Coal ash is the less notorious byproduct of coal-fired power plants. It’s the residual solid waste that comes from burning coal. While it doesn’t have the same deleterious reputation of its airborne counterparts, tens of millions of tons of ash does up in landfills each year. Now researchers from Drexel University believe they have found a use for the powdery residue — one that could help make concrete more durable and crack-free.