A series of rubbled hills runs alongside Highway 89 about thirty miles north of Flagstaff. They look fairly unremarkable, but really, they’re remnants of mines that once produced pozzolan.
Pozzolans are a class of silica- and aluminum-rich materials commonly derived from explosive volcanic deposits. The early Romans knew of their value, incorporating them into aqueducts and buildings. The word pozzolan comes from a town in Italy.
Added to Portland cement, pozzolan reacts with hydrated lime. It’s beneficial because it replaces some of the cement that’s needed in the concrete end product. And it creates a more durable, long-lasting concrete that’s less permeable to water—a desirable quality in materials used for dams, bridges, and canal linings.
From the late 1950s into the early 1960s, pozzolan was mined in the volcanic field near Flagstaff and used as a key additive to the concrete for Glen Canyon Dam. When that source played out, Sugarloaf Mountain on the San Francisco Peaks was also tapped.
The hills were bulldozed, the material was milled onsite and then trucked nearly ninety miles to the Glen Canyon damsite at Page, Arizona. More than 200,000 tons of it were added to the nearly five million cubic yards of concrete that went in to the 710-foot-high dam.
Pozzolan has also been artificially produced from fly ash left over at coal-burning power plants, but that source is declining. Good-grade natural pozzolans are also considered environmental friendly—potentially cutting in half the carbon dioxide emissions and other greenhouse gasses generated during cement production.