Rising from coal ash waste to cure concrete

A by-product of coal-fired power plants is the tens of millions of tons of coal ash that ends up in landfills each year. Now researchers from Drexel University, the National Institute of Standards and Technology and the University of Antwerp have developed a way to turn this waste ash into a lightweight aggregate that can speed up the curing process for concrete and make it more durable and crack-free. Their discovery was recently reported in the journal Cement and Concrete Composites.

Concrete is made from a mixture of fine powder and coarse rock particles, called aggregates, bonded by a mineral glue called a ‘cementing matrix’ made of cement and water. The aggregates form the strong internal structure of the concrete as the cementing matrix hardens to bind the ingredients together in a process called curing. For concrete to reach its maximum durability, the cement must mix thoroughly with water during the curing process so it all dries — and cures — at the same time.

“This is a very important part of the process because if the concrete dries too quickly during its curing, due to added water shortage, it can form cracks and other flaws. These drying shrinkage cracks cause the surface to be susceptible to aggressive fluid ingress, creating concrete durability problems such as corrosion, salt damage or freeze-thaw damage,” said Yaghoob Farnam, PhD, an assistant professor in Drexel’s College of Engineering and principle investigator of the research.

To ensure even curing there are a number of things concrete contractors might have to do, including constantly spraying the concrete, covering it with a membrane to keep it moist, submerging it in water or creating pools of water on its surface. All of these strategies consume time and resources and are complex enough that flaws could creep into the process. To help prevent this, in the last decade researchers have developed an internal curing concept that uses porous lightweight aggregate to aid the curing process. The aggregate can maintain a consistent level of moisture inside the concrete to help it cure evenly from the inside out.

“The solution we came up with involved recycling this waste product, coal ash, into a porous, lightweight aggregate with excellent performance characteristics that could be produced at a lower cost than current natural and synthetic options,” Farnam said. “This material and process would not only benefit the concrete industry by improving the quality of their products, but it could also help keep coal ash out of landfills.”

The material the researchers came up with is called ‘spherical porous reactive aggregate’ — SPoRA for short. It is made by combining the ash with chemicals that facilitate aggregate sintering and bonding, forming them into tiny spheres and then baking them at 1160°C for a few minutes.

The end product is an aggregate pellet that can hold almost half its weight in water, which is better than traditional lightweight aggregates. And, as importantly, it can release that water at a regular rate from inside to the cementing matrix as it cures.

In the paper they report that two types of SPoRA perform better than some traditional lightweight aggregate materials — shale, clay and slate, and foamed glass — on measurements of shape, porousness, relative weight, and ability to absorb and release water. These are all key metrics related to its ability to integrate with the cement mixture and release its moisture at the right time and in the right part of the structure as it forms.

“As the concrete begins to cure on the outside, the aggregate pellets are also releasing their moisture to help it cure from the inside out as well,” said Mohammad Balapour, a doctoral researcher in Farnam’s lab and lead author of the paper. “This approach can help to maximise the durability of the concrete. And the SPoRA-making process is simple enough to produce aggregates of any size and water capacity, so we believe it could be used for a number of applications in the construction industry.”

Recycling a waste product like coal ash not only reduces the cost of making lightweight aggregate, it also ensures that concrete producers will have access to it.

Image credit: Drexel University.