World Cement - November 2018

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between the cement paste and aggregate particles.
This improved bond results from enhanced paste
characteristics that also increase compressive strengths.
The St. Pete-Clearwater Airport apron pavement
replacement in Florida and Ten Hudson Yards in
New York City are examples of projects that benefit from
the high flexural strengths achievable with slag cement.
Reduced permeability
Slag cement is a hydraulic cement, it reacts with water
to harden by forming calcium silicate hydrate (CSH).
CSH is the glue that provides strength and holds
concrete together. Slag cement also reacts with calcium
hydroxide, the byproduct of cement hydration, to form
additional CSH. The increased CSH produced modifies
the pore structure of the paste, resulting in lower
permeability. The level of improvement is proportional
to the percentage of slag cement in the mixture,
normally between 25 – 65%. Lower permeability
reduces chloride ion ingress and thus reduces the
corrosion potential of the structure. Projects such as
JFK International Airport Runway Reconstruction and
St. Croix Crossing Bridge in Louisiana used slag cement
to help reduce chloride ion penetrability to less than
2000 couloumbs, by the Rapid Chloride Permeability
Test (ASTM C1202).
Reducing thermal stress in mass concrete
One of the most difficult challenges in designing mass
concrete structures is limiting the concrete temperature
differential between the centre and the surface of
the concrete. If this differential becomes too large,
thermal cracks can develop in the concrete. Slag cement
has been used successfully to substantially reduce the
temperature of mass concrete. When used at high
replacement rates, slag cement will provide lower heat
in mass concrete than concrete produced with low heat
cement. Examples include 75% slag cement replacement
in the I-895 Pocahontas Parkway footings in Richmond,
Virginia, and 70% slag cement replacement in the
Mississippi River Bridge.
Sulfate resistance
Waterborne sulfates, found in some soils, seawater, and
wastewater treatment plants, may react with hydration
products of portland cement to cause cracking or
softening of the paste. The use of slag cement decreases
the likelihood of sulfate attack by reducing the total
amount of potentially reactive hydration products
in the system. Additionally, slag cement reduces the
permeability of the concrete and limits the ability of
sulfates to penetrate. Examples of mitigating sulfate
attack with slag cement include the William Preston
Memorial Bridge in Maryland and the city waste water
treatment plant in Clyde, Ohio.
Improved workability/finishability
Concrete mixes with slag cement tend to have a
smoother and more workable consistency than when
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