Making “green” concrete: A challenging process for a better world

Northeast Michigan is at the center of a green technology research project that could make a “dirty art” cleaner for a healthier planet.

Alpena Community College’s World Center for Concrete Technology has been used by visitors from around the world for training in the art and science of making concrete blocks for construction. Now it is the home of a National Science Foundation (NSF) funded research project examining a possible industry change to making “green” concrete.

Since ancient times, people have used mixtures of minerals from the earth, aggregate (gravel) and water to make various types of the hard, heavy building materials we depend on for safe, stable, long-lived structures and pavement. It is an energy intensive process, however, resulting in greenhouse gas emissions, mainly in the form of carbon dioxide (CO2).

Making Portland cement alone requires large amounts of heat which requires fuel. Cement is the product of intensely heating various blends of materials such as limestone, chalk, shale, clay and sand almost to melting, then cooling the mix to form pebbles and rocks called “clinker.”

Fig.1: Basic cement kiln 

A basic cement kiln. These are often 20 to 25 feet in diameter and from 150 to 300 feet in length. Preheating pre-calcination are not shown here. 

The clinker is ground with gypsum or other minerals to a fine powder. This is calcination—the process of making Portland cement, which is the binder of water, rocks/gravel and sand in concrete; or, the main ingredient in mortar to hold bricks or blocks together. There are many types of Portland cement and mortar depending on the engineering requirements for its various uses.

How much energy does this take? The United States Environmental Protection Agency table below shows the older wet process and the newer, more efficient dry process and the dry process using kiln waste heat as a pre-heater and pre-calciner, which starts the cement making process before the kiln. In addition to carbon dioxide from burned fuel, the cement forming process (calcination) breaks down the calcium carbonate (CaCO3) in limestone to liberate free carbon dioxide.

Fig.2: Amount of heat input by kiln type

 Kiln type

Heat input
(million btu/ton of cement)

Wet

5.5

Long dry

4.1

Preheater

3.5

Preheater/precalciner

3.1

Source: EPA, 2007

So, all this is to show that, even with making great strides compared to the 1950’s, cement making is still a pretty dirty art. Art, indeed. In fact, the complete set of perhaps a thousand or more chemical reactions in cement and concrete making is only now becoming more completely understood, thanks to a five year Massachusetts Institute of Technology (MIT) research project.

In part two of this article series, “Concrete and greenhouse gasses: Exploring the relationship,we’ll discuss how the world-wide concrete block industry and Alpena Community College figure into a green future for concrete.

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