PHOSPHOGYPSUM

1. INTRODUCTION

Phosphogypsum (PG) is a waste by product obtained by the processing of phosphate rock for the production of phosphoric acid in the fertilizer industry by Wet acid method. Phosphate ores are naturally radioactive substances and their radioactivity originates mainly from 238U and 232Th. The PG is mainly contains the compound of CaSO₄ .2H₂O but also contains impurities such as H₃PO₄, Ca(H₂PO₄)₂.H₂O, CaHPO₄.2H₂O and Ca₃(PO₄)₂, residual acids, fluorides (NaF, Na₂SiF₆, Na₃AlF₆, Na₃FeF₆ and CaF₂),sulphate ions, trace metals (e.g. Cr, Cu, Zn and Cd), and organic matter as aliphatic compounds of carbonic acids, amines and ketones, adhered to the surface of the gypsum crystals. The Fig 1.1 below shows the by-product phosphogypsum.

Fig.1.1. Phosphogypsum

The quantity of PG produced is very large such that for each 1 ton of phosphoric acid production, there is a coproduction of 5 tons of calcium sulphate (PG) ^[1].

Phosphoric acid can be produced by two methods a) Dry thermal method and b) wet acid method. In dry thermal method produces element phosphorus using an electric arc furnace. The wet acid method includes the treatment with sulphuric acid.

Fig 1.2 shows the schematic representation of phosphate process.

The process included are given below chemical equation 1.1

Ca₅ F (PO₃)₄ + H₂SO₄ +10H₂O = 3H₃PO₄ + 5CaSO₄ .2H₂O + HF……… (1.1)

The wet process is economic but generates a large amount of PG (5 tons of PG per ton of phosphoric acid produced) ^[1]. The nature and characteristics of the resulting PG are strongly influenced by the phosphate ore composition and quality.

Wet processing causes the selective separation and concentration of naturally occurring radium (Ra), uranium (U) and thorium (Th). About 80% of 226 Ra is concentrated in PG while nearly about 86% U and 70% of Th end up in the phosphoric acid.

Fig.1.2. Schematic Representation of Phosphate Process

1.1. PROPERTIES OF PHOSPHOGYPSUM

Properties of PG may vary with respect to some of the factors given below

1) Nature of the phosphate ore used

2) Plant operation efficiency

3) Disposal method

4) Age

5) Location and depth of land fill or stack where the PG is dumped

PG is a powdery material has no plasticity. It is mainly of calcium sulphate di hydrate, (>90% gypsum and sodium fluorosilicate (Na2SiF6). Due to the residual phosphoric, sulphuric and hydro-fluoric acids in the porous PG it is an acidic by product of pH <3. PG from filter cake usually has a free moisture content of 25-30%. The vertical hydraulic conductivity of PG has been reported to range between 1×10-3 and 2 ×10-5 cm/s. The free water content may vary greatly depending upon how long the PG has been allowed to drain after stacking and on local weather conditions. PG solubility is depending upon its pH, and it is highly soluble in salt water. Its particle size density ranges between 2.27 and 2.40g/cm3 and its bulk density between 0.9 and 1.7 g/cm3.

While in morphological point of view PG has a predominant particle size ranging between 0.250 and 0.045 mm in diameter, depending on the sources of the phosphate rock and the reactor conditions sowed that PG has a marked crystal structure, mostly rhombic and hexagonal forms.

2. TREATMENTS

In view of the above mentioned characteristics of PG ( more than 90% gypsum) and its attractive economic potential, PG have wide applications in construction field and soil stabilisation. But due to its increasing concerns about environmental pollution some treatments are carried out in PG for its purification. Treatments are classified as chemical treatment, physical treatment and thermal treatment.

2.1. CHEMICAL AND PHYSICAL TREATMENTS

For making the PG harmless and making suitable for applications methods such as washing, wet sieving and treatment with citric acid are done.

2.1.1. Treatments with Citric acid

PG samples were thoroughly shaken with 2-5% aqueous citric acid solution in a mechanical shaker for 15-25 min at 30 ºC, filtered through a Buchner funnel and washed with 0.5-1 % aqueous citric acid solution then washed with plain water two to three times. The purified gypsum samples were dried at 42 ºC and analysed for residual impurities.

Impurities of phosphates present in PG combine with aqueous citric acid and form water soluble phosphoric acid (H3PO4), whereas fluorides combine with aqueous citric acid and form water-soluble sodium citrate (Na3(C6H8O7), hydrofluoric acid (HF), hydrofluorosilicic acid (H3SiF6), hydrofluoroaluminate (H3AlF6) and hydrofluoroferrate (H3FeF6) compounds as per the following chemicals equations 2.1.

Reaction with phosphatic compound are shown below

C₆H₈O₇ + 3Ca(H₂PO₄)2.H₂O = Ca₃(C₆H₅O₇)2 + 2H₃PO₄  ……… (2.1)

C₆H₈O₇ + 3CaHPO₄.2H₂O = Ca₃(C₆H₅O₇)₂ + 2H₃PO₄ + 2H₂O

C₆H₈O₇ + Ca₃(PO₄) = Ca₃(C₆H₅O₇)₂ + 2H₃PO₄

Reaction with fluoride compounds are shown below

2C₆H₈O₇ + 3NaF = Na₃(C₆H₅O₇)₂ + 3HF

C₆H₈O₇ + 3Na₂SiF₆ = Na₃(C₆H₅O₇)₂ + H₃SiF₆

C₆H₈O₇ + 2Na₃AlF₆ = Na₃(C₆H₅O₇)₂ + H₃AlF₆

C₆H₈O₇ + 2NaFeF₆ = Na₃(C₆H₅O₇)₂ + H₃FeF₆

2C₆H₈O₇ + CaF₂ = Ca₃(C₆H₅O₇)₂ + 3HF

All the above products are water soluble and can be easily removed with a stream of water.

2.1.2. Wet Sieving Method

This process of purification process is based on wet sieving and hydro cyclone trials. In this method the PG sample was firstly wet sieved through a 300 micron sieve and then washed and dried at 42º C.

Results showed that impurity concentrations were lower in the fine fraction that passed through the sieve (85%) than in the coarse fraction retained in sieving (15%), dropping from 1.28 to 0.41% for P2O5, from 1.80 to0.57% for fluorides, and from 1.58 to 0.34% for organic matter.

In case of hydro cyclone purification treatment, PG was mixed with water (in a proportion of 1:3 by volume) for 30, 50 and 65min in order to solubilize the impurities. Under flow and over flow samples were collected from the PG slurry and dried at 42ºC. Comparatively to the over flow samples the under flow samples were characterised by high pH values 5.8 – 6, a high SO3 content, and a lower level of P2O5, fluorides, organic matter and alkalis. These results are similar to those obtained with wet sieving treatment.

The PG purification process employed by, consisting of washing PG with water, sieving it through a 100 micron sieve, and calcining it at different temperatures (low and high). In order to improve the setting time and the compressive strength of the resulting material, accelerators (such as calcium hydroxide) were added to the calcined PG.

2.1.3. Production of Beneficiated PG

Based on the wet sieving technic a pilot plant is used to produce beneficiated phosphogypsum. Pilot plant consists of a mixer, 300 micron vibratory screen, centrifuge, rotary drier, ball valves and centrifugal pumps. The slurry tanks, settling tube and centrifuge were connected with each other through pipe fitted with control valve system. The rotary drier was fitted separately with a hopper for supplying PG and an automatic discharge for PG. The slurry tank was filled with requisite quantity of water and churning was started. The unprocessed PG was introduced into the mixer in proportion of 1:3 by volume of PG and water respectively. The gypsum water slurry was allowed to enter through the side valve slurry pump for uniform mixing.

The gypsum slurry was pumped into the vibratory screen to remove coarse fraction. Majority of gypsum passing through the screen is shifted to the centrifugal pump to remove excess of water containing water soluble P2O5, F, organic matter and alkalies. The gypsum containing 25-26% free moisture was dried in LPG run rotary drier at 110-120ºC to get dry beneficiated phosphogypsum of free moisture content of below 2%. Fig.2.1 Shows the Pilot Plant for Beneficiated PG.

Fig. 2.1. Pilot Plant for Beneficiated PG

2.2. THERMAL TREATMENT

PG washed with water and not washed was calcined at temperatures of 170, 600, 750, 850, and 950 ºC for 3 h. The calcination process was carried out using an electrical oven. The composition of PG oxides after calcination was determined by chemical analysis and is presented in Table 2.1. The compound composition also determined composition after calcination using the X-ray diffraction technique and results are given in Table 2.2.

Table 2.1. Composition of PG by Chemical Analysis

Table 2.2. Compound Composition by X-Ray Diffraction

3. PG AS A SOIL STABILIZER

Chemical stabilisation is the process of modification of the properties of a locally available soil to improve its engineering performance. Phosphogypsum is used with cement and fly ash (FA) as a chemical stabilisation material to improve the Engineering properties of selected soils. PG and FA are industrial by-products generated by phosphoric fertilizer industry and thermal power plant. PG can be stabilised with class C fly ash and cement for potential use in soil stabilisation.

FA is a pozzolanic material and has been classified into 2 classes F and C, based on the chemical composition. Class F fly ash is produced by burning anthracite and bituminous coal and class C fly as is produced by burning lignite and sub bituminous coal.

The major difference between class F and class C, fly ash is in the amount of calcium and the silica, alumina, and iron content in the ash. Class C fly ash in addition to having pozzolanic properties, also has some cementitious properties and it has been successfully used as part of the binder in stabilised base applications.

The addition of Portland cement or fly ash to PG yields slightly higher maximum dry density and optimum moisture content values for stabilised PG mixtures than those of unstabilised PG samples. These studies recommended that cement with C3A content less than 7% can be used with PG.

Generally the clay content of the soil increases the quantity of cement required for stabilization. Some researchers recommend that the range of cement contents can be selected as 5–15%. It is also recommended that stabilized base mixtures containing PG should be designed as close as possible to optimum moisture content and maximum dry density conditions, as determined by either the modified or standard Proctor test method.

3.1. EXPERIMENTAL STUDY

Representative soil samples were collected at 1.5 m depth from deposits in Balikesir, Turkey. The properties of the soil samples designated as soil I and soil II are shown below Table 3.1

Table 3.1. Properties of Soil

Soil samples can be classified as an A-7-5 soil in the AASHTO soil classification system. The soil samples are MH/silt and CH/clay type according to the Unified Soil Classification System. The cement used was Portland composite cement. The physical, chemical and strength properties of Portland composite cement (C) are presented in Table 3.2.

Table 3.2 Chemical Composition of Materials

The percentage of cement retained on sieve no. (45 micron sieve) was 6.9%.FA was obtained from Soma Seas Thermal Plant in Manisa, Turkey. The Soma FA was produced from lignite coal and contains significant amount of CaO with a lime content of 15.34%. The chemical composition of FA is given in Table 3.2.

According to ASTM C 618 Soma FA can be classified as class C fly ash due to its chemical composition. This fly ash in addition to having pozzolanic properties also has some cementitous properties. The total amount of SiO2, Al2O3 and Fe2O3 is 74.32% which is an amount larger than the value given by ASTM standard for type C class fly ash. The amount of SO3 with 0.99% is less than the value given by the standards. Pozzolanic activity index (PAI) of Soma FA is 88% at 28 days and this value satisfies the ASTM C 618 limit (75%). PAI also meets the TSI and EN criteria that are 75% and 85% at 28 days and 90 days, respectively. The retained on the sieve 45 micron was 16% which was less than 40%.

3.2. PROCEDURE

PG is generated as a filter cake in the wet process and is pumped in slurry form to holding ponds. The wet PG may need to be spread out in fairly thin layers for a few days. For this reason the appropriate amount of PG and soil were air dried. The air dried soil was first passed through a 425 micron standard sieve before tests. The required amount of stabiliser measured as a percentage of dry soil was added to soil and mixed thoroughly to produce a homogenous soil blends. Then the appropriate amount of water calculated by weight of the soil mass was sprayed on the soil blends.

The samples were moulded at maximum dry density and optimum moisture content in accordance with TSI procedure. Atterberg limits, standard proctor tests and unconfined compression strength tests were carried in the soil were stabilisers are added at variable percentages. Compaction characteristics and the description of soil mixtures are given in Table 3.3 and 3.4 respectively.

Each soil samples used in the unconfined compressive strength tests was statically compacted in the cylindrical in the mould (38 mm in diameter and 76 mm in diameter high) at the optimum moisture content and maximum dry density. For curing, the samples were closely wrapped and placed in laboratory room where the temperature was maintained around 21ºC. The samples cured for 2 days and after curing unconfined compressive strength test was conducted.

3.3. RESULTS AND DISCUSSIONS

The effect of cement stabilisation and cement and PG stabilisation on the consistency limits are shown below in Table 3.3.

Table 3.3. Effects on Consistency Limits with C and PG

It can be observed that a reduction in plasticity of stabilized soil as a result of increase in liquid limit values. Treatment with cement and phosphogypsum generally reduces the plasticity of the soils. Plasticity index was not determined for each soil with addition of 10% and 15% of cement.

Generally 2.5–5% of cement and 2.5–5% of phosphogypsum show the optimum amount to reduce the plasticity of soils. Fig.3.1. shows the effect of the addition of cement and cement–phosphogypsum mixtures on the compaction characteristics of the soils.

Fig. 3.1. Variation of Compaction Characteristics of Soil Stabilised with

Cement/Cement –PG, a) Maximum Dry Density b) Optimum Moisture content

An increase in dry unit weight and a decrease in optimum moisture content occurred as the cement and phosphogypsum contents increased for all soils. The increase in dry unit weight is generally accepted as an indicator of improvement. The maximum dry unit weight and optimum moisture content and consistency limits of soils mixed with fly ash and fly ash–phosphogypsum mixtures are reported in the Table 3.4.

Table 3.4 .Consistency Limits with FA and PG

The addition fly ash generally decreases the plasticity index. Fly ash reduces the plasticity index of high plasticity soils but has little influence on the plasticity index of low plasticity fine soils. This behaviour is attributed to smaller particle size, higher specific surface area and less crystallinity that make the clay minerals more susceptible to lime.

The effect of fly ash and fly ash–phosphogypsum on maximum dry unit weight and optimum moisture content of stabilized soils are shown in Fig.3.2.

Fig. 3.2. Variation of Compaction Characteristics of Soil Stabilised with FA/FA –

Phosphogypsum, a) Maximum Density b) Optimum Moisture Content

Maximum dry unit weight and optimum moisture content decreases with increasing of fly ash and phosphogypsum content. There is also a substantial decrease of optimum moisture content at 5% of FA for soil II and after then the value remains relatively constant. By the addition of 5% of fly ash alone causes the increase in optimum moisture content of soil II.

The maximum dry unit weight decreases with increasing fly ash content because of the lower specific surface gravity of the fly ash than that of the soils. Some researchers also indicate that the reduction in dry unit weight occurs because of both particles size and specific gravity of soil and stabilizer. Fig.3.3 shows the unconfined compressive strength test results of soils stabilized with cement, fly ash and phosphogypsum. Unconfined compressive strengths of untreated soils were in all cases lower than treated soils.

Fig. 3.3. Unconfined Compressive Strength of Soil stabilized with Addition of (a) Cement and Cement–Phosphogypsum, (b) Fly ash and Fly ash–Phosphogypsum.

There is significant gain strength with addition of cement. The gain in unconfined compressive strength is dependent on the cement content. The cement content has significantly higher influence than fly ash content. A high increase in unconfined compressive strength occurred with 15% of cement content for soil II.

4. CONSTRUCTION & BUILDING APPLICATIONS

Manjit and Mridul at 2000 developed the production of stable, high strength anhydrate cement according to USA standards. PG was exposed to 500, 600, 700, 800, 900, and 1000 ºC for 4h, and after cooling, the different anhydrates obtained were grounded in a ball mill. X –Ray diffraction and scanning electron microscopy studies showed that compressive strength improved in all cases with curing time and mainly exceeded the USA standard value of 17 Mpa at 28 days. The maximum compressive strength (38.90Mpa) was obtained within the use of a mixture of Na2SO4 and FeSO4 as activator ^[1].

Mridul at 1996 reported that a cementitious binder made with calcined PG (hemihydrate), fly ash and hydrated lime in proportions of 40, 40, and 20% respectively, and cured for 28 days at 50ºC can have different applications as a building material.

The binder materials cured at 27 ºC exhibited higher strength, better water resistance and lower porosity, which is attributed especially to the high amount of hydration products such as ettringite and tobermorite. The durability test involved various cycles of alternate wetting and drying on the one hand, and heating and cooling on the other hand by increasing the temperature from 27 to 60 ºC. When the cementitious material was cured at 50 ºC, contrarily to those cured at 27 ºC, a lower fall in strength and weight loss was observed with the alternate wetting and drying cycles. Moreover, in the case of alternate heating and cooling cycles no strength variation or weight loss was noted ^[1].

4.1. CEMENT INDUSTRY APPLICATIONS

H. Taybi et al had tested the effect of PG as a mineralizer on the burning temperature of clinker and as a set controller in Portland cement as well as its effect on cement properties and its effect on cement setting time and mechanical properties. Mehta & Brady at 1997 proposed to reduce the temperature of clinker formation by adding and mixing the PG with the raw mix before clinkering [1].

Cement were made by inter grinding PG and reagent gypsum with the raw mix to obtain a clinker with 2% SO3 content, which was burning at the temperature 1285 ºC for 1 h and then cooling by air. On the other hand, by grinding PG and the reagent gypsum added directly to the clinker. The data indicated that the cement prepared with PG which was mixed with the raw mix before clinkering required the lowest time of milling (55min) while the one made by incorporating PG directly to the clinker required 90 min. This confirmed the negative effect of the impurities on the grinding process [1].

4.1.1. Effect of PG in Portland cement

It is shown that the setting time of the cement is prolonged and the compressive strength of the cement was reduced to a greater extend as compared to the natural gypsum. The prolongation of setting can be attributed to the formation of the protective coatings of Ca3(PO4)2 and CaF2. As a result the hydration of cement is suppressed. These layer no longer as a barrier for the water to react with the cement. Chemical equation 4.1 shows the formation of protective layer.

H₄P₂O₇^2- + 4OH- = 3H₂O + 2PO₄^3- ………………… (4.1)

2PO₄^3- + Ca²⁺ = Ca₃(PO₄)₂

SiF₆^2- + 6OH^- = 6F^- + 6 SiO₃^2- + 3H₂O

17 6F^- + Ca²⁺ = CaF₂

On addition of beneficiated phophogypsum the retardation of setting time reduced and the compressive strength increased due to the removal of impurities. Table.4.1 shows the properties Portland cement with PG.

Table 4.1. Properties of Portland Cement with PG

The cement made by incorporating PG before clinkering and in comparison with that made by adding directly to cement plant clinker allowed very efficient energy savings by reducing the heat energy required for the clinker process and power consumed for the grinding process.

The addiction of 5% PG at 900ºC to 90% Portland cement in the presence of 5% silica fume improves the hydraulic properties of cement as well as its mechanical properties for up to 90 days ^[1] . Different PG percentages between 0 and 10% were mixed with the raw material and heated for 30 min at a rate of 30ºC/min up to1450 ºC to produce clinker. The incorporation PG resulted in an increase in the initial limestone de carbonation temperature from 750 ºC for the clinker raw material without PG to 900 ºC. At 950 ºC an adverse effect was observed due to the catalytic effect of PG which accelerates the lime combination process and the free lime content was more significant in the mixture with a lower PG content. When the heat temperature reached 1000 ºC the mixtures containing a higher amount of PG showed that lowest free lime proportion demonstrating that de carbonation was completed.

However the clinker forming temperature was decreased in relation to the PG content. PG with 10% PG contained a higher alite percentage of (56%) and negligible free lime content (0.4%), in spite of the low burning temperature of 1200 ºC the ordinary clinker contained 52% alite at 1450 ºC [1].

In comparison with the ordinary clinker the PG clinker was characterised by a high SO3 content and the presence of small crystals and a low amorphous phase. The grinding time was decreased from 26 min for ordinary clinker to 19 min for the PG clinker [1].

H.Taybi et al was attempt to incorporating PG in the manufacture of Portland slag cement. The process consists of burning PG for 2 h in a muffle furnace at 200, 400, 600, 800 ºC with a heating rate of 10 ºC/min. after calcining, PG is cooled, crushed and ground in an agate mortar machine. It is the sieved through a 90 micron sieve before mixed and homogenised with Portland cement clinker and blast furnace slag.

The results obtained indicate that increasing the temperature of the PG treatment reduced the P2O5 and fluorides content. Consequently the initial and final setting times of the pastes were decreased while their mechanical strength improved and also high degree of hydration. Therefore the hydraulic properties of Portland slag cement can be effectively with the incorporation of 6% of thermally calcined PG at 800 ºC.

The combination of calcined PG– fly ash–lime hollow blocks act as an alternative to conventional wall bricks. The procedure involved firstly mixing calcined PG and fly ash in different weight proportions and then incorporating the hydrated lime slurry to the mix. The study showed that the compressive strength of the hollow blocks increased by extending the curing time from 24 to 120 days. For a specific proportion of PG (20–30%), the compressive strength was improved when increasing the amount of fly ash, reaching a maximum value at a fly ash content of 35–40%. Also, for the same proportion of fly ash, when the PG content was reduced from 30 to 20% the hollow blocks exhibited lower strength. Concerning the durability of PG –fly ash– lime hollow blocks in sulphate environments, it was reported that they have sufficient strength for use as a construction material [1].

On the other hand, studies have been focused on the development of several applications of PG with cement, fly ash and lime in the construction industry. The potential of using calcined PG as an activator of fly ash–lime binders to increase early strength. PG used was heated at 135 ºC for 3 h and then crushed in a roller mill for about 8 min before being incorporated in different proportions (0, 5, 8,10,12 and 15% by weight) to the fly ash and lime mixture [1].

The specimens obtained were cured in two conditions, some at room temperature and the others firstly at 45 ºC and 90% humidity for 12 h and then at room temperature. The study showed that the specimens showed different behaviour according to their curing conditions. In the case of the specimens cured at room temperature, compressive strength was significantly improved by the incorporation of 8% calcined PG. Also, for the same fly ash content (75%) and an increasing PG content, the early strength (1, 3 and 7 days) improved, while the late strength (after 7 days) decreased. The strengths at different ages of samples cured first at 45 ºC in 90% humidity were considerably improved.

The mineralogical study of the specimens with calcined PG confirmed the formation of ettringite and gypsum, which accelerated the pozzolanic reaction of fly ash and consequently greatly enhanced the mechanical strength [1].

5. ENVIRONMENTAL IMPACT OF PHOSPHOGYPSUM

PG management is one of the most serious problems currently facing in the phosphate industry. The storage of PG without any prior treatment requires large land areas and can cause serious environmental contamination of soils, water and the atmosphere. Negative atmospheric impacts can be caused by the erosion of PG piles and the release of highly polluting substances, due to the presence of hazardous vapours containing heavy metals, sulphates, fluorosilicates.

5.1. ATMOSPHERIC CONTAMINATION

The impact of PG radionuclide concluded that 90% of Po and Ra originally present in phosphate rock remain in PG, whereas the remaining U percentage is well below 20%. Thus the potential problem of PG piles is the emanation of 222Rn from the alpha-decay of 226Ra, a radionuclide classified by the USEPA as a Group A human carcinogen, whose common presence in PG led to the regulation of PG disposal under the National Emission Standards for Hazardous Air Pollutants (NESHAP) and the National Emission Standards for Radon Emission from PG Stacks.

The EPA ruling restricts PG exceeding 370 Bq/kg of 226Ra from being used on agricultural soil. The maximum regulatory limit of 222Rn exhalation (the flux density of 222Rn gas entering the atmosphere from the surface of a 226Ra-bearing material) is 0.74 Bq/m2/s [1].

As an example PG stacks located on salt marshes in Huelva (Spain) contain about 100 Mt of PG (area of approx.1200 ha, with an average height of 5 m) and are generally not completely water tight or even covered with any inert material, leading to a local gamma radiation level between 5 and 38 times the normal rate (0.74 Bq/m2/s).

5.2. WATER CONTAMINATION

Another matter of concern is the leachability of hazardous elements from PG and thus the contamination of groundwater under lying PG stack. Since PG waste is generally transported and disposed as aqueous slurry, PG piles can be affected by tidal variations and dissolution/leaching of the elements naturally present in the PG can occur.

Dissolved elements may be deposited in nearby soils or transferred to waters and finally to living beings. When PG was exposed to natural weather conditions (rain) the maximum 226Ra activity in the leachate was 0.53 Bq/l while the minimum was 0.07 Bq/l. Most 226Ra values determined in the leachates exceeded the limit value of 0.1 Bq/l prescribed by the Bureau of Indian Drinking Water Standards.

The laboratory results indicated that rainwater leached less 226Ra (0.09–0.28 Bq/l) than distilled water (0.08–0.38 Bq/l). When PG was exposed to natural weather conditions (rain) the maximum 226Ra activity in the leachate was 0.53 Bq/l while the minimum was 0.07 Bq/l. Most 226Ra values determined in the leachates exceeded the limit value of 0.1 Bq/l prescribed by the Bureau of Indian Drinking Water Standards.

6. CONCLUSION

Since it is a waste by-product obtained in the fertilizer industry it should be safely utilised rather than dumping due to its negative impact. PG is a harmful radioactive element and the the potential problem of PG piles is the emanation of 222Rn from the alpha-decay of 226Ra and the leachability of PG in to ground water. Thus PG stock piles possess a negative impact to the surroundings.

The study about the phosphogypsum concluded that.

1) Treatment with phosphogypsum, fly ash and cement generally reduces the plasticity index. Principally, a reduction in plasticity is an indicator of improvement.

2) The maximum dry unit weight of phosphogypsum stabilized soils increases with increasing phosphogypsum content. Besides this, fly ash content decreases the maximum dry unit weight.

3) Unconfined compressive strengths of unstabilized soils were lower than the stabilized soils. The cement content has a significantly higher influence than the fly ash content.

4) The anhydrite cement produced from PG has lower energy requirements than other traditional building materials

5) Cement prepared with the PG which was mixed with the raw mix before clinkering required the lowest time of milling (55 min).

6) PG also used as mineralizer, decreased the retarding effect from 155 min for the control sample to 75 min.

7) The addition of 5% of calcined PG at 900 ºC to 90% Portland cement in the presence of 5% silica fume improves the hydraulic properties of cement as well as its mechanical properties for up to 90 days.

8) The hydraulic properties of Portland slag cement can be effectively with the incorporation of 6% of thermally calcined PG at 800 ºC.

9) In comparison with the ordinary clinker the PG clinker was characterised by a high SO₃ content and the presence of small crystals and a low amorphous phase

10) Even though the mechanical strength of the concrete is decreased with the addition of PG, it can be used for low strength building applications such as floor tiles, low strength hollow bricks etc.

7. REFERENCE

1. H. Tayibi et.al (2009), “Environmental Impact and Management of
Phosphogypsum”, Journal of Environmental Management ,Volume 90, Page No. 2377–2386

2. M.M. Smadi et.al (1999), “Potential Use of Phosphogypsum in Concrete”, Cement and Concrete Research, Volume 29, Page No. 1419–1425

3. N. Degirmenci et.al (2007), “Application of Phosphogypsum in Soil
Stabilisation”, Building and Environment, Volume 42, Page No. 3393–3398

4. M. Singh et.al (2002), “Treating Waste Phosphogypsum for Cement and
Plaster Manufacture”, Cement and Concrete Research, Volume 32 Year 2002 Page No. 1033–1038

5. Manjit Singh and Mridul Garg (2002), “Production of Beneficiated Phosphogypsum for Cement Manufacture”, Journal of scientific and industrial research, Volume 61, page 533-537.