Concrete and greenhouse gasses: Exploring the relationship

Researching green technology and understanding the emission of greenhouse gasses in relation to the production of concrete blocks is a key part of the construction business.

"Green" concrete blocksThe World Center for Concrete Technology (WCCT) at Alpena Community College (ACC) is a cooperative effort between the concrete industry, primarily Besser Company, and ACC. The title may sound ambitious until one realizes that Besser Company furnishes concrete block and precast equipment – up to complete turnkey factories – in more than 115 countries. Many of the block-making employees and managements of those plants come to the Center in Alpena for training.

Besser Company is providing some of the grant match for the National Science Foundation (NSF) grant to Alpena Community College for the carbon capture and sequestration research in concrete block-making processes.

The green aspect of concrete block and precast building materials comes in changing the curing process for the concrete. Curing is the natural process of forming the chemical bonds that make the concrete hard and durable. Controlling and adding moist heat to the curing process produces concrete of greater mechanical strength, hardness and impermeability to water – without as many breaks and failures – fairly important in a structural element such as a beam or wall. Moisture is key in allowing the hardening reaction to take place, called hydration, in which water molecules and the cement form crystal structures.

Though producing Portland cement has a rather high energy use, transportation energy requirements for concrete may be low since it is often made from aggregate, sand and water near a pour site.

Concrete blocks are the ubiquitous, easily transported, versatile components of many structures. After concrete is pressed into a mold, blocks are cured for 12 to 18 hours at 165 to 170 degrees Fahrenheit.

Normally, plants like those Besser Company uses for product use a steam process to keep the blocks moist. Producing the steam requires still more energy.

According to WCCT staff, McGill University in Montreal, Quebec reported that curing blocks can occur in a high carbon dioxide (CO2) atmosphere without additional heat. The NSF grant is to perform basic experiments on CO2 curing and resulting quality of the blocks.

It seems that the blocks actually take up CO2 in the process. Could this be a way to fix or sequester carbon, making the total cement/concrete block manufacturing process more carbon neutral? How much CO2 can be sequestered and how much of a CO2 saturated atmosphere does it require? Are there trade-offs in block strength or other quality standards?

Some of the staff believe CO2 impregnated blocks may have superior strength, but only testing will tell.

There are many other questions that must be answered to make the technology change practical – not the least of which are: How does one obtain the CO2 and handle it without adding much more energy? What type of curing chamber would be cost effective? This is only the ground floor, so to speak, and Alpena Community College is clearly on it.

See “Making “green” concrete: A challenging process for a better world” for part one of this series.

Photo: Green (also called “fresh”) concrete blocks from the molding machine, ready for curing. Each set of six green blocks weighs over 400 lbs.

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