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In the transportation sector alone, concrete has many different applications,
including pavements, bridges, barriers, sidewalks, and much more.
How can a single product, be the right solution for so many different applications,
designed to withstand a wide range of weather conditions?
The answer is partly through the use of a variety of Supplementary Cementitious Materials...
or SCMs…in varying amounts, directly, or as components of blended cements, in the concrete mixture.
Concrete is made of paste and aggregates, or rocks.
The paste that binds fine and coarse aggregate is made of cement and water.
When first placed, the mixture is plastic, but soon a chemical process known as hydration begins,
causing the paste to harden, and transforming the mixture into a hard,
durable, rock-like material known as concrete.
Prior to the 1980’s, portland cement was used in paving mixtures for conventional concrete
as the sole binder.
Portland cement is manufactured through a closely controlled chemical combination of
calcium, silicon, aluminum, iron and other ingredients from limestone, clay, and other
raw materials.
These ingredients, when heated at high temperatures combine to form a rock-like substance that
is ground into the fine powder, commonly known as portland cement.
Since the 80’s, use of SCMs, particularly in blended cements, continues to increase
due to a number of technical and environmental benefits.
Blended cements are produced by grinding together or blending portland cement with one or more
SCMs, such as pozzolans like the industrial by-product fly ash and silica fume,
natural pozzolans, slag cement, and limestone.
Pozzolans are a broad class of materials which themselves are not very cementitious,
but as a fine powder, and in the presence of water, react chemically with calcium hydroxide
to form compounds possessing cementitious properties.
The two most commonly used SCMs in blended cements are fly ash and slag,
which are the recycled waste products of thermal and steel plants.
Fly ash is the airborne residue from the coal combustion processes and blast-furnace slag
results from the reduction of iron ore in a blast furnace to form iron.
Roberto Carrillo: "Essentially the slag cement is an environmentally
friendly product.
It is a recycled material from the steel-making process.”
“If the material was not granulated, it would be put into a pit, basically, used as
air-cooled slag or put into a landfill.”
Precious minerals like limestone, clay and silica are saved by using these by-products.
Pollution and greenhouse gases are also controlled by reducing the production of cement, which
is an energy intensive product.
Concrete producers may choose to blend SCMs with portland cement at the concrete plant,
rather than use blended cements.
However, this has the potential for more variability and inconsistent quality due to the less controlled
blending environment.
The advantage of blends produced and supplied by the cement supplier is that there is a
greater level of quality control over the final product.
The disadvantage is that it offers the concrete producer less flexibility to respond to changing
concrete placement conditions.
The composition of blended cement can be a binary system, made with portland cement and
one other material, or a ternary combination of portland cement and two or more materials.
Blended cements are classified based on the type and amount of material blended with portland cement.
Type IP(X) – The P indicates this is portland-pozzolan cement in which X denotes the target percentage
of pozzolan, typically 15 to 25 percent, and up to 40 percent.
Type IS(X) – The S indicates this is portland-slag cement in which X denotes the percentage of
typically 30 to 50 percent, and up to 95 percent.
Type IL(X) – The L indicates this is portland-limestone in which X denotes the percentage of limestone,
typically 10 to 12 percent, and up to 15 percent.
Type IT(AX)(BY) – The T indicates this is a ternary blend, consisting of portland-pozzolan,
portland-slag, or portland-limestone.
A and B denote the material and can be P for pozzolan, S for slag, or L for limestone.
X and Y denote the percentage of each.
Blended cements are broadly accepted by highway agencies throughout the United States.
The variation of the blend affects the properties and results,
providing both technical and environmental benefits.
Blended cement reduces water demand and therefore water-cementitious ratio can be reduced.
It also improves workability of the mixture.
Blended cements are typically finer and less permeable, often resulting in improved durability,
long-term performance, and reduced lifecycle costs.
The SCMs in blended cements are effective at reducing transport of chloride ions through
the hardened concrete, thus reducing the chance of corrosion of embedded steel.
Andrea Breen: “The more we can let that strength develop,
the more dense and less permeable the mix is.
Then the concrete can protect itself from things like de-icer salts and freezing and
thawing water.”
Due to the presence of SCMs, blended cements are a cost effective way to protect the concrete
from durability distresses such as alkali-silica reactivity (ASR) or external sulfate attack.
In hot-weather conditions, the prolonged setting time and reduced heat of hydration reduces
built-in curl and associated early age cracking.
The production of blended cements also offers environmental advantages.
By adding mineral admixtures with ordinary portland cement, the energy, which would have
otherwise been utilized in the production of the replaced portland cement is saved.
Blended cements provide improved workability… enhanced long-term strength and durability…
and reduced greenhouse gas emissions to create a cement product that is the best solution
and the best use of resources for any application.
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