Using ground granulated blast-furnace (GBF) slag to partially replace portland cement in concrete mixtures can provide many benefits. When added in the right proportions, the mineral admixture improves the workability of fresh concrete and increased the strength and durability of hardened concrete.

GBF slag is a nonmetallic byproduct of iron production consisting primarily of silicates and alumi-nosilicates of calcium and other bases. It's produced by rapidly chilling molten iron blast-furnace slag using a water-quenching process to form a glassy, sand-like granulated material. The granules are then ground to a fineness of less than 45 microns, which is somewhat finer than most portland cements.

In concrete mixtures, GBF slag exhibits both pozzolanic and cementitious properties. It contains the same chemical components as portland cement, but the amounts of each may differ. Both materials also are hydraulic, which means they harden by chemically reacting with water.

As with other mineral admixtures, such as fly ash and silica fume, GBF slag can significantly affect certain properties of both fresh and hardened concrete. Contractors placing GBF-slag concretes need to be aware of these effects and adjust their procedures accordingly. And specifiers should know how to correctly proportion concrete mixes containing GBF slag to meet specific performance requirements.

This chart shows the effects of increasing doses of GBF slag on concrete compressive strengths. Concretes made with slags meeting ASTM C 989 Grade 120 requirements achieve higher compressive strengths after seven days than plain portland cement concrete. The greatest 28-day strengths can be achieved with blends of 35% to 65%. For this data, the total cementitious materials content was 517 pounds per cubic yard, and the concretes were air-entrained.

Effects on Fresh Concrete

Workability. Many contractors report noticeable improvements in the pumping and placing properties of concretes containing GBF slag, both with and without water-reducing admixtures. These improvements are due to the smooth, dense surface characteristics of the slag. Research has shown that cement pastes containing GBF slags exhibit different rheological properties than pastes containing portland cement alone (Ref.1). Little, if any, water is absorbed by the slag during initial mixing. The paste, therefore, is more fluid, resulting in greater workability.

Setting time. Because GBF slag hydrates more slowly than portland cement, use of the slag generally retards concrete setting time. The degree of set retardation depends on such factors as the proportion of slag used (the higher the slag dose, the slower the setting time), the water-to-cementitious-materials ratio, and the initial curing temperature. Typically, initial setting time is extended one-half to one hour at temperatures of 73° F; little change occurs at temperatures above 85° F (Ref.1). Adding accelerators to GBF-slag concretes can reduce or eliminate this set retardation.

Heat of hydration. Using GBF slag can reduce heat buildup in a concrete structure because it lowers the heat of hydration. This reduction in temperature rise is especially beneficial in concretes used for massive structures. The extent to which slag lowers heat of hydration depends on slag fineness (finer grades have less effect) and the amount of slag used (using more slag results in greater temperature reductions).

Curing. Temperature and moisture conditions during curing have similar effects on the setting properties and strength development of concretes containing GBF slag and those made with only portland cement. However, curing times may need to be longer for GBF-slag concretes, since they typically develop strength more slowly. As with all portland-cement-based concretes, best results occur when the concrete is cured continuously with water for as long as possible, then allowed to dry sufficiently before the first freeze-thaw cycle.

IN SOME CASES, THE SULFATE RESISTANCE OF GBF-SLAG CONCRETES IS EQUAL TO OR GREATER THAN CONCRETES MADE WITH TYPE V SULFATE-RESISTING CEMENT.

Effects on Hardened Concrete

Permeability. When portland cement hydrates, the principal binding material produced is calcium-silicate hydrate (CSH) However, up to one-fifth of the hydration products may be calcium hydroxide, or free lime. In hardened concrete, the presence of calcium hydroxide may contribute to some durability problems. When GBF slag is mixed with portland cement and water, more CSH binder is produced. That's because the hydration of GBF slag consumes calcium hydroxide and uses it to produce additional CSH binder. Pores in concrete normally containing calcium hydroxide are filled, in part, with CSH, resulting in a denser cement paste and reduced permeability. The higher the GBF slag content, the lower the concrete's permeability.

As seen in this graph, GGBF slag significantly reduces the alkali-silica reactivity of concrete when added in dosages of 40% to 65%. The concretes here were tested in accordance with ASTM C 227, "Test Method for Potential Alkali Reactivity of Cement-Aggregate Combinations (Mortar Bar Method)."

Strength. Mineral admixtures often are essential to the production of high-strength concrete. Compressive strengths of more than 14,000 psi can be achieved by using the right proportions of GBF slag in a concrete mixture.

ASTM C 989, "Ground Granulated Blast-furnace Slag as a Constituent in Concrete and Mortars," provides for three strength grades of GBF slag. The grades are based on a slag-activity index: Grade 80 (low activity), Grade 100 (moderate activity), and Grade 120 (high activity). The numbers in the grade designations roughly correspond to the relative compressive strength at 28 days that standard mortar cubes made with GBF slag (blended with an equal mass of Portland cement ) achieve compared to the strength of a plain Portland cement mortar mixture.

Concretes made with Grade 120 GBF slag achieve higher compressive strengths after seven days than plain portland cement concrete. Strength gain at early ages (one to three days) is less, due to the slower rate of hydration. The proportion of Grade 120 GBF slag used affects the strength and rate of strength gain. The greatest 28-day strengths can be achieved with blends of 35% to 65%.

When using GBF slag primarily to increase concrete strength, make sure the slag meets ASTM C 989 Grade 120 requirement. Lower slag grades tend to result in reduced concrete strengths at all ages.

Resistance to sulfate attack.

GBF slag can improve concrete's resistance to sulfate attack, primarily by reducing the amount of reactive elements (such as calcium) needed for expansive sulfate reactions. Research shows high resistance to sulfate attack in concrete where the GBF-slag proportion exceeds 50% of the total cementitious material content (Ref.1). In some cases, the sulfate resistance of GBF-slag concretes is equal to or greater than concretes made with Type V sulfate-resisting cement.

Improvements in sulfate resistance are greatest in concretes where the alumina content of the GBF slag is less than 11%. If using slag to increase concrete's sulfate resistance, find a supplier that produces a material with low average alumina contents.

Alkali-silica reactivity. GBF can significantly reduce the potential of concrete to expand due to alkali-silica reaction. The additional CSH produced by GBF slag decreases reactivity by chemically tying up alkalies in concrete.

Determining optimum slag dosage rates is important to maximize the reduction in reactivity. Using slag contents that are 40% to 65% of the total cementitious material content can virtually eliminate expansion (Figure 2).

Color. GBF slag is lighter in color than most portland cements and will produce a lighter-colored concrete after curing. However, don't be surprised to see concretes containing high slag contents turn green or blue-green two to four days after placement. This temporary discoloration is due to a complex reaction of sulfide sulfur in the slag with other compounds in the cement. After curing materials are removed and concrete is exposed to the air for a few days, the surface usually will turn light gray or nearly white, depending on the type of portland cement used.

Proportioning GBF-slag concretes

GBF-slag concretes are proportioned by most of the same methods used to proportion portland cement concrete. However, the unique properties of GBF slag may result in:

• Lower water demands (3% to 5%) for a given slump

• A need to slightly increase the dosage rate of air-entraining ad mixtures

• The use of about 25% less high range water reducer to produce flowing concrete (Ref.1)

• No loss in workability when more coarse aggregate is used

The optimum dose of GBF slag to use in a concrete mix depends on performance requirements, materials used, and other factors. Slag dosages replacing 25% to 50% of the portland cement in mix are common. However, in concretes requiring special properties, such as sulfate resistance of low temperature rise, blends containing more than 50% slag may be needed. Conversely, when early strength gain is required for quick form removal, blends containing less than 50% should be used. As with all other concretes, GBF-slag concretes can be affected by different portland cements, admixtures, curing conditions, and placing practices. Make trial batches of all mix designs being considered.

The price per ton of many ASTM C 989 Grade 120 slags is comparable to that of portland cement. Improved 28-day strengths and workabilities achieved through a 25% or 50% portland cement replacement allow for potential reductions in overall cement contents and water-reducer levels without sacrificing concrete performance.

Concrete Construction Anne Balogh