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Q. In the past few years, we have noticed that mix design specifications have become more complex. Many documents contain terms such as admixtures, additives, and agents. What is the difference in these terms?

A The origin of these terms dates back to when most concrete recipes were three-part mixes: aggregates, portland cement, and water. Engineers, producers, and contractors considered any other ingredient added to the batch recipe to be an additive or admixture. With the introduction of liquid admixtures, engineers adopted the word, agent, to describe an additive that provided a particular workability characteristic to fresh concrete.

In time, the industry developed a working difference between these terms. And for the most part, the practical difference between the terms is the time at which the ingredient is introduced into the mix recipe.

According to Mike Calderon, a research engineer at CTLGroup, Skokie, Ill, in the U.S., admixture refers to an ingredient other than water, aggregates, cement, or fiber added to a cementitious mixture during the mixing process. This ingredient is added to modify the mixing, setting, or hardened properties.

When the same ingredient is added to a cement during the manufacturing process, the industry calls it an additive.

If this isn't confusing enough, cement engineers refer to pozzolanic materials and hydraulic material other than portland cement as mineral admixtures. The reason is mineral admixtures have a different effect on the recipe. These ingredients contribute additional mineral oxides to the paste.

But with the growth of more complex mix designs, researchers and engineers are attempting to further define the role of these important materials. Many researchers now refer to powdered materials such as fly ash, silica fume, ground granulated blast-furnace slag, and pozzolans as supplementary cementitious or supplementary cementing materials.

Perhaps the best answer to your question is provided by the editors who review ACI publications and update the ACI Concrete Terminology online dictionary. The Web resource contains terms common to the concrete industry that have been defined and balloted by the ACI technical committees and approved by the Technical Activities Committee. You can find this at www.concrete.org, under the Technical tab.