How Fly Ash Concrete Functions

When you think of green building materials, coal is likely the last thing you would consider. Typically, coal is neither green nor associated with construction. In fact, coal is a major contributor to greenhouse gas emissions and air pollution globally. However, in an ironic turn, coal has also paved the way for the creation of fly ash concrete, a new construction material that could help reduce carbon emissions in the industry.

Fly ash is a byproduct of burning coal, and U.S. environmental regulations require power companies to capture and dispose of it properly. Disposal is challenging due to the large quantities produced by coal-fired power plants, as well as the heavy metals in coal that make fly ash a potentially hazardous material. Coal plants in the U.S. have a less-than-perfect history of handling coal ash disposal, with several high-profile coal ash spills in recent years.

To address both of these issues, the construction sector came up with the innovative idea of using fly ash to create concrete, replacing Portland cement (the primary ingredient in most concrete) with fly ash. Surprisingly, fly ash offers many properties that make it an ideal component for concrete: it is strong and durable (though its long-term performance is still being monitored), and of course, it's affordable. Furthermore, as a recycled material, fly ash helps reduce the need for concrete companies to mine other materials for concrete production.

It seems like a win-win, right? While fly ash concrete appears promising in many ways, there are still ongoing concerns about the safety of being surrounded by too much fly ash. Coal contains harmful substances, and questions remain about whether heavy metals could leach from concrete made with coal ash. Concerns have also emerged about whether using fly ash could expose builders to lawsuits and leave them without insurance coverage. Continue reading to learn more about how fly ash concrete is produced and whether it is truly safe.

What is fly ash?

In the U.S., over 50% of our electricity comes from coal-fired power plants, where coal is pulverized into a fine powder, placed in a boiler, and burned at extremely high temperatures. The heat generated produces steam, which is used to spin a turbine and generate electricity. While burning coal is a highly polluting process, modern coal plants are equipped with pollution-control devices that prevent particulate matter from escaping the smokestacks. When this equipment is cleaned, we are left with coal ash [source: Dewan].

Due to the widespread use of coal power, fly ash is one of the most abundant industrial byproducts on Earth. Rather than storing it in ponds that could eventually spill, it makes sense to repurpose it as a construction material. Not only does using fly ash in concrete prevent harmful heavy metals from being released back into the environment, it also reduces the need for cement manufacturers to mine new raw materials.

As expected, coal ash is a mixture of toxic chemicals, containing the same heavy metals found in coal, only in higher concentrations. Fly ash commonly contains heavy metals like arsenic, lead, and selenium, which are known to cause cancer and other health issues. Due to these known risks, environmentalists have long campaigned to have fly ash classified as a hazardous material, but they have faced strong opposition from the energy and now the construction industries.

Fly ash, due to its heavy metal content, poses a significant challenge for power companies when it comes to proper disposal and storage. It is usually mixed with water and stored in ash ponds, which can be vulnerable to storms, leading to dangerous spills. Presently, there are about 600 coal ash sites across 35 U.S. states, many of which have suffered from spills in recent years [source: Merchant]. The largest fly ash spill in U.S. history occurred in 2008 at the Kingston Fossil Plant, situated around 40 miles west of Knoxville, Tennessee. Around one billion gallons of fly ash sludge and water were released into surrounding land and water systems, creating an environmental disaster [source: Living on Earth].

A possible solution to the fly ash storage and disposal issue, which could help avoid spills like the one in Tennessee, is to repurpose fly ash as a construction material. When incorporated into concrete, fly ash is generally considered to be safe. Although it may contain known carcinogens, these harmful effects are believed to be neutralized once the fly ash is mixed with concrete.

How Fly Ash Is Used

Now, let’s break down the chemistry of fly ash and what makes it unique. We often refer to fly ash as an excellent substitute for Portland cement, but what exactly is Portland cement? It’s the most commonly used cement type worldwide and the main ingredient in most concrete varieties. Portland cement primarily consists of lime and silica, along with aluminum and iron. It typically includes a blend of limestone, chalk, shells, and various clays, shales, and sands. Manufacturing Portland cement is a resource-intensive and energy-consuming process, which is why finding a suitable alternative, like fly ash, is highly beneficial [source: Portland Cement Association].

Geopolymer concrete is the scientific term for fly ash or any type of concrete derived from synthetic aluminosilicate materials, which are compounds made of aluminum, silicon, and oxygen. When fly ash is mixed with cement, it doesn't merely act as a filler. Instead, it chemically interacts with the other compounds in Portland cement, becoming part of the concrete structure. Rich in alumina and silicate, fly ash reacts with alkaline solutions to form a binding aluminosilicate gel. This results in a high-quality alternative to conventional Portland cement [source: van Riessen].

In modern concrete mixes, it's common to replace up to 25 percent of the cement with fly ash, and some manufacturers are increasing that to 50 percent. However, higher fly ash concentrations require more extensive testing because the outcomes can vary significantly depending on the mix.

From a construction perspective, the primary consideration regarding fly ash concrete is its performance, and it delivers impressively. Adding fly ash enhances the concrete's strength and durability, as modifying its composition results in a tougher material. This concrete is less porous than standard Portland cement, offering superior resistance to corrosion and early deterioration [source: Ideker]. Additionally, fly ash concrete is more resistant to acid and fire, and it shows enhanced compressive and tensile strength compared to regular Portland cement.

Environmental Benefits of Fly Ash Concrete

Fly ash concrete is a remarkable paradox. While fly ash originates from one of the largest sources of air pollution and carbon dioxide emissions, it is still regarded as an environmentally friendly material. Why? The answer lies in its nature as a recycled byproduct. Since power plants will continue to burn coal and generate fly ash, repurposing this byproduct for construction makes practical sense. This recycling process not only helps reduce waste but also cuts down on energy usage and costs in the building industry.

Producing cement from scratch is a highly energy-intensive process. Conservative estimates suggest that concrete manufacturing is responsible for 5 to 8 percent of global greenhouse gas emissions, with the actual percentage potentially being higher. Over 2.6 billion tons of Portland cement are made annually, and this figure is expected to grow as the global population increases, unless a more sustainable alternative is found.

This is where fly ash concrete comes into play. Fly ash concrete has the dual ability to reduce global carbon emissions while simultaneously creating stronger, more durable infrastructure that doesn't need frequent rebuilding. In fact, one study even pointed out that fly ash bricks can absorb carbon dioxide from the environment, adding another environmental benefit [source: Liu].

The U.S. Green Building Council supports fly ash's environmental benefits. The renowned authority on sustainable building practices, Leadership in Energy and Environmental Design (LEED), acknowledges the use of fly ash concrete to replace at least 40 percent of conventional Portland cement [source: Portland Cement Association]. Life-cycle assessments, which evaluate the entire production process, show that replacing Portland cement with fly ash concrete can cut carbon emissions by up to 90 percent. This is primarily due to the fact that fly ash is a recycled material. Substituting it for Portland cement prevents excess carbon dioxide emissions and, due to its superior durability, reduces the need for frequent replacements [source: Salton].

Potential Hazards of Fly Ash

However, fly ash is not without its concerns. Since it is a byproduct of coal, which contains hazardous heavy metals and toxins, there are worries that buildings constructed with fly ash concrete could pose risks to human health.

In 2008, the biggest setback for fly ash occurred after a catastrophic spill at the Kingston Fossil Plant in Tennessee (see sidebar). A few weeks later, another smaller spill in Alabama kept fly ash in the media spotlight. These incidents, along with the subsequent cleanup and extensive news coverage, brought fly ash to the attention of the Environmental Protection Agency (EPA) and fueled calls from environmental activists for stricter regulations. Since then, the EPA has been deliberating on whether to classify fly ash as hazardous waste. Although the agency has stated that such a classification wouldn't affect fly ash used in concrete, industry experts remain uncertain [source: Purdy].

But can it truly be safe to reside in a structure made from coal byproducts? The safety of fly ash in concrete has been the subject of intense debate. While geopolymer concrete has been shown to leach toxic substances like arsenic, chromium, and selenium, a study suggests that adding calcium may reduce the leaching of these metals [source: Sanusi]. Nonetheless, many concerns persist about what will happen to these dangerous elements in fly ash once the concrete degrades and is disposed of [source: Post].

The concrete industry fears that even a law that only designates stored fly ash as hazardous could harm the reputation of fly ash concrete, placing it alongside dangerous materials like asbestos or lead-based paint. Even if fly ash is proven to be safe for use in concrete, a hazardous classification could expose construction companies to a higher risk of lawsuits.