Understanding Slag Cements
Slag cements are a product of the steel production process. Slag cement is manufactured by finely grinding granulated blast-furnace slag, a glassy by-product of iron production, and blending it with Portland cement to create a product that has a number of advantages over mixes created using Portland cement alone. The most common use of slag cement in North America is as a partial replacement for Portland cement in concrete. This practice has been growing in popularity since being introduced in the late 1800s and is now accepted by ACI, AASHTO and ASTM. The ASTM C 989 specification covers seven types of slag cements, from Type 100 to Type 700. The designations refer to percent slag content in the finished product; Types 100 through 300 contain 70-95% ground granulated blast furnace slag (GGBFS) by mass.
We have been active with this blog since 2007 and the amount of information shared has been considerable. We have spent time with the basics of slag cement, the benefits that slag cement can provide and applications of slag cement. We have also covered some historic information about the product.
We are going to take a few minutes here to explain what slag cement is, where it comes from and why we feel it provides so many benefits. Slag is a by-product of steel production. It is produced during the “smelting” process where iron ore, limestone, coke and other ingredients are melted together in a blast furnace to make steel. During this process, the limestone combines with impurities in the iron ore to form the glassy granular material we know as slag.
The composition of slag varies depending on the chemical composition of the raw materials used in making steel. Typically, there is a high percentage of calcium oxide, silicon dioxide and aluminum oxide contained in slag. The calcium oxide acts as an activator for silicates and aluminates in portland cement while contributing little or no additional strength beyond portland cement alone. The aluminum oxide and silicon dioxide are chemically combined as aluminosilicates (also called hydraulic components)
Slag Cement: An Overview
Slag cement, often called “ground granulated blast-furnace slag” (GGBFS), is one of the most consistent cementitious materials used in concrete. It is actually a byproduct of iron production. When the iron is processed using a blast furnace, slag and iron both collect at the bottom of the furnace.
The molten slag must first be separated from the molten iron. The molten slag then runs into a granulating process where it is rapidly chilled to form pellets, which are ground into a fine powder for use as a cementitious material. This material has many benefits over other cementitious materials and has been used since the early 1900s in concrete construction. In fact, ASTM C989 (Standard Specification for Slag Cement for Use in Concrete and Mortars) was first approved in 1924.
In recent years, new applications have proven even more promising for slag cement: producing higher strength concrete at all ages; reducing permeability while increasing resistance to sulfate attack, abrasion and competitive chemical attack; lowering heat of hydration to reduce thermal cracking; improving workability; and extending longevity of concrete structures.
Slag cement, also called ground granulated blast-furnace slag (GGBFS), is a hydraulic cement derived from iron production. It is a low-energy product with low emissions of greenhouse gases. It improves strength, durability, and resistance to chemical attack on concrete.
Slag cement has been used in concrete since the early part of the 20th century. The earliest recorded use was documented in Germany when it was first used in concrete construction in 1905. Between 1911 and the early 1930s, the U.S. saw its use increase by nearly 1,000 percent.
Since then, slag cement has been used successfully around the world as a replacement for portland cement in concrete in virtually all types of construction projects. Current uses for slag cement range from large government projects such as bridges, dams and highways to private sector applications such as parking lots, residential driveways and sidewalks as well as commercial applications such as office buildings and warehouses.
Slag cement has been in use for over 100 years and is among the oldest of man-made cements. Although it was originally used for its durability, strength and low heat of hydration characteristics, slag cement has gained broader acceptance for all types of concrete construction, yielding improved long-term performance, reduced energy consumption, easier placing and finishing and enhanced sustainability.
Slag cement is a recovered industrial by-product of an iron blast furnace. Molten slag diverted from the iron blast furnace is rapidly chilled, producing glassy granules that yield desired reactive cementitious properties when ground into powder. The resulting material is collected and transported to a grinding facility where it is ground into a fine powder similar to Portland cement.
Slag cement, originally known as granulated blast-furnace slag, begins with the production of iron. The heart of the process is the blast furnace that refines iron ore into iron. The ingredients are heated to nearly 1500 degrees Centigrade to form two components: iron and molten slag. When the liquid slag comes in contact with the quenching medium (water), it rapidly solidifies and crystallizes forming glassy granules that resemble beach sand. Slag cement is a recovered byproduct of this process and is one of our most consistent products.
The production of iron in a blast furnace is a continuous process: Iron ore, coke and fluxes are fed into the top of the furnace; air (oxygen) (200 lbs/ton) is blown in through openings near the bottom; and molten iron and molten slag flow out periodically from separate tap holes. As the iron begins to melt, it collects at the bottom of the furnace in a pool called “hot metal.” Molten slag floats on top of the hot metal until it is tapped off separately at an opening near the bottom of the furnace, usually around 1 hour after tapping off hot metal.
Slag cement, or ground granulated blast-furn
Slag cement is a recovered industrial by-product of an iron blast furnace. Molten slag diverted from the iron blast furnace is rapidly chilled, producing glassy granules that yield desired reactive cementitious properties when ground into fine powder. Slag Cement Association promotes the increased use of this industrial by-product in concrete construction.
Slag cement is a hydraulic binder that, like portland cement, produces durable concrete. However, unlike portland cement, slag cement has latent hydraulic properties and provides additional benefits in the fresh and hardened state of concrete compared to portland cement.
PCA has published extensive research summarizing the effects of slag cement on durability and technical performance. Scales of Use also provides summaries of more specific research on the performance of slag cement in concrete.
Anns is a contributor at PanyamCements. We are committed to providing well-researched, accurate, and valuable content to our readers.