The primary pozzolan used in concrete in recent years has been fly ash, a residual from coal-burning power plants. Production has diminished as coal plants close or convert to natural gas. The Western U.S. is facing supply shortages of fly ash, creating big problems for concrete suppliers, including contract delivery issues, cost, capacity limits, quality, and performance. Significant additional closures and conversions are scheduled, creating a bigger deficit in an already constrained market.