Aparna KumarRajalakshmi School of Architecture

4/20/2013201

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PhosphogypsumAs a building material

Several million tons of phosphogypsum is produced across the world and is generally stacked up as a waste material due to its radioactive nature. Studies are being done to put this to use in several fields including construction of buildings as a partial substitute for Portland cement..

INTRODUCTION

The wet process phosphoric acid is manufactured by reacting ground

phosphate rock with sulphuric acid in a series of mixing tanks. After

separation by filtration, the phosphoric acid is the liquid product and

phosphogypsum (calcium sulphate) is the solid by-produce waste. For

each ton of wet process phosphoric acid, there are approximately 5 tons

of phosphogypsum produced.

Phosphogypsum is pumped with recycled transported water to disposal

sites or stored as "stacks". Over a billion tons of phosphogypsum have

been stacked already and over 40 million new tons are produced each

year. For this purpose vast stretches of land are used up.

Phosphogypsum has been subjected to extensive investigation at the

University of Miami, since May 1, 1983, under the sponsorship of the

Florida Institute of Phosphate Research. Phosphogypsum was found to be

a good construction material. With proper moisture content and high

compaction force phosphogypsum can achieve compressive strength over

1,000 pounds per square inch.

Current construction practice in building secondary roads consists of

mixing generally granular soil subgrade with find-grained soils transported

to the site. Granular soil such as sand is abundant throughout Florida.

However, lack of adequate sources of fine-grained soils such as clay has

been a major concern of road builders. Laboratory tests indicate that

phosphogypsum and sand mixtures have higher compressive strength

than that of clay and sand mixtures. Since phosphogypsum is abundantly

available in the State of Florida, the construction cost of a road utilizing

phosphogypsum and sand mixtures would be much lower than that of a

road built with clay and sand mixtures.

Engineering properties of phosphogypsum, cement and sand mixtures

were found to behave similarly to that of conventional cement mortar.

Consolidation by compaction is particularly advantageous for the

phosphogypsum- based concrete because of the contribution of

phosphogypsum to strength development. In other words,

phosphogypsum can also be used as a partial substitution of Portland

cement when phosphogypsum based cement mortar is placed by

compaction. Like conventional cement mortar, phosphogypsum-based

cement mortar is high in compressive strength while low in tensile

strength. Abaca and steel fibres can be added to the mixtures to improve

cracking resistance and ductility.

AS A BUILDING MATERIAL

Building products such as brick, block, panel and roof tile were fabricated

in the materials laboratory of the University of Miami, utilizing

phosphogypsum and phosphogypsum mixtures. Two concrete canoes and

one modular building were constructed utilizing phosphogypsum based

cement mortar. A phosphogypsum-based concrete parking lot ramp was

also built at the University of Miami.

The modular house built with phosphogypsum-based mixture was used for

a radiation emission study with three different experimental techniques to

monitor indoor radon level. All measurements indicate that radiation

exposure inside the house is low

The total world-wide production of phosphogypsum is approximately 120

million tonnes/year (39% of which is in the United States and Canada)

corresponding to almost twice the world's present day demand of natural

gypsum. Currently, only 4% of the phosphogypsum amount is utilized by

the gypsum and cement industries, whereas the bulk of the production

has to be disposed of and, depending upon the economic and ecological

circumstances, the procedures may consist of: wet stacking (pumping of

slurried waste to the sedimentation ponds); dry stacking (transportation to

the disposal site by trucks or belt conveyors) and discharge to river and

sea.

It has been found that phosphogypsum subjected to consolidation by

compaction can be transformed into a solid of valuable strength.

Observing the fact that piles of phosphogypsum (over 200 feet high)

stand very well by themselves, it would be logical to assume that this

material has appreciable shear strength. Phosphogypsum subjected to

compaction force, can be transformed into a solid of valuable strength.

Laboratory study on consolidation of phosphogypsum consists of impact

compaction (Standard Proctor method and Modified Proctor method) and

static compaction.

Although phosphogypsum possesses significant strength under air dry

conditions, low strength is observed when it contains excessive moisture.

Moisture susceptibility of phosphogypsum is observed when specimens

are tested after placing them in the moisture room for different periods to

absorb varying amounts of moisture.

It can be seen that moisture content affects phosphogypsum strength to

such an extent that even a small increase in the amount of moisture

produces a significant drop in strength.

ENGINEERING PROPERTIES OF PHOSPHOGYPSUM AND SAND MIXTURES

FOR SECONDARY ROAD AND PAVEMENT

The secondary road with phosphogypsum and sand mixtures will be

placed directly over a generally fine sand subgrade. A thin asphalt 58

concrete surface layer will be added on the top of the phosphogypsum

sand mixture base to provide a better performance for the expected

traffic volume and to meet the requirement specified in the computer

program. Therefore, the pavement will consist of three layers: a) an

asphalt concrete surface layer with thickness of 1 in., b) a base layer

composed of the optimum phosphogypsum-sand mixture (as determined

previously) with a thickness to be determined and given later according to

AASHTO Standard, based on the experimental results, c) a deep clay-sand

subgrade.

ENGINEERING PROPERTIES OF PHOSPHOGYPSUM-BASED CEMENT MORTAR

Portland cement is the most common binder used in the construction

industry. Its applications range from high strength concrete to soil

stabilization, depending on the cement content and aggregate properties.

Mixtures containing portland cement, dihydrate or hemihydrate

phosphogypsum in combination with fine aggregate (crushed limerock)

were investigated. The characteristics of the product obtained, primarily

depend on mix ratios and placement technique. The latter plays an

important role because it is closely related to the water demand and the

properties of phosphogypsum. Specimens were prepared by three

different methods: dynamic compaction, static compaction and high-

frequency vibration. It was found that for preparation by compaction and

low cement content, phosphogypsum contribution to strength was

remarkable when testing air-dried specimens. Such characteristic is due

to the self-adhesive property of gypsum when subjected to a

compressional force. The emphasis of this chapter is placed upon

determining strength properties in unconfined compression and tension

(splitting or modulus of rapture) of phosphogypsum-cement-fine

aggregate mixtures with particular attention to testing conditions in terms

of water content. Studies presented in previous chapters, have

established that not only plain phosphogypsum, but also phosphogypsum-

based mixtures are highly influenced by the moisture content at the time

of testing. The following specimen conditions at the time of testing will be

addressed:

1) Air-Dry: indicating that the specimen had been removed from the

curing environment (sealed or 100% humidity room) and maintained in

open-shelf, laboratory conditions 69 for 2 days. The resultant moisture

content of all specimens was below 3%.

2) Sealed: indicating that the specimen was tested with the moisture

content resulting from sealed curing. Sealed curing was obtained by

tightly wrapping the specimen with a plastic membrane after removal

from the mold. The 28-day moisture content was approximately 2.5%

lower than the moisture content of the mix at the time of compaction.

3) Soaked: indicating that the specimen was removed from the curing

environment (sealed or 100% humidity room) and submerged in water for

a 2-day period unless otherwise specified.

CONCLUSION:

Phosphogypsum as a building material would provide as an effective

solution as a substitute for Portland cement, as a mixture for concrete for

applications in infrastructural purposes such as laying of roads,

construction of buildings, etc.

The major advantages are that it is cost effective due to its abundance

and easy time efficient since it dries and sets quicker than regular

cements and concrete mixtures. But the major drawback is its rapid

decrease in strength due to the presence of moisture which may be

overcome by the addition of asphalt in small quantities the other

drawback is the radioactive nature of the material. Though it is very low,

people may be reluctant to use it.

THE END