A by-product of blast furnaces that produce iron, slag cement is made by grinding granulated iron blast-furnace slag to cement fineness. Lafarge North America's plant in Sparrows Point, Md., was the first U.S. operation built specifically for granulated slag cement production. Prior to construction of this facility, slag was chiefly considered unusable waste material; however, the benefits of slag cement have been demonstrated in recent decades with respect to both energy conservation and mix properties.

Coming online in March 1981, the Sparrows Point plant produces NewCem, Lafarge's granulated slag product, which meets ASTM C-989, Grade 120, and AASHTO M-302 standard specifications of ground granulated blast furnace slag for use in concrete and mortars. NewCem serves as a portland cement substitute, replacing up to 70% of portland cement in concrete mixes. In the last decade, it has been used in an estimated 40 million cu yd of concrete.

The Lafarge plant produces approximately 850,000 tons of slag cement annually. It was designed and built to handle slag produced by Sparrows Point “L” blast furnace, one of the largest in the western hemisphere. In addition to processing slag from the steel mill, Lafarge also imports blast furnace slag to meet its goal of producing 3,000 tons of blast furnace slag cement per day. Because the water granulation system at Lafarge produces a consistently high-quality slag without any intermediate or lower grade materials, all of the blast furnace slag can be processed into slag cement.

Granulated blast furnace slag is highly abrasive and easily wears through unprotected piping components in the processing system. Designing the Sparrows Point plant, engineers searched for a product that could handle a high volume of harsh, abrasive slag/water slurry. Specifying basalt-lined pipe manufactured by Abresist Corp., Urbana, Ind., eliminated the majority of problems attendant upon harsh slag processing. Approximately 2,000 ft. of 19-in. diameter basalt-lined pipe were installed during construction of the plant in 1981.

When Sparrows Point relined the “L” blast furnace and relocated the slag runners in 1999, Lafarge moved its granulator and a portion of piping to accommodate the changes. During the relocation, engineers examined the 18-year-old system and found much of the original pipe in good condition.

Plant operators used the opportunity to replace pipe in highly inaccessible, critical areas subjected to the most pounding. About half of the pipe installed in 1981 between the agitation tanks and the pipe bridge was replaced. A portion of the original pipe was found in good condition and left in place, moving harsh slag slurry to this day. Lafarge Maintenance Coordinator Allan Graham says, “The Abresist has continued to perform for us. We have replaced a few elbows since 1999, but never the same elbows twice.”

BASALT PIPE AT WORK

Starting with a combination of iron ore, sinter, coke and limestone or dolomite fluxes, the blast furnace produces molten iron and a molten slag composed primarily of calcium silicates, a chemical combination of lime, silica, alumina and magnesia. The production of slag cement begins as soon as the blast furnace finishes tapping the molten iron, which has a greater density than slag and is drawn off using a dam structure, flowing from the tap hole down a runner in the cast-house floor for further processing. Separately from the iron, the molten slag is tapped off the furnace down one of four hot runners and out of the furnace building.

To ensure product quality, the Lafarge granulating system was built as close to the blast furnace as possible. With cooling, the viscosity of molten slag increases, making it more difficult to granulate. Accordingly, Lafarge maintains four hot runners and four blow boxes. At a rate of approximately three to five tons per minute, the hot runners channel the molten slag from the furnace to one of the blow boxes, where it is granulated or, if need be, diverted to a pit for air-cooling.

Rapid cooling for proper granulation is necessary for slag to become hydraulic. It is cooled from the molten state at 1500°C to a temperature less than 100°C in water. The cooling process is critical, since higher glassy composition and resultant hydraulic potential are dependent upon accelerated cooling rates. Lafarge quenches the molten slag through high-volume water sprays, maintaining a water:slag ratio of 10:1. This shock cooling instantly vitrifies molten slag into a glassy sand-like material with a glass content of 95% to 98%.

After quenching, the granulated slag and water slurry flows to the agitation tanks in refractory-lined channels called cold runners. Lafarge has two cold runners and one agitation tank on each side of the furnace. Each agitation tank is serviced by two 15,000-gpm slurry pumps. The agitation tanks keep the slag suspended in the slurry so that it can be pumped to one of five filter beds on site. A constant level of slurry is maintained in the agitation tank.

The Abresist straight pipe on the bridge between the agitation tanks and the filter beds was also examined in 1999. Although not in a critical area like the elbows and bends, the bridge pipe was checked and found intact. Showing little wear, none was replaced.

In 1989, an Abresist basalt lining was installed in the agitation tanks, since it had not been specified when the plant was built. The basalt lining appeared to work better than any material used previously, yet it did not completely withstand the impact. Graham reports, “The corrosive nature of the atmosphere at the plant also erodes the tanks from the outside. The Abresist panels were not worn, but some of the tiles had been knocked off the panel due to impact.”

When the agitation tanks were rebuilt in 1999, Lafarge decided to use Abresist's Alresist high-density alumina ceramic tiles. Rated nine on the Mohs scale of hardness, the Alresist tiles were manufactured as curved panels that bolted into the tanks, providing the wear protection needed to withstand the abrasion and impact of slag. According to Graham, the Alresist has worked well: “We have only had to replace the odd tile that came loose.”

From the pipe bridge, basalt-lined piping empties the slag slurry into a distribution box. Constructed with a ceramic-tiled lining, the distribution box has a series of gates to allow the operator to direct the slurry to one of five 2,580-cu.-ft.-capacity filter beds, where a gravel filter reduces moisture content to approximately 12%. A series of drainage pipes channels the water from the filter bed, leaving the granulated slag behind. The granulated slag is removed by a bridge crane and transported to the storage silo by conveyor. From there, the raw slag is trucked to the grinding plant for processing.

All of the slag could be dried, Graham contends, but one dry silo and one wet silo are maintained in order to mix the slag so that material feed to the ball mill is neither too hot nor too cold. By this means, optimum performance from the mill is assured.

The ball mill grinds the slag into a powder. During grinding, product fineness is closely monitored on an hourly basis to ensure that the hydraulic activity of the slag is uniform and exceeds the stringent strength requirement of ASTM-C-989 specifications. Lafarge stores the finished NewCem product in two 20,000-ton-capacity silos, distributing it by truck, rail, and barge to construction industry customers along the eastern seaboard.

BENEFITS, USES OF SLAG CEMENT

Advantages of slag cement include improved workability and pumpability of plastic (unhardened) concrete. In hardened concrete, the use of slag cement increases 28-day strengths, reduces permeability and heat of hydration, promotes sulphate resistance, and controls the alkali-silica reaction. Using slag cement during hot weather is especially beneficial, because set times in the concrete are lengthened. By contrast, slag replacement rates are usually lowered in cold weather due to its effect on set time.

Slag cement can be used as a partial cement replacement in all types of construction. A suitable proportion depends on job requirements and the condition and desired characteristics of the concrete. NewCem is reportedly preferred by many engineers and concrete suppliers who produce high performance concrete, because it provides higher compressive and flexural strengths in addition to improved workability and finishability, lower permeability, and resistance to sulfates, chlorides and alkali-silica reaction. It offers more consistent plastic and hardened properties while producing a lighter color. Common applications for slag cement also include backfilling operations and waste fixation.

This article was adapted from materials provided by Abresist Corp., (+1) 800-348-0717

GGBFS USE GROWS EXPONENTIALLY

Since the late 1800s, ground granulated blast furnace slag has been used as a component of blended hydraulic cement. In Europe, typical applications include traditional structures and those exposed to seawater; in the U.S., its use extends to general construction. As a result of granulated blast furnace slag's high quality and the efforts of all manufacturers to conserve energy, the use of blast furnace slag has grown significantly in the U.S. since the 1970s.

According to the Slag Cement Association, slag cement consumption has almost tripled since 1996 and is growing faster than any other product in the cementitious materials industry. Association statistics show that in 2003, 3.1 million metric tons of slag cement were shipped for use in construction projects. The tonnage is a combination of slag cement shipped as a separate product (conforming to ASTM C989) and as a component of blended cement (conforming to ASTM C595). The term slag cement refers to 100% ground granulated blast furnace slag (GGBFS) in North America.