ABSTRACT:

This paper summarizes the results of a laboratory-testing program aimed at evaluating the properties of cement-stabilized phosphogypsum mixes as potential materials for base and sub-base construction, as well as seeking a nonpollutant alternative to discard large quantities of the material. Phosphogypsum is a solid byproduct of phosphoric acid production, a major constituent of many fertilizers, whose chemical and radioactive properties may cause environmental problems. As a general rule, 5 tons of phosphogypsum are generated for every ton of phosphoric acid. This magnifies the problem of dealing with growing phosphogypsum stockpiles. This paper discussed the physical characterization of phosphogypsum, and the influence of cement content, curing time, and compaction moisture content on its unconfined compressive strength and initial tangent modulus. The laboratory results indicate that cement-stabilized phosphogypsum mixes have potential applications as road base and sub-base materials. This research was developed under the sponsorship of FAPESP.

INTRODUCTION

The production of phosphoric acid, a major constituent of many fertilizers, results in a solid byproduct called phosphogypsum. This material, which consists mainly of calcium sulfate (CaSO4.nH2O), can exist in at least three forms, depending on the value assumed by n, the number of water molecules present in phosphogypsum crystals. The n value depends on the phosphoric acid production process used, i.e., 2 for the hemihydrate-dihydrate process and the dihydrate process, — for the hemihydrate process, and 0 for the anhydrate process. The composition of phosphogypsum includes a large variety of impurities such as radioactive elements (whose effects of long-term exposure are presently unknown), and traces of heavy metals and other chemical elements that can contaminate soil and water. It also has an acidic characteristic, and runoff from stockpiles are a potential threat to surface and groundwater. Thus, due to its chemical and radioactive characteristics, phosphogypsum is an environmentally aggressive element.

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