Recycled coal ash makes better concrete
Coal ash is the residual solid waste produced by burning coal. Although a portion of it can be exploited as loose but reactive fill material to replace concrete, millions of tons of the stuff still end up in landfills each year. Civil engineer Yaghoob Farnam (Drexel University), his doctoral student Mohammad Balapour , and coworkers from Drexel, NIST, and the University of Antwerp have now found a way to use that leftover waste. They developed a method that turns it into a concrete additive that improves the material’s durability by promoting a uniform curing process.
Concrete is a mixture of fine and coarse particles, collectively known as aggregate, that forms a structural framework held together by a matrix of cement and water. For the concrete to remain crack-free, the mixture must dry slowly—over a period of days—and uniformly. Farnam, Balapour, and their colleagues realized they could tailor the properties of coal ash to produce a porous, lightweight aggregate that absorbs and desorbs water more efficiently than the traditional aggregates of shale, clay, slate, and foamed glass. The researchers made the new material, spherical porous reactive aggregate (SPORA), by blending the ash with sodium hydroxide to chemically bind it, packing the wet mixture into spherical pellets 16 mm across, and then sintering the pellets at 1160 °C for less than 10 minutes.
Computed tomography revealed the structural origin of SPORA’s performance. Stacked together, two-dimensional x-ray projections, such as the one shown here, of the newly sintered SPORA pellets capture the 3D pore structure (white). Those pores hold almost half a pellet’s weight in water—an amount that exceeds the capacity of traditional aggregates. And because the pores are open and interconnected, they release the water from the pellet’s interior. That’s important because as concrete begins to cure on the outside, the SPORA pellets release most of their own moisture so that the concrete’s interior cures at the same time. (M. Balapour et al., Cem. Concr. Compos., 2019, doi:10.1016/j.cemconcomp.2019.103428 .)