Blended Cements

What Are Blended Cements?

ACI 116, Cement and Concrete Terminology1, defines blended cements as hydraulic cements "consisting essentially of an intimate and uniform blend" of a number of different constituent materials. They are produced by "intergrinding portland cement clinker with the other materials or by blending portland cement with the other materials or a combination of intergrinding and blending."

How Is Slag Cement Used In Blended Cements?

Slag cement can be used to produce blended cement that complies with ASTM C595, Standard Specification for Blended Hydraulic Cements2. Type I (SM), or slag-modified portland cement, contains less than 25 percent slag by mass of the finished cement. Type IS, portland blast-furnace slag cement, contains between 25 and 70 percent slag by mass of the finished cement. These cements are often produced by blending the pre-ground constituents at the cement plant, or in terminals equipped with blending equipment.

Blended cements produced by intergrinding of the constituent materials use granulated blast-furnace slag, the glassy granular material that results in slag cement when ground to a fine powder. Slag granules are added to the grinding mill along with portland cement clinker and the materials are ground simultaneously.

Slag cement can also be used as a constituent in hydraulic cements produced under ASTM C1157, Standard Performance Specification for Hydraulic Cement3.

What Are The Benefits Of Blended Cements?

Blended cements can be produced to provide the benefits in performance that are also available when slag cement is used as a separate component of the concrete mix. By vary- ing the proportions of the blend, attributes such as sulfate resistance and resistance to alka- li silica reaction can be attained with blended cement. A blend designed for a specific proj- ect requirement can also be produced. For concrete producers, blended cement may allow them to take advantage of the benefits of slag cement despite storage constraints.

Can Other Materials Be Used With Blended Cements?

Concrete can be produced with blended cement containing slag plus other cementitious materials (most commonly fly ash or silica fume) added at the batch plant. These are considered ternary systems. Ternary systems can be designed to attain performance characteristics that may be difficult to achieve in a binary system.

How Are The Cements Blended?

There are a number of systems that are used to make blended cements. Some systems are capable of "on-demand" blending, while others may blend the materials in a fixed percentage into a storage silo. All of the systems meter the constituent products in the desired proportions, and then blend them to a uniform mixture. In most cases proportions can be adjusted to produce blends that optimize the desired properties in concrete.

IS blended cement (33 percent slag content) was used to produce a noticeably lighter concrete masonry unit on the right.

Airport paving at Spirit of St. Louis Airport in Chesterfield, MO, placed with blended cement (Type I (SM)) and Class C Fly Ash added separately.

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