In 1977, Pennsylvania tested an 800-foot road base made with power-plant scrubber sludge; its strength rose from 720 to 1,014 PSI over seven years

In 1977, an 800-foot section of a state secondary road in southwestern Pennsylvania became a test site for a road-base material containing dewatered flue gas desulfurization (FGD) scrubber material.

The demonstration used a pozzolanic road base made from lime, fly ash, bottom ash and dewatered FGD scrubber material. The entire base was placed in a single lift in one day and had a compacted thickness of 10 inches.

It was then covered with a 2-inch bituminous concrete binder course and a 1-inch bituminous concrete wearing surface.

The road survived severe freeze-thaw cycles

The demonstration remained fully successful over seven years of monitoring.

Despite severe freeze-thaw cycles, the road developed no potholes or broken areas. Periodic coring also showed that the base remained intact.

The average unconfined compressive strength of cores increased during the monitoring period. It measured 720 pounds per square inch (PSI) after one year, 840 PSI after three years and 1,014 PSI after seven years, as per a FHWA report.

How FGD scrubber material was prepared

FGD scrubber material can be used in stabilized road bases and subbases when it meets required strength and durability criteria.

Before stabilization or fixation, the material is dewatered. It can then be treated with dry reagents such as quicklime, Class F or Class C fly ash, or Portland cement to produce a compactable material.

If the material does not achieve the required performance on its own, additional fixation reagent can be added. Aggregate, usually coal bottom ash, can also be blended into the mixture.

Properly designed fixated FGD material can develop strength and durability comparable to conventional stabilized base materials.

Similar projects followed in other states

Pennsylvania was one of several locations where fixated FGD scrubber material was used in stabilized road bases or subbases.

Florida had at least 12 projects between 1977 and 1989. The applications included roads, parking lots, a landfill access road and other facilities. At least 100,000 tons of fixated FGD base were placed, with performance reported as highly satisfactory.

Texas had two demonstration sections constructed in 1993. After two years of monitoring, both FGD scrubber sludge base sections showed significantly higher strength and stiffness than a control section made with crushed iron ore gravel.

In Ohio, a 250-foot test section was placed in 1995 as part of a 750-foot demonstration haul road at the Gavin power plant. The road was used extensively by heavy trucks and had survived one winter and more than a full year of use without noticeable distress.