Ashcrete is one of many substitutes for traditional concrete that have been conceived as a result of the increasing need for sustainability. Ashcrete was created by Pliny Fisk III as an attempt to mitigate both the high rate of carbon dioxide production during cement production and the disposal of fly ash, a residue of coal-based energy production. It was intended to be more environmentally friendly when compared to cement concrete, but like most materials, it comes with its own set of pros and cons.
Concrete has long been used as a building material, but in recent times has been considered unsustainable and environmentally hazardous. However, the issue does not necessarily lie with concrete, but with its composition. Concrete is traditionally a mixture of cement, sand, gravel, and water, out of which cement is the main drawback. Cement production, which requires large quantities of coal combustion, is responsible for 9% of carbon dioxide emissions globally.
As the demand for concrete increases, so does the demand for cement, making traditional concrete an unsustainable, environmentally destructive material by proxy. Luckily, research and experimentation have led to multiple alternative options to cement, producing concrete substitutes of varying characteristics and properties. One of them is Ashcrete.
Ashcrete is intended as an answer to the high demand for cement to create strong, durable structures. Ashcrete is a mixture of fly ash, borate, bottom ash, and a chlorine compound. About 93% of the resulting substance is made of recycled material, making it immediately more environmentally viable.
Additionally, fly ash, the predominant component, is produced by power plants. With the increase in consumption of coal, there is a surplus of this waste product. Ashcrete has been found to be stronger and more durable than traditional concrete.
A Detour into Fly Ash
Fly ash is the main component of Ashcrete. It is used to replace cement in the concrete mixture due to its natural pozzolanic properties. This means that, like volcanic ash, fly ash reacts with calcium hydroxide in the presence of water to produce a strong, sturdy material that is stronger than regular cement. To give you an idea of how durable pozzolans are, the Roman aqueducts and the Pantheon were both constructed using pozzolana, or volcanic ash.
Two main types of fly ash are used:
- Class C, which has high calcium content and low carbon content (less than 2%). This class is pozzolanic and is the main component of ashcrete.
- Class F – these have low calcium and 5-10% carbon. This class does not bind as well, due to the low calcium content, but does produce volatile reactions that impart resistance to concrete if added into the mixture.
Fly ash is produced due to the burning of coal, which means that CO2 would still be produced. However, relatively speaking the amount of CO2 emitted for one ton of fly ash would still be less than that produced during the manufacture of one ton of cement by a significant factor – almost 90% fewer emissions.
Advantages of Ashcrete
- Cost-effective – Fly ash is much cheaper than cement since it is a by-product of coal combustion. Therefore, the overall cost of production for ashcrete is lesser than traditional concrete.
- Low embodied energy– Embodied energy is the measure of how much energy is consumed in producing and transporting material. Ashcrete has low embodied energy, especially when compared to traditional cement, which requires a very energy-intensive manufacturing process.
- Durability– Ashcrete is stronger and much more durable, while also having a greatly reduced permeability. This is due to the fine particles of fly ash and their pozzolanic properties, which reduce cracking and bleeding after setting. Overall, it has better tensile and compressive strength.
- Workability– the spherical particles of fly ash in ashcrete allow for lesser water usage in the hardening process. This in turn allows for greater workability and pumpability, producing clean edges and smooth finishes. It also remains workable for longer, allowing for more intricate detailing.
- Resistance – Ashcrete has been shown to have higher acid and fire resistance. Fly ash also causes increased resistance to drastic temperatures and corrosion. Furthermore, ashcrete performs better than traditional concrete in shrinkage and slump tests, for lesser quantities of water.
- Water-saving– Ashcrete requires less water to begin the hardening reaction, in contrast to cement concrete which requires large quantities of water. It thus aids in water conservation. Furthermore, ashcrete has been shown to trap CO2 from the air, reducing carbon emissions.
Cons of Ashcrete
- Slower strength gain and longer setting time – while the final product is stronger than cement concrete, ashcrete takes longer to reach maximum strength due to the presence of fly ash, and this might add more time to the construction process.
- Seasonal limitations – During pouring ashcrete is more susceptible to low temperatures, which significantly lengthen setting times and strength gain. Many regions even ban the use of fly ash in winter. Furthermore, the amount of air-entraining required for fly ash concrete is higher.
- Sustainability – Ashcrete requires fly ash, which is made from coal production. While it does contribute less to global warming and pollution, it still utilizes an unsustainable method of production in the long run. Furthermore, currently ashcrete contains a chlorine compound, which is hazardous. Research is ongoing to find an alternative to this chlorine-based compound. Storage of fly ash is also an issue that is pending review.
Ultimately, with a few tweaks, ashcrete could prove an attractive alternative to traditional concrete in construction, since it has greater strength and a lower carbon footprint, especially in comparison to traditional concrete. Ashcrete has not yet gained prominence in construction outside the U.S. However, dam builders have increasingly preferred this material due to its strength and resistance to water damage.
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