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Phosphogypsum for Secondary Road Construction

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Phosphogypsum is a by-product of the phosphate fertilizer industry. Its main constituent is calcium sulfate dihydrate. According to recent statistics, the phosphate industry in Central Florida annually produces 33 million tons of phosphogypsum and has over 500 million tons stockpiled. It is estimated that by the year 2000, Florida could accumulate over one billion tons of phosphogypsum. The continued accumulation of phosphogypsum has created urgent pressures to find useful applications for this by-product.

The population increase in Florida is one of the highest in the country. Each year many new roads and many existing roads have to be built and upgraded, respectively. However, traditional road building materials, such as limerock, shellrock, shell and clay are in short supply in many parts of Florida. Significant tonnages of aggregates used in road construction are now imported from foreign countries. The U. S. Bureau of Mines has forecasted that Florida will have to import all its aggregate by the year 2000. The lack of adequate sources of locally available rock materials has prompted William C. Kenley, County Engineer, Polk County, and Ronald W. Williams, County Commissioner, Columbia County, to take the initiative in finding alternate material for building roads.

The experimental projects are to provide alternate methods of rebuilding county and other secondary roads in Florida. It is intended to provide comparable or better material to repair or replace existing roads with the best possible utilization or locally available aggregates.

The experimental project in Polk County consists of the construction of one and one-half miles of secondary road utilizing phosphogypsum. The Parrish Road located one mile east of Fort Meade and South of U.S. 98 was selected for the experiment because the road was convenient to the supply of phosphogypsum and easily accessed from U.S. 98.

The experimental road section in Columbia County, a two-mile rural road, is known as the White Springs Road, located south of SR 136 between I-75 and U.S. 41 southwest of White Springs.

In recent years, the University of Miami had performed extensive laboratory research into the engineering properties of phosphogypsum as well as phosphogypsum mixtures with portland cement, flyash, lime and sand under the sponsorship of the Florida Institute of Phosphate Research.

Phosphogypsum when subjected to compaction can be transformed into a solid of valuable strength. Therefore, it can be used very effectively as a binder to stabilize on-site soil and to replace shell and clay in secondary road and parking lot construction.

The design, supervision and testing of the road projects were conducted by a collective effort of the University of Miami and the Florida Department of Transportation. The experimental roads were built by the Polk County Division of Public Works and the Columbia County Department of Public Works, respectively. Evaluation of the construction crews on the projects is highly positive.

The experimental projects call for a thorough environmental impact investigation which includes the pre- and post-construction sampling of air, soil and groundwater including drinking water. Environmental monitoring as described were conducted by the University of Miami, in cooperation with the Florida Department of Environmental Regulation and the Florida Department of Health and Rehabilitative Services. Test results as obtained indicated that there was no leaching occurring into the groundwater and drinking water samples. Radiation monitoring during the construction of the roads, indicated no health hazards, either to the construction crews or the residents living in the areas.

Economic analysis on the construction of the experimental roads conducted by the University of Miami, indicated tremendous saving on the construction cost of utilizing phosphogypsum as compared to the traditional method of construction.