slag acts similarly to cement since it
possesses hydraulic properties. Ground granulated blast furnace
slag, also known as cementitious hydraulic slag, is often used in
concrete requiring maximum durability, higher strength, fire-resistance,
better insulation, and lighter weight.
Ground Slag Cement
(GS-Cem)
The prime ingredient in GS-Cem is barren slag, which is produced
from Teck Cominco's Trail Operations, in British Columbia, Canada,
as a by-product of the lead smelter. The slag undergoes a slag cleaning
process, resulting in a "barren" slag which is rich in
reactive silica and chemically active metal oxides. The barren slag
is transported to a specialty plant in Calgary, Alberta, where it
is interground with a concrete additive called Econo-Set—a
proprietary product which enhances early strength development, bleed
water control and concrete finishability. When used as an SCM in
concrete, GS-Cem increases concrete durability due to its highly
pozzolanic properties. In addition, the use of GS-Cem also increases
sulphate resistance of concrete, controls alkali-silica reactivity,
increases the ultimate strength of concrete, and improves the water
retention of the plastic concrete mix. GS-Cem is typically added
to concrete at a rate of 15-20% by the weight of cement, replacing
an equal amount of Portland cement in the concrete mix with no other
mix adjustment required.
Metakaolin
Metakaolin is produced by calcination of kaolin (clay mineral) at
650-800°C. Kaolin is the by-product of oil sands operations.
Metakaolin is a highly reactive pozzolan with a high specific surface,
which makes it very suitable as a cementing material in concrete.
Metakaolin particles are nearly 10 times smaller than cement particles,
which results in a denser, more impervious concrete. Using metakaolin
in concrete increases the durability of concrete (i.e. resistance
to chemical attacks, sulphate, ASR expansion, and freeze-thaw cycles).
Metakaolin also enhances several mechanical properties (i.e. early-age
compressive strength, and flexural strength) of concrete.
Natural Pozzolan
Pozzolan is a siliceous or alumino-siliceous material that, in finely
divided form and in the presence of moisture, chemically react with
the calcium hydroxide that is released by the hydration of Portland
cement to form compounds possessing cementitious properties. Natural
pozzolans include diatomaceous earth, metakaolin, rice husk ash,
volcanic ash, or calcined shale, all of which are natural materials
that may also be calcined and/or processed.
Silica Fume
Silica fume is a residue from the manufacturing of silicon and ferro-silicon
metals. Silica fume particles are 100 times smaller than cement
grains, so they can be used to fill in the spaces between cement
grains, minimizing permeability in concrete. Silica fume particles
have a high surface area, making this pozzolanic material an ideal
supplement to produce high-strength concrete. Using silica fume
also results in improved freeze-thaw resistance in concrete.
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