US firm turns toxic coal ash into powerful cement

April 23, 2025

A US-based startup focused on recycling coal ash, reducing cement emissions, and decarbonizing the concrete industry has given coal ash a surprising second life by helping decarbonize concrete, one of the world’s dirtiest building materials.

PHNX Materials, a cleantech startup headquartered in California, developed a method to extract valuable compounds from fly ash, which is a finely divided toxic byproduct made from the combustion of pulverized coal, making it suitable for use in concrete.

With the production of one kilogram of cement releasing roughly one kilogram of CO₂, Krish Mehta, co-founder and CEO of PHNX Materials, says their process could significantly reduce the carbon footprint of cement, a material responsible for about 8 percent of global CO₂ emissions.

“Ash can be used to replace up to 30 percent of cement,” Mehta adds, explaining that while large amounts of fly ash exist, most of it is too contaminated to be used effectively in concrete without proper treatment.

Durable infrastructure

Cement, the binding agent in concrete, emits large amounts of CO₂ both from the high-temperature kilns used in its production and from the chemical reaction that forms it. By displacing cement with processed fly ash, builders can create more sustainable concrete without compromising performance.

PHNX’s system targets sulfur and carbon, common impurities in fly ash that concrete makers typically reject. Removing those contaminants not only purifies the ash for construction use, but also yields marketable byproducts like sulfur and aluminum. The startup is also exploring the extraction of rare earth elements, which are in high demand for clean energy technologies.

The company reportedly just closed a USD 2.5 million seed round led by early-stage investment firms known for backing science-driven and climate-focused startups such as Divergent Capital, KdT Ventures, and Overture, with participation from longtime cleantech investor Jane Woodward.

The idea of using ash in construction isn’t actually brand new to the industry, and goes back centuries to the time when the ancient Romans used volcanic ash in their concrete, which was made by mixing volcanic ash – known as pozzolana – with lime and water to create a durable mortar.

In more recent history, U.S. transportation departments have routinely used coal ash to enhance the durability of infrastructure projects. For instance, California’s Caltrans mandates at least 25 percent fly ash in its concrete mixes. Fly ash helps prevent destructive chemical reactions in concrete that can cause it to expand and crack, especially critical in infrastructure expected to last decades.

A decarbonizing mission

However, as coal plants have shut down across the U.S., 21 of which in the last two decades, fly ash has become harder to source. Coal, which once reigned supreme by generating more than half of the country’s electricity, has seen its dominance steadily fade with the rise of cleaner energy sources, now accounting for just 15 percent of the power grid.

However, the resulting shortage has forced concrete makers to reduce fly ash content to around 8 percent, substituting it with more cement, which is not only pricier, but more polluting. That shift not only compromises the strength and longevity of concrete but also raises its carbon emissions.

“When you spend a billion dollars on a highway or on a bridge, you want it to last the next 100 years,” Jorge Osio-Norgaard, PHNX Materials co-founder and CTO, explains. “Fly ash helps achieve that goal.”

PHNX’s technology aims to tap into the estimated 843 fly ash landfills across the U.S., transforming what was once a toxic liability into a climate solution. The company will sell cleaned ash to concrete manufacturers and sell extracted elements to other industries, sulfur for fertilizer, aluminum for manufacturing, and potentially rare earths for electronics.

“Unlocking this hidden supply is the fastest and most scalable way to decarbonize concrete,” Mehta concludes in a statement.