Q.We have a job that is attempting to gain a high rating as a “green” building. They want to use “supplementary cementitious materials” in the concrete to get points for using recycled materials. Is this going to create problems for us?
A.We asked Jan Prusinski, executive director of the Slag Cement Association, to give us a primer on this topic.
Supplementary cementitious materials (SCMs) are finely divided mineral products that substitute for a portion of portland cement in concrete and enhance many concrete properties.
The three principal SCMs used in the United States are slag cement, fly ash, and silica fume. Since these are all byproducts of an industrial process, using SCMs in concrete reduces both landfill disposal and the use of virgin materials in concrete. Additionally, using less portland cement in concrete decreases the energy and greenhouse gas embodied in the constituent materials of concrete, making concrete a more sustainable material.
Slag cement (ground granulated blast-furnace slag), is produced from the nonmetallic minerals tapped from an iron blast furnace in an integrated steel mill. Slag has been used in blended cements and mortars in the United States since the late 19th century, and has been available as a separate product since the early 1980s. Fly ash is generated from coal-burning electric power plants and is removed from flue gases. It has been available in quantity since the 1930s and has been used in concrete since the 1940s. More recently, in the mid-1980s, silica fume became available commercially in the United States. It is captured from the gases of electric arc furnaces that produce silicon metal or ferrosilicon alloys used in integrated circuit manufacturing.
Slag cement, like portland cement, is a hydraulic cement—it chemically combines with water to set and harden. Fly ash and silica fume, on the other hand, are pozzolans, which do not possess cementitious value by themselves. Instead, they react with the calcium hydroxide formed during the portland cement-water chemical reaction, resulting in additional cementitious material.
When properly proportioned in concrete, each SCM can increase strength and improve durability. Slag cement and fly ash also can reduce water demand in fresh concrete and enhance concrete workability/finishability. Without these materials, significant advances in concrete technology accomplished in the last half-century—such as low-heat mass concrete, high-strength concrete, high-performance concrete, and self-consolidating concrete—may not have been possible. These materials also play a key role in economically mitigating concrete durability problems—such as alkali silica reaction, sulfate attack, and steel reinforcement corrosion.