Concrete is a general term used to describe a mix of different sized aggregates (such as sand and small rocks) held together in a matrix by a binder, also called a cement. Concretes are mixed wet and then dry to form a hard, durable material. The binder used in conventional concrete is called Portland cement. The binder in a geopolymer concrete mix, on the other hand, is a geopolymer cement.
What is geopolymer cement?
Geopolymer cements are a class of material that combine an aluminum silicate with a chemical activator such waterglass. Aluminum silicates include a variety of naturally occurring clays as well as industrial byproducts such as blast furnace slag and fly ash from coal combustion.
Why use geopolymer cement concrete (GCC)?
Concrete has been used for thousands of years all over the world. It’s popular because it can be poured to fill most any space and take a variety of forms but hardens to a strong, long-lasting material. Many ancient concrete buildings still stand, including the Roman Pantheon with its unreinforced concrete dome spanning over 140 feet. The trend continues today with more than 5 billion tons poured per year worldwide. The problem with conventional Portland cement concrete (PCC) is that it comes with a high environmental price. Large kilns burning fossil fuels are required to produce Portland cement causing air pollution and carbon emissions. The result is that PCC is responsible for at least 7% of our worldwide carbon footprint and therefore is a significant contributor to human induced climate change.
Geopolymer cements don’t need to be fired in production nor do they give off CO2 during curing. Though they do need to be heated at low temperatures to cure, the energy required to produce GCC is considerably less than that required for PCC mixes, resulting in an up to 90% reduction in carbon emissions. What’s more, GCC can be made from a variety of aluminum silicate materials including raw clays allowing for the use of regional and local materials in their production. This further reduces their environmental impact and increases their practicality in a variety of situations. The summary: GCC mixes can be designed to have similar physical and mechanical characteristics to PCC mixes with only a fraction of the environmental footprint.
Though geopolymer research has been underway for several decades, as far as was we know UrbanEden is the first project in the world that has used geopolymer cement concrete as a building envelope solution. We think GCC is potentially a world-changing technology and are excited to be able to present it to such a large cross section of the public through the Solar Decathlon competition.
How is geopolymer cement concrete produced?
Geopolymer cements are a completely different class of material from Portland cement, with a very different chemical composition. However, GCC can be produced using the same equipment as PCC. We manufactured the precast GCC panels used for UrbanEden at a local precast concrete manufacturing facility. The main difference in the production cycle is that GCC has to be heated (in our case to 170degF) for a period in order to cure. This can be accomplished in a variety of ways, including heated forming beds and forced air heating. For UrbanEden, we embedded tubing in the concrete before pouring, then surrounded the panels in insulation and ran hot water through the tubes to bring the panels to temperature to cure. On the interior wythes, these tubes are then repurposed as part of the building’s heating and cooling system.
What is fly ash and is it safe?
Geopolymers can be produced using aluminum silicates from a variety of sources, including naturally occurring clays and industrial byproducts. We chose one such material, fly ash, for use in UrbanEden. Fly ash is comprised of the fine particulates that rise with flue gases when coal is combusted. Based on the composition of the coal that is burnt, fly ash can contain a variety of potentially toxic mineral elements including arsenic, lead, and mercury ranging in relative volume from traces to small percentages. Previously in the US, when coal was burned, fly ash was simply allowed to enter the atmosphere causing considerable toxic pollution. Through environmental regulation, technologies were implemented in the past decades to capture much of this ash.
Since close to 50% of our electrical production comes from coal, we now have a lot of captured fly ash on hand. Presently, most fly ash is put into landfills where it is in danger of leaching into the water supply. Another option is to recycle it for re-use. Since the 1930’s fly ash has been used as an additive in concrete mixes, replacing a portion of the Portland cement binder. Geopolymer cement concretes present a major new direction in fly ash recycling. Not only do GCC mixes use much more fly ash per unit volume than PCC, they have a completely different chemical make-up which results in a drastic reduction in CO2 emissions.
Is it safe? Can you use a potentially toxic material to build a healthy, sustainable house? From our point of view, a project dedicated to creating a cleaner environment is the perfect place to deal with an existing industrial pollutant. To be honest, we’d rather live in a world without fly ash stockpiles, but they are here so what are we going to do with them? Geopolymer cement concretes are an exciting option because they take a byproduct of a heavily carbon emitting industry, coal, and use it to replace a carbon emission heavy industrial product, Portland cement.
But is it safe? UNC Charlotte is an international leader in research quantifying the risks associated with heavy metals in GCC. To read a more detailed risk assessment of the fly ash in the UrbanEden concrete system written by Dr. Brett Tempest, the materials science professor whose lab generated our mixes, please see the Information Bulletin attached below.
The summary is that though GCC made with fly ash is an untested building technology, many thousands of buildings have been built in the US over the past several decades utilizing PCC with fly ash content. As a result of research and testing, it is generally accepted that buildings constructed of these mixes pose no health risks to occupants. Discussion of potential hazards when they do occur generally center around production, demolition, and disposal…periods when the concrete has particulate components. It makes sense to assume similar results for GCC buildings.
However, GCC should be tested on its own merits. In batteries of tests conducted by scientists at UNC Charlotte, GCC made with fly ash was finely ground and aggressively leached in conditions simulating worst case scenarios for demolished concrete. In these tests, measured levels of heavy metals were well within established safety guidelines (since no specific standards apply to concrete, an EPA standard for heavy metal limits in soils around residential buildings was used) . These tests would apply to demolished GCC, ground and disposed of. It seems reasonable to assume the amount of exposure for occupants of a building constructed of precast GCC would surely be even much lower because the fly ash is locked in concrete and not available as a particulate.
Even in the face of this strong evidence, we don’t want to belittle the concern over the safety of GCC mixes using fly ash. More study is needed. After the competition, UrbanEden will return to UNC Charlotte as a research lab where its performance on this and other important issues can add to the body of scientific knowledge. For this and many other reasons, we feel that the Solar Decathlon is the perfect venue to introduce geopolymer cement concrete to the world of inhabited buildings. If at the end of the day, fly ash is deemed a health hazard in concrete used in buildings, there are plenty of other readily available aluminum silicate materials that can be used to create equally useful GCC mixes.