Florida turned scrubber waste into road base in 12 projects by 1989; 100,000 tons earned highly satisfactory ratings, transforming an industrial byproduct into useful road base

Florida used fixated flue gas desulfurization (FGD) scrubber material in 12 road-base projects between 1977 and 1989, placing at least 90,000 metric tons, or about 100,000 tons, of the material.

The projects demonstrated that processed FGD scrubber material could be incorporated into stabilized road bases. Performance in Florida was reported as highly satisfactory, with no freeze-thaw concerns.

How scrubber material became road base

FGD scrubber material can be used as a stabilized base or subbase after processing. It can be used in much the same way as lime-fly ash or cement-stabilized base materials.


The material can be used in its as-produced condition if it develops the required compressive strength and meets durability requirements. If it does not, Portland cement, lime or fly ash can be added. Coal bottom ash or another aggregate can also be blended into the material.

In Florida, crushed lime rock was widely available and commonly used for road-base construction. Laboratory testing by the University of Florida found that a lime-fly ash-dewatered FGD scrubber sludge mixture known as Poz-o-Tec had better bearing strength characteristics than lime rock base, as per an FHWA report.

2 Florida projects used the material

The projects were built in central Florida and included one truck haul road, three parking lots, a landfill access road, a private access road, a heavy steel storage yard, a utility plant access road and four city or county roads.

The base courses ranged from 152 millimetres (6 inches) to 305 millimetres (12 inches) in thickness.

The process began with wet sludge

FGD scrubber material begins as a wet slurry. After dewatering, calcium sulfite scrubber material can have a toothpaste-like consistency and about 25% moisture.

Dewatering is generally carried out using belt filter presses or centrifuges. The material is then stabilized or fixated with dry reagents such as quicklime, Class F fly ash, Class C fly ash or Portland cement.

Fixation produces a compactable material containing at least 65% to 70% solids. Additional reagents or aggregate can be added when necessary to meet performance requirements.

Why it can work in roads

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

After seven days, compressive strengths can range from 1,400 to 2,800 kPa for pozzolanic-stabilized mixtures and from 4,100 to 6,200 kPa for mixtures stabilized with Portland cement or Class C fly ash, as per the FHWA report. Some mixtures can exceed 13.8 MPa after one year or more.

Durability can be evaluated through freeze-thaw and wet-dry testing. The source notes that mixtures reaching between 2,800 and 4,500 kPa of compressive strength within seven days will generally pass durability tests.

Florida was not the only state to use FGD material

Other states also used fixated FGD scrubber material in stabilized bases and subbases.

In Pennsylvania, a 244-metre (800-foot) section of road base containing lime, fly ash, bottom ash and dewatered FGD scrubber material was installed in 1977. After seven years, the road had no potholes or broken areas, and the base remained intact despite severe freeze-thaw cycles.

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

In Ohio, a 77-metre (250-foot) test section was installed as part of a demonstration haul road in 1995. The road was used extensively by heavy trucks and showed no noticeable signs of distress after more than one year of use.

Construction required careful handling

Construction methods for fixated FGD scrubber material are broadly similar to those used for conventional pozzolanic stabilized bases and subbases.

Plant mixing is recommended because it provides better control over the amount of each material and produces a more uniform mixture. In-place mixing of dewatered FGD material with reagents is not recommended because of its toothpaste-like consistency before fixation.

After mixing, the material should be delivered to the job site and compacted as soon as possible. Proper compaction and moisture control are important for achieving the required design strength.

The material is generally placed in compacted layers between 100 millimetres (4 inches) and 20 to 22 centimetres (8 to 9 inches) thick. Smooth-drum steel-wheeled rollers can be used for compaction.

What still needs to be studied

Although field installations containing fixated FGD scrubber material have generally been successful, FGD sludge is considered unusual and difficult to handle before processing.

The source calls for more laboratory mix-design work and additional field installations using FGD material with Portland cement, self-cementing fly ash and unconventional aggregates such as coal bottom ash, reclaimed paving materials, nonferrous slags and mineral by-products.

Longer-term monitoring and documentation are also needed to demonstrate to the highway engineering community that properly designed fixated FGD material can produce quality road bases.