Why is concrete potentially toxic?
This was news to us and maybe to you…concrete manufacturing has evolved over the ages, and the components of modern concrete, besides being energy and carbon-intensive, include some potentially toxic materials. We’ll try to explain it, starting first with the fact that Concrete is not the same as Cement.
Concrete is not the same as Cement: although many people use the words interchangeably, this is only because they don’t know what these substances are composed of. Think of them this way: Cement is the “glue” that holds concrete together. Therefore, cement is a very important ingredient of concrete. Portland Cement, a popular type of cement, is made by heating limestone and clay at very high temperatures in rotary kilns (about 2640 degrees F), which is when it produces “clinkers”, which are dark-colored stones. Then, the clinkers are ground up and mixed with gypsum to make Portland cement. The name “Portland” comes from the cement’s similarity to the high-quality building stone quarried on the Isle of Portland in Dorset, England. Bricklayer Joseph Aspdin patented the process for making the material in 1824, which his son William later refined. Portland cement is hydraulic, which means that it has a chemical binding reaction with water. (Back to Basics: Cement vs Concrete) If you are interested in learning more about concrete and cement, this blog by Robert Riversong gives the extensive history of concrete, including the civilizations that have built with it and the dilemma we are in today, faced with concrete projects that have failed within 100 years of their construction, sometimes very quickly and spectacularly.
Cement is one of the main building blocks of mortar and concrete. “Cement is then brought to sites where it is mixed with water, where it becomes cement paste,” explains Professor Franz-Josef Ulm, faculty director of the MIT Concrete Sustainability Hub (CSHub). “If you add sand to that paste it becomes mortar. And if you add to the mortar large aggregates — stones of a diameter of up to an inch — it becomes concrete.” (Explained: Cement vs. concrete — their differences, and opportunities for sustainability) These substances can often be found in the same area of a home-improvement store: if you open a bag of cement, it looks like a fine powdery material. If you open a bag of mortar (which is used to “glue” bricks, masonry blocks, or tile on slab), it looks similar but a gritty component added. If you open a bag of “ready-mix” cement, it has the powder, grit and larger gravel/stones all together, ready to be mixed with water.
What makes concrete so strong is the chemical reaction that occurs when cement and water mix — a process known as hydration. “Hydration occurs when cement and water react,” says Ulm. “During hydration, the clinker dissolves into the calcium and recombines with water and silica to form calcium silica hydrates.” (Explained: Cement vs. concrete — their differences, and opportunities for sustainability)
Now we’ll add one more term: “pozzolan”. Pozzolans are siliceous or alumino-siliceous materials that react with lime and water to form the hard paste that holds together the aggregate in concrete. The term comes from the Italian city of Pozzuoli near Mount Vesuvius (because volcanic soil is a natural pozzolan used to make lime concrete hydraulic). Here’s where it’s a little confusing: regular sand, although it has silica in it, is not a pozzolan, meaning it doesn’t produce a chemical reaction. Portland cement is not also a pozzolan, but pozzolans are frequently added to create Portland Pozzolana Cement (PPC). PPC is a blended cement containing 15% to 35% pozzolanic materials, which makes it more resistant to corrosive environments than plain portland cement. PPC is used for dams, bridge piers, and wall coatings.(Pozzolan Cement) Natural pozzolans include processed clays or shales, volcanic ash, and other powdery compounds. A synthetic pozzolan, fly ash is a byproduct of coal combustion, which is used to generate power. Fly ash has been substituted for some of the cement in concrete intermittently since the building of the Hoover Dam.
The production of 1 ton of Portland Cement produces roughly 1 ton of CO2, which is a high amount of carbon emissions. Although the use of fly ash in concrete is touted as beneficial to strengthen the concrete, “dispose” of the fly ash, and reduce carbon emissions, it has a major downside: the fly ash can contain a lot of heavy metals and dioxins. The heavy metals have been shown not to leach into the surrounding environment, but dioxins are troublesome. Dioxins are highly toxic, with a toxicity 130 times that of cyanide and 900 times that of arsenic. In one study, a method of reducing dioxins from municipal solid waste incineration (MSWI) fly ash before using it to make Ultra High Performance Concrete (UHPC) was to heat it to 600-900 degC for 4 hours. The content of dioxin within the fly ash was greatly reduced as a result of the high-temperature treatments, as compared to samples treated at room temperature. More specifically, the content of 17 dioxins was reduced to 1/2700 to 1/200, as compared to the levels of the room temperature samples. (Performance of ultra-high-performance concrete incorporating municipal solid waste incineration fly ash) However, pre-treating fly ash to reduce dioxins is not mandatory. We would encourage anyone who is considering building with concrete made with fly ash to read this whitepaper, which lays out the pros and cons in an excellent way. As the paper puts it, however: “Simply stated, there is no clear conclusion” regarding whether using fly ash in concrete is safe long-term.
Another source, “Is fly ash an inferior building and structural material” reads like a debate, giving both sides of the question of using fly ash in concrete. The article has many interesting examples, two of which are as follows:
From the affirmative answer to the article’s question: “...the use of fly ash in construction materials is far from safe. For example, some buildings in the United States, Europe, and Hong Kong have been found to have an increase in toxic indoor air contamination which is in direct relation to fly ash that has been used as an additive in concrete to make it more flowable. In a high rise building in Hong Kong, researchers suspect that the combination of fly ash and granite aggregation in concrete causes the building to be "hot" with the radioactive gas radon when the air-conditioning systems are shut down at night and on weekends. As a result, night and weekend workers may be exposed to higher and potentially dangerous radon levels.
One especially troubling component of fly ash is dioxin, one of the best-known contaminates of Agent Orange, the notorious defoliant used in the Vietnam War. On July 3, 2001, the British Broadcasting Corporation (BBC) featured a report on its Newsnight program about highly contaminated mixtures of fly ash and bottom ash (the ash left at the bottom of a flue during coal burning) that included heavy metals and dioxin. The mixtures had been used throughout several London areas to construct buildings and roads. Tests showed that the dioxin content of the fly ash was greater than 11,000 ng/kg (nanograms per kilogram), which is much greater than the 200 ng/kg left as a result of the use of Agent Orange. (In fact, 30 years after the end of the Vietnam War scientists still find elevated dioxin levels and birth defects in human tissues in Vietnam.)
In addition to the many hazardous compounds already contained in fly ash, the use of ammonia to condition fly ash adds another environmental/health problem. Ammonia can be adsorbed by the fly ash with the flue gas train in the form of both free ammonia and ammonium sulfate compounds. During later transport and use of this fly ash, the ammonia can desorb, which presents several concerns,” with workers, the environment, and people who are exposed to the final concrete product everyday.
Fly ash from source to source is inconsistent in regards to physical properties: size, chemical makeup and toxic materials all vary from batch to batch, just as the coal that it came from. Therefore, concrete companies should test the fly ash before incorporating it into concrete in order to at least know its strength properties, but this step might be overlooked when deadlines or costs are prioritized. “... to reduce costs, cement manufacturers have been known to use too much fly ash (typical construction specifications permit substituting fly ash for just 15% of the cement) in the production process. As a result of one such case, after an earthquake in Taiwan in 1999, many buildings collapsed. Problems with fly ash used as a fill material in cement construction has also been documented in the United States. In Chesterfield County, Virginia, at least 13 buildings built around 1997 developed problems, including floors heaving upward and cracking, because fly ash fill that had been exposed to moisture was used in their construction.”
Many people today are worried about climate change, or the anthropologic factors that are changing our planet’s environment. Since increasing amounts CO2 (a “greenhouse gas”) in the atmosphere have been implicated as causing our atmosphere to retain more heat, and processes like cement manufacturing have a high CO2 output, the substitution of fly ash to replace some of the cement seems to be a good environmental choice. From the other side of the debate: “Fly ash in the concrete improves concrete flow, and furnishes a better-looking, stronger, longer-lasting, and more durable finished product. Fly ash is readily available and relatively inexpensive. From an environmental standpoint, using fly ash in concrete reduces the amount of this byproduct of the burning of coal, which would otherwise be destined for burial in landfills.”
The advantages of concrete made with fly ash (FA-concrete) grow from its physical and chemical characteristics. Fly ash particles are small, smooth, and round in shape, attributes that allow them to move readily around the aggregate to create an FA-concrete mixture with fewer voids. On the chemical side, fly ash makes a critical contribution by reacting with the lime (calcium hydroxide) that results when cement mixes with water. The fly ash reacts with the lime to make the same binder, called calcium silicate hydrate, that is created when cement and water mix. In other words, hydrated cement yields the concrete binder along with lime, and fly ash uses that lime to make more binder. Fly ash, therefore, can be used to replace at least some of the cement in a concrete mixture. The strength enhancement in concrete made with fly ash has been confirmed over numerous studies, which actually increases over 10 years after installation. The strength is created because the fly ash continuously reacts with excess lime, removing voids that were created with water and replacing them with more binder. Therefore, thinner, lighter concrete castings can be made with FA concrete to replace the same bulkier, heavier components made with traditional concrete.
We’ve all seen cracked concrete that exposes the steel rebar imbedded in it, which in turn allows the rebar to rust and allow the concrete to crack even more. One of the enemies of reinforced concrete is salt. Both seawater vapor in coastal areas and deicing salt in colder regions can quickly ravage concrete. In various tests, FA-concrete demonstrated a greater resistance to the effects of chloride than did traditional concrete. (“Is fly ash an inferior building and structural material”)
In conclusion, we only get to choose what type of building materials are around us if we own or build our own homes, and in that case, concrete is often used at least as a foundation. If you’re building a new home, you should know about fly ash in concrete and check out the ingredients your supplier uses, to make sure the recipe is non-toxic. For a non-toxic, more carbon-friendly version, you can check out our article on hempcrete (or hemplime as some call it), which is becoming more popular.
Photo by Etienne Girardet on Unsplash
