The manufacturing industry is the largest source of human-created CO2 – of which, Portland cement is a major contributing factor (Exhibit 1). For every ton of cement produced, approximately a ton of CO2 is emitted (Exhibit 2). This process contributes up to 7% of global CO2 emissions annually.
At one time, almost all coal fly ash was landfilled, leading to millions of tons of waste in impoundments across North America. Now, more than half (60% in 2019) of the fly ash produced gets used beneficially in concrete instead of being disposed of – a recycling success story. Fly ash was first used as a cheap filler, but engineers learned quickly that ash can give concrete important engineering properties, such as higher long-term strength, reduced heat of hydration, resistance to alkali-silica reaction, and more. Over the past 60 years, the benefits of pozzolanic materials have been rediscovered and refined. The concrete industry has come to rely on fly ash as the go-to pozzolan.
A global transition to cleaner energy sources is underway. Coal-fired power plants are closing (or switching to natural-gas and renewable energy) at a record pace, creating an unintended problem for the concrete industry. There are now fly ash shortages in many markets!
Lack of fly ash supply is becoming an engineering problem for concrete, but it’s also a CO2 problem. Without cheap SCM in the form of fly ash, Portland cement content in concrete will likely need to increase. Higher Portland cement content means higher CO2 emissions.
Fly ash has been especially useful to the concrete industry because quality ash has traditionally been available near many concrete markets at a low price. Existing alternatives to fly ash include metallurgical slags, natural pozzolans such as volcanic ash, and metakaolin. Unfortunately, existing products cannot replace fly ash at large scale due to limited supply volume, geographical constraints, and most concisely: higher delivered price.
Many new technologies are being deployed that could help address the fly ash gap. For example, CO2 injection during concrete mixing can provide extra strength (leading to 10%+ reduction in cement content). Portland Limestone Cement achieves about 10%+ reduction in cement content by partial replacement with unheated limestone. These, and other new technologies, have their place, but the performance and environmental impact of fly ash as an SCM is arguably larger than alternatives since it routinely replaces 10-30% of Portland cement content in modern concrete, an impressive and underappreciated fact.
Terra’s solution is to manufacture a cost-competitive and eco-friendly synthetic “fly ash” that looks and performs like traditional Type F fly ash: Opus SCM.Learn More
Yes, but we shouldn’t. “Manufacturing” a ton of fly ash requires burning about 14 tons of coal, which causes 31 tons of CO2 emissions1!
The value of electricity produced by burning coal is worth several times more than the ash produced (supposing it is suitable quality for concrete), so the fly ash is traditionally considered a by-product.
It may be more useful to think of fly ash as a co-product of coal energy because fly ash has become a market commodity in its own right with distributors, resellers, and a formerly robust supply chain. Concrete industry demand for fly ash has never been higher, yet supply is dwindling. Our response to this problem is the development of Opus SCM: a “synthetic fly ash” that can be produced locally without burning coal.
1Calculation assumes: i) a coal ash content of 9% (typical of bituminous and subbituminous coal, the most frequently used coal in the US), ii) the use of a furnace (dry bottom) that produces the highest percentage of fly ash (80%), iii) carbon emission estimate for burning 1 ton of bituminous and subbituminous coal: 2.21 ton CO2/ton coal.
Improved durability of concretes produced with pozzolanic materials (vs. only Portland cement) and significantly increased longevity is expected from advanced cements.
Existing Portland cement and fly ash can be moved great distances for use in concrete production. Haul distances over 100 miles can be common. Terra’s cementitious products for use in concrete can be produced close to end-use markets, substantially reducing heavy truck movements from the roadways yielding three core benefits:
Glassy cementitious reagents (made from mine tailings themselves in Terra’s process) are a promising route for economical permanent chemical and physical stabilization of mine tailings. Possible applications are site dependent, but include the following: reducing cement requirements for cemented backfill, stabilizing sulfide concentrates and other tailings with high-leaching potential, producing valuable construction materials from mine tailings, remediating legacy sites, and more.
Photo in heading block by Jefferson Sees on Unsplash