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Slag

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Cement shortages are resulting in increased usage of slag as a concrete admixture.  Slag has been used for quite a while in the DC area,  because it comes out of Baltimore and Delaware,  but apparently the slag people have not done good marketing,  because a lot of engineers seem to consider it as an inferior substitute for fly ash.  In reality,  slag has better properties than fly ash, although it is also more expensive.  Until recently, it was about the same price as cement.

Slag usage is apparently catching on in California; the slag is coming from China.   Some basic info on slag:  

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Slag

Ground granulated blast furnace slag is a nonmetallic byproduct of iron production that contains essentially the same chemical compounds as portland cement, but in different amounts.  It is produced by rapidly water-quenching molten slag to form a glassy, sand-like material. The slag granules are then ground to a fineness of less than 45 microns, which is somewhat finer than Type I portland cement.

ASTM C989, "Specification for Ground Granulated Blast-Furnace Slag for Use in Concrete and Mortars" lists three grades of slag:  Grades 80, 100, and 120.  These roughly correspond to the expected percentage of 28 day strength of a mortar cube made with 50% slag / 50% portland cement compared to straight portland cement mortar.   Grade 120 is typically specified for use as a concrete admixture.  Slag dosages replacing 25% to 50% of the cement are common; higher dosages are sometimes used for concretes requiring special properties, such as high sulfate resistance or extremely low temperature rise.  Slag is not classified as a pozzolan because it has cementitious properties; however, it has pozzolanic properties in that it will react with calcium hydroxide to form additional C-S-H.

The properties of slag concrete in both its plastic and hardened states will vary with different material combinations, job conditions, and placing practices. Slag particles have a smooth, dense surface and as a result, absorb little water during mixing and initial curing. The paste is thus more fluid, which results in concrete with greater workability as well as better pumping and placing properties.  The concrete tends to have improved uniformity and is easier to consolidate and finish.

Because slag is finer than portland cement, it creates a denser, less-permeable concrete. The increased C-S-H formed as a result of the pozzolanic reaction further reduces permeability, which increases durability.  The concrete will also typically have a higher ultimate strength than a straight portland cement mixture.  Slag does not hydrate as fast as cement, so the temperature rise during hydration is less.  This is especially beneficial in mass concrete, but can also be beneficial in slab on ground construction during hot weather.

Slag improves concrete's resistance to sulfate attack; depending on the chemical composition of the slag, the sulfate resistance of concrete with  50 % slag /50% Type I cement can be equal to that of concretes made with Type V (sulfate-resisting) cement.  Slag can also reduce problems with alkali-silica reactivity; using 40 to 65% slag can virtually eliminate alkali-silica reactions.

The slower hydration may be a problem during cold weather, however.  The setting time of the concrete will typically be retarded; initial setting may delayed by as much as an hour, depending on the slag dosage and the initial curing temperature.  Accelerators can be used to reduce or eliminate the set retardation but overall strength development is slower, particularly with high volume applications.  Concrete strengths at 1, 2, and even 7 days, may be lower; slag is not recommended in cold weather if early strength is required.  Strength development is very sensitive to curing temperatures and slag dosage; predicting ultimate strength based on seven-day compressive strength results is difficult because the 7 to 28 day ratio can vary greatly.

Slag is lighter in color than most portland cements and will produce a lighter-colored concrete after curing; however, concretes containing high slag contents may initially turn a bluish-green.  This is due to a reaction between sulfides in the slag and compounds in the cement.  The bluish color will disappear after the concrete is exposed to the air for a few days.


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Gail Kelley