The Manufacturing Process
Manufacturing Skyway Slag Cement requires two basic processes: granulation and grinding.
Blast-furnace slag is produced in a blast furnace during the reduction of iron ore to iron. It consists of non-metallic minerals, which are tapped slag from the blast furnace while molten. By processing blast furnace slag into Skyway slag cement, the material is diverted from landfills, and the heat energy of the blast furnace is converted into chemical energy in the slag cement for future use as a cementitious material.
This process transforms molten slag into granules. In the blast furnace at U.S Steel’s Gary Works (Gary, IN), slag is produced when the mineral components of the metal refining process separate from and float on the molten iron. The iron is drawn off to be further refined into steel, and the slag is tapped off to be granulated.
Granulation occurs when the molten slag (2,650° F to 2,800° F) is quenched with large quantities of water. This results in granulated blast furnace slag (GBFS). The granules are similar in size and appearance to beach sand. GBFS is a non-metallic, non-crystalline glass, which has similar components to portland cement clinker. After granulation, the material is drained and transported to Skyway Cement Company’s Chicago plant to be ground into Skyway slag cement.
This process transforms slag granules (GBFS) into powdered slag cement, which is a type of hydraulic cement (i.e., it chemically reacts with water to harden and gain strength).
The Skyway plant first dries the GBFS to about 2.5% moisture. The dried granules are then pre-ground with two high-pressure roll presses. Each machine consists of two large rolls, forced together by high-pressure hydraulics. The granules are fed between the rolls and crushed. To increase the fineness through this circuit, a portion of the crushed granules is recirculated through the press. The pre-ground material exiting the press is fed to the ball mill, a large tube 15 feet in diameter and 50 feet long, partially filled with steel balls. When rotated, the balls further grind the material to ASTM C989, AASHTO M302 and Grade 100 specifications. After the product is ground, it is called ground granulated blast furnace slag (GGBFS) or slag cement. The slag cement is stored in weather-proof silos for shipment by either truck, rail or barge.