GGBF Slag Cement
Like fly ash, use of blast-furnace slag in concrete has been practiced in Western Europe and Japan since the early 1940s. Blast furnace slag is the byproduct of the manufacture of molten iron, resulting from the fusion of limestone and other fluxes with the ash from coke and silica and alumina from iron ore.
While air-cooled slag has been used for decades for noncritical applications such as railroad track ballast, or landfilled or left in heaps at steel mills, in a processed state as ground granulated blast-furnace slag (GGBF) it takes on much higher value as an admixture to concrete.
According to ASTM, GGBF slag is a cementitious, glassy, granular material formed when molten blast-furnace slag is rapidly chilled by immersion in water. This chilling creates a granular product that is then ground to spec and used as an admixture in concrete where it provides improved performance over conventional concrete.
A major mass pour for a bridge pier spread footing was a major application of GGBF slag cement for the Missouri DOT. There, GGBF slag cement – in a 70-percent substitution for Type II portland cement – was being used to lower the heat of hydration in a 2,600-cubic-yard pour for a pier footing for the Phase 1 extension of Page Avenue (S.R. D) in St. Louis County. Ultimately more than 100,000 yards of the mix – representing about 10,000 tons of slag cement – would go on the job over a year and a half.
Control of heat of hydration was the prime purpose for GGBF slag cement being used. The mass pour concrete mix design called for 50 calories per gram heat of hydration.
The ready-mix producer wanted to achieve crack-free concrete, and devised a 70-percent slag cement/30-percent Type II low-heat cement mix that met the criteria. For this project, MoDOT had a crack specification of 0.03 millimeters.
The pours were averaging 4,100 psi in 28 days, and 4,500 in 56 days. In addition to the 70/30 slag/Type II cement content, the mix design included MoDOT-approved river sand and No. 67 (3/4-inch) MoDOT gradation D limestone, using W.R. Grace air entrainment admixture. Four additional mass pours of this size would be placed.
The same mix design as the mass footings was used for the columns atop the footings, with the exception that a high-range water reducer was added to enable placement of a concrete with a 5- to 6-inch slump around the steel. The spec was written as a performance spec, enabling the contractor to meet or exceed the specification of the heat spec, which specified 160oF maximum internal temperature. That’s extremely hard to do conventionally, without going to extremes such as insulated forms and liquid nitrogen.
Option of Quaternary Mix
While ternary mixes are more and more common, a quaternary mix is seen less often.
A quaternary mix was used in the Revive Wacker Drive project in Chicago. This was a unique quaternary mix, with all four elements of cement, slag, Class F fly ash and microsilica added. The city was looking to attain the optimum mix for durability, and the proportions and composition of the mix complement one another to attain a 75- to 100-year life cycle on this project.