6.What are the enviromental benefits of using slag cement?

Production of slag cement creates a value-added product from a material—blast furnace slag—that otherwise might be destined for disposal. Not only does the making of slag cement lessen the burden on landfills, but it also reduces air emissions at steel plants through the granulation process (as compared to the traditional air cooling process). Use of slag cement in concrete reduces the environmental impact of concrete by:

Reducing greenhouse gas emissions by eliminating approximately one ton of carbon dioxide for each ton of portland cement replaced.

Reducing energy consumption, since a ton of slag cement requires nearly 90% less energy to produce than a ton of portland cement.

Reducing the amount of virgin material extracted to make concrete.

Reducing the “urban heat island” effect by making concrete lighter in color thus reflecting more light and cooling structures and pavements with exposed concrete.

The Environmental Protection Agency recognizes the environmental benefits of using slag cement in concrete. It has classified slag cement as a “recovered” product under the Resource Conservation Recovery Act (RCRA), and has issued a procurement guideline requiring its specification on most federally-funded projects.

7.What specifications govern the use of slag cement in concrete?

Slag cement, when used as a separate component in a concrete mixture, is specified through ASTM C 989 Specification for Ground Granulated Blast-Furnace Slag for Use in Concrete and Mortar. When used as a component in blended cement, one of two specifications are used: ASTM C595, Specification for Blended Hydraulic Cements or ASTM C1157, Standard Performance Specification for Blended Hydraulic Cement.

8.How much slag cement do I include in a concrete mixture?

The amount of slag cement added to a concrete mixture, as a percentage of cementitious material by weight, normally ranges from 20 to 80 percent. The amount for a specific project depends on several factors including application, early and later age strength requirements, durability requirements and ambient temperature to name a few.

Most general or structural concrete applications (flatwork, paving, foundations, walls, columns, floors, etc.) typically use between 25 and 50% slag cement. Optimum slag cement percentage for maximum strength development is generally between 40 and 50 percent. A specification based on concrete strength at 28 days may be able to use less total cementitious material (portland + slag cement) than a similar plain portland mixture is mixture strength is optimized.

If durability parameters are specified (e.g. permeability, sulfate resistance, alkali-silica reaction (ASR) resistance) up to 70 percent slag cement may be required. For instance, one combination of portland and slag cement, in combination with a highly reactive aggregate may need as much as 70 percent slag cement to mitigate ASR, while a less reactive aggregate, combined with a lower alkali cement and slag cement may only need 25 percent.

Precast and prestressed concrete is an excellent application for slag cement at levels between 20 and 50 percent. If heat curing is used (or ambient curing temperatures relatively high) higher levels within this range are common. Using higher levels of slag cement in precast, provides the additional benefits of a more flowable mixture with smoother surface finish (fewer bugholes), and a whiter concrete appearance, often favored by designers and architects.

Mass concrete utilizes the highest potential levels of slag percentage to provide effective heat mitigation and reduced thermal stress. Mass applications use between 50 and 80 percent slag cement, with the thickest, most massive placements normally in the 65 to 80 percent range. Some slag cements in massive placements may not provide sufficient heat mitigation at the 50 to 65 percent levels because slag cement is activated by heat, and may react faster than desired because of the heat generated by portland (and slag) hydration.

9.What is the difference between slag cement and slag aggregate?

Slag cement is the hydraulic cement that results when molton slag from an iron blast furnace is rapidly quenched with water, dried and ground to a fine powder. The rapid quenching ("granulation") "freezes" slag cement in a glassy state and imparts cementitious properties to the product when ground finely.

Blast furnace slag aggregate comes in two forms:

Air cooled blast furnace slag, results when molten slag from a blast furnace slag cools slowly by ambient air (as opposed to rapid quenching), and is processed through a screening and crushing plant for use principally as a construction aggregate. Air cooled slag is not cementitious.

Pelletized or expanded slag results when molten slag is quickly cooled using water or steam. It produces a lightweight aggregate that can be used for concrete masonry, lightweight fill, or can be ground into a cementitious product.

10.What’s the best equipment to produce slag cement?

Nowadays, the slag vertical roller mills are widely accepted by customers, because it has the advantage of energy-saving, environment friendly, simple process, high intelligence.