production of plaster from gypsum …dl.uctm.edu/journal/node/j2014-3/12-rawajfen-293-302.pdfthe...

Albara I. Alrawashdeh, Aiman Eid Al-Rawajfeh, Areej A. Al-Bedoor, Ehab M. Al-Shamaileh, Mahmood Najy Al-Hanaktah

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Journal of Chemical Technology and Metallurgy, 49, 3, 2014, 293-302

PRODUCTION OF PLASTER FROM GYPSUM DEPOSITS IN SOUTH JORDAN: IMPROVEMENT OF THE SETTING TIME

Albara I. Alrawashdeh1, Aiman Eid Al-Rawajfeh1, 2, Areej A. Al-Bedoor1

Ehab M. Al-Shamaileh3, Mahmood Najy Al-Hanaktah4

1 Tafila Technical University (TTU), P.O. Box 179, 66110 Tafila, Jordan2 Jordan Atomic Energy Commission (JAEC), P.O. Box 70, Shafa Badran 11934, Amman, Jordan E-mail: [email protected] The University of Jordan, Amman 11942, Jordan 4 Rawabi for Mining Co. W.L.L., Tafila, Jordan

ABSTRACT

In this work, the improvement of the setting time (S.T.) of the Plaster of Paris produced from gypsum deposits of Jabal Mulaih, in Tafila, south Jordan, was investigated. The setting time of the reference sample was about 10 min and the final setting time reaches 20 min. Two approaches were investigated. The first one provided the application of some additives as potential retarders, while the second one required the control of the size of calcined gypsum materials produced in the crusher. Tens of samples were prepared and the setting time was measured by the Vicat methods, electrical conductance measurements, and temperature change. The morphology, bending breaking and burning resistance were studied, too. The influence of the water to plaster ratio, gypsum powder size, water tem-perature, water quality, admixtures, and aging time on the setting time was investigated. It was found that the setting time was delayed with increasing the water amount, the powder size, the admixture amount, the aging time, and decreasing the dissolved salts amount. The shape of the dihydrate gypsum crystal changed with the type and amount of the admixture. The inhibiting effect in each growth direction was found different depending on the mechanism of the process. All admixtures improved the bending breaking value of the plain reference sample. The decrease of the material size increases the setting time, the bending breaking and improves significantly the burning resistance.

Keywords: gypsum calcination, setting time, admixtures, morphology, bending breaking.

Received 28 June 2013Accepted 11 April 2014

INTRODUCTION

The productive chain of the civil construction industry is one of the activities of the society which has interferes most directly with the environment. It consumes vast re sources (materials/energy), generates residues and is responsible for high emissions of CO2. Therefore, the sustainable construction is in search for agglomerates of minor energetic consumption as well as the adequate use of industrial and agro-industrial wastes. The industrial process of plaster production issues less anhydrous carbon (CO2) and reveals the lowest energy consumption when compared to that of cement and lime.

That is why it is regarded as “green” material. However, hardened gypsum plasters are intrinsically soft, they have appreciable water solubility and show high wet creep. Therefore their use is mainly restricted to indoor applications. Plasters are produced on the ground of gyp-sum by a process involving dissolution and subsequent precipitation of the dihydrate (DH) from the solution [1].

The benefits of using gypsum as a building con-struction material refer to: (i) the processing technology involved in converting the raw material gypsum into the end product gypsum plaster is simple and energy effec-tive consumption (the conversion temperature of making gypsum plaster is in the range from 120oC to 165oC,

Journal of Chemical Technology and Metallurgy, 49, 3, 2014

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while clay bricks and cement require 900oC and 1450oC, correspondingly); (ii) its properties connected with quick in implementation high efficiency in thermal and acoustical insulation, fire resistance, humidity control.

The setting process of gypsum is of great impact on the subsequent steps. The profile of the setting curve must be synchronized with the sequence of the latter. It must be well balanced and has to compensate for gypsum raw material variations. The setting time of plaster is approximately 8 - 10 min. It is much shorter than that needed commercially. The typical factors influencing gypsum setting performance refer to the particle fine-ness, the impurities present the raw material source (natural or flue-gas-desulfurized (FGD) gypsum). They all have an impact on the gypsum crystal nucleation.

Retardation of the setting process may be achieved by:

- Reducing the speed of hemihydrate dissolution by increase of the viscosity of the mixing liquid, e.g. by adding glycerin,

- Inhibition of crystal growth brought about by for-mation of a coat on the crystallizing salt surface, e.g. by addition of colloidal substances, such as glue, caseine (milk protein), and sulphite liquors,

Adsorption of admixtures, such as:Organic polar substances, e.g. tartaric acid, citric

acid, and ethanol. Their activity may be explained as fol-lows. A face of gypsum crystal with Ca2+ ions of strong electric field adsorbs polar particles on its surface. The adsorbed layer inhibits the growth of the crystal face considered Ca2+ and SO4

2- ions are subsequently incor-porated in the remaining faces and the crystal achieves a habit hexagonal plate. Cations of higher electric charges, e.g. Fe3+, Al3+ contribute to clear increase in the number of plate crystals.

Sodium phosphates or hydrogen phosphates (e.g. Na3PO4, Na2HPO4) added to calcium sulphate hemi-hydrate react with CaSO4.0.5H2O producing calcium phosphates. The latter inhibit the growth of crystal nuclei through precipitation.

Several organic and inorganic retarders are used to delay the setting time of cement-based materials. The mechanism of hydration retarding is different for each type of the retarder used. For example, some retarders such as glucose and citric acid delay gypsum hydration as they can be hydrated easier than gypsum [2, 3]. Citric acid acts as one of the most effective retarders among the

chemical additives. NaHCO3 produces gas to obstruct the hydration [4, 5]. Natural fibers are also used as they can promote adsorption of organic molecules on the active crystal growth sites [6]. Recently [7], Thailand coconut coir fibers (CCF), saw dust (SD) and tobacco waste fiber (TWF) were used as additives. It is worth adding that natural fibers are useful for reinforcing construction material. Besides, their use can be very beneficial in terms of low cost and waste utilization. Inorganic additive such as fly ash (FA) is found to play an important role on solidification of FA–lime–gypsum building material [8].

The size of gypsum fed to the kiln is followed. It is found the material is not homogeneous, the size of the crushed gypsum changed from powder to big stones (up to 30 mm). When the temperature is increased to reach the calcination conditions required for the big sized ma-terials, the powdered one converts to anhydrite or other products. The reverse is true when the temperature is decreased to suit the calcination conditions required for the powdered materials. The big size material remains uncalcined and provides seeds for the subsequent set-ting process.

The aim of the present study is to investigate the effect of different organic and inorganic, synthetic and natural retarders and that of particle size on controlling the setting time of the plaster.

EXPERIMENTAL

In a typical process the gypsum mined (normally by open mining) of high purity (> 88 %) is transported from the mine to the factory. There the material is crushed in a primary crusher. The homogeneity required is achieved in a secondary crusher. Then the material is stored in a silo to be feed to a rotary calcination kiln. The calcina-tion is carried out at a temperature > 130oC for a period between 15 - 30 min. It is followed by cooling down and further milling.

Influence of admixturesGypsum samples were provided to study the effect

of admixture and aging time on the setting time (ST). Raw and the calcined materials were used. The setting time reference sample of no admixture was measured daily to ensure its stability with time. The ST was found constant up to 2 weeks. Then it began to increase to reach


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a value twice as high in 4 weeks. In a typical ST experiment using the Vicat apparatus,

an accurately weighed 170 g sample of gypsum was added to 120 ml of water and kept for soaking for 1 min. Then it was stirred manually for 1 min at 20oC, and transferred to a typical Vicat mold (an upper diameter of 6 cm, a base diameter of 7 cm and height of 4 cm). Distilled, tap and seawater was used in the experiments. The Vicat apparatus had a needle of length of 5 cm and weight was (with the plunger support rod) of 280 g. The base glass plate was of a surface of 100 mm2. The rod is kept at a constant initial Vicat value of S = 71 [m/m]. The rod and the needle were left to fall freely in the sample molded. The readings were done at an interval of 1 min.

In the admixture experiments, the accurately weighed admixture amount was dissolved in water prior to the mixing with gypsum. The setting time was measured in some cases by the plate casting method. The diameter and the resistance to the sharp knife cutting were the criteria used. The examined samples were com-pared to other competitor products available at the local Jordanian market, mainly Jordanian and Saudi Arabian products. Natural and synthetic admixture samples were used. They contained Jordanian Tripoli, tartaric acid, citric acid, ethylene di-amine tetra- acetic acid (EDTA), Mg/Al hydrotalcite layered double hydroxide (LDH), a synthetic clay, polyvinylalcohol (PVA) and a com-mercial retarder. The electrical conductivity (EC) was measured by a conductivity meter (Wilkens-Anderson Co., USA). In a typical experiment, ca 10 g of a gypsum sample were mixed with ca 100 ml of water containing a defined amount of the admixture. Then EC was followed with time. Scanning electron microscopy (SEM) was applied using LEITZ AMR 1000A SEM. The composi-tion of the gypsum samples was analyzed using ARN 9800 X-ray fluorescence. The bending breaking of the samples studied was measured by DHZ-5000 (China) breaking instrument.

Influence of plaster powder sizeSamples were sieved to a certain size aimed at study-

ing the powdered doze effect in the experiments. Then they were calcined in a laboratory oven at 150 - 170oC for 30 min. An additional experiment was performed using ca 6 tons of homogeneous size material calcined at the Rawabi factory. ST was measured following the procedure described above.

RESULTS AND DISCUSSION

Setting time of the reference gypsum samplesFig. 1 shows the change of the penetration depth S

of the Vicat needle with the time of the evolution of the crystallization process of DH of the reference sample with no admixture added. It shows that when hemi-hydrate is mixed with water, it goes into the solution, making it supersaturated with respect to gypsum. This supersaturation remains constant during the induction period, during which clustering and nuclei formation takes place. As soon as the nucleating embryos achieve a critical size, a rapid crystallization takes place [9]. Fig. 1 shows as well that the setting time is ca 10 min, while the final setting time reaches 20 min.

The rate of hydration is determined using Tem-perature Rise Set (TRS) data which measures the heat evolved from the hydration of HH to DH. The smoother the temperature curve, the longer the setting time. Fig. 1 shows sigmoidal curve generated in the course of the DH crystallization process.

The electrical conductivity (EC) curve of freshly diluted reference plaster slurry of no admixture is shown in Fig. 2. Three stages were observed. The three stages referred to dissolution of HH plaster, nucleation of DH gypsum and DH gypsum crystal growth [7] are well outlined. Fig. 3 illustrates the comparison of Rawabi product to other competitors at the Jordanian market (the samples investigated in this study were mainly Jordanian and Saudi Arabian products).

Fig. 1. Time dependences of S and temperature of the refer-ence sample studied.

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Parameters affecting the setting timeThe process of gypsum hydration and setting is af-

fected by a number of factors:l the temperature during gypsum paste preparation;l the water-gypsum ratio;l the method of gypsum mixing as well as its mixing

intensity and time;l the fineness of grinding;l the purity of gypsum.Fig. 4 illustrates the effect of water to plaster (W/P)

ratio. Two ration were examined; 0.860 and 0.706. It is evident that the crystallization of gypsum is affected by the water amount used in the setting process. The setting time is increased from 10 to 15 min with W/P ratio increase from 0.706 to 0.860.

The reference sample which gives ST of 10 min is additionally sieved to 150 mm. As shown in Fig. 5, the

ST decreases from 10 min to 7 min. It is expected as the smaller size means a higher surface area and hence faster dissolution and crystallization.

Fig. 6 shows the water temperature influence on the setting time. The curve referring to the cold water shows a longer setting time. The water temperature effect is illustrated in Fig. 7.

Water used in mixing plaster should be as pure as possible. If water is drinkable, it probably is suitable for mixing of plaster slurries. When water for industrial use is taken from contaminated sources and is high in organic impurities, it will lengthen the setting time of the plaster. Large amounts of soluble salts such as sodium chloride, sodium sulfate, and magnesium sulfate, which maybe in the water, can migrate to the surface of the mold during drying. The resulting efflorescence forms hard spots on mold surfaces that can result in problems with the mold or cast. Other chemicals in the water may

Fig. 3. Juxtaposition of the reference sample to other prod-ucts studied in respect to S vs time dependence.

Fig. 2. Change of the electrical conductance of the refer-ence sample with time.

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This work (No admixtures)Jordan 1Saudi 1Jordan 2Saudi 2

The Target

Fig. 4. Water to plaster vs the setting time dependence.

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W/P= 120/140 = 0.860

W/P= 120/170 = 0.706

Fig. 5. Effect of the gypsum powder size on the setting time.

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W/P= 120/170 = 0.706 Sieved (150um)


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react with the gypsum to produce these soluble salts. In general, any compound that has a greater solubility than gypsum can produce efflorescence. Fig. 8 shows the effect of water quality on the setting time. The tap

water provides a longer setting time when compared to that of the seawater, the longest ST is observed when distilled water is used. The effect observed is attributed to the presence of ions such as Ca2+ and SO4

2-.They shift in turn the reaction towards faster crystal formation and decrease the solubility of the dehydrate.

Fig. 9 shows the effect of citric acid and anhydrite (CaSO4). Citric acid acts as a retarder of gypsum plas-ter hydration only in presence of other cations to form citrate. The cations required have to be extracted from a foreign compound – (for an example, calcium carbonate or hydroxide which are present as impurities) or cations available in the water used. The ions usually adsorb at the hemihydrate crystals hindering the nucleation [10].

Fig. 8. Water quality effect on the setting time.

Fig. 6. Effect of water temperature on the setting time.

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Fig. 7. Effect of water temperature on the setting time.

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Influence of water temperature

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Tap waterSeawater (Aqaba Gulf)

Fig. 10. Effect of the gypsum product aging time on the setting time.

Fig. 9. Effect of different materials on the setting time.

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Rawabi (No Admixtures), Aging = 30 days

Anhydrite acts as an accelerator as it absorbs water at a higher rate than the dihydrate obtained.

In a trial of adding the dust of the calciner kiln on the setting time, the fine dust increases the setting time up to


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certain value. Increasing the amount of dust will increase the ST to a certain level but much amounts will not have a significantly effect. The dust was saturated with the vapor and it may transfer the high-temperature vapor to the storing silos and help in calcining the amount if materials that reach the silos without calcination. Fig. 10 shows the aging time effect of the gypsum product on the setting time. Reference samples are analyzed to check the setting time after 30 and 60 days from produc-tion. The setting time increases from 10 min during the first 14 days to a ca 20 min after 30 days. No further change is observed.

Admixtures influence on the setting timeNatural and synthetic admixture samples are exam-

ined as potential retarders (Fig. 11): tartaric acid, citric acid, ethylene di-amine tetra-acetic acid (EDTA), Mg/Al hydrotalcite layered double hydroxide (LDH), synthetic

clay, Jordanian Tripoli, polyvinylalcohol (PVA) and a com-mercial retarder. Different weight percentages are used.

Fig. 12 illustrates the effect of the retarders studied on the reference sample setting time. The setting time is shifted from its reference value of 10 min to 15, 16, 17, 21, 22, 31, and 37 min by the addition of 0.12 % LDH, 0.12 % EDTA, 0.12 % Tripoli, 0.012 % citric acid, 0.12 % PVA, 0.012 % tartaric acid, and 1.5 % commercial retarder, respectively. The effect of paracetamol on the setting time is shown in Fig. 13. The ST increases from 15 min to 18 and 22 min in case 0.07 to 0.3 % of par-acetamol is added to the plaster, respectively.

It is worth adding that during the first stage admix-ture, the electrical conductance (EC) values are lower than those of the pure plaster slurry. Pure plaster slurry curve shows an abrupt change sooner that of the sample containing tartaric acid. The three stages in the second curve are significantly expanded (Fig. 14). Fig. 15 sum-

Fig. 11. Compounds added: tartaric acid, citric acid, EDTA, Mg/Al LDH, Tripoli, PVA, Paracetamol.

Tartaric acid

Citric acid

EDTA

Mg/Al LDH

SiO2 (Tripoli)

Polyvinylalcohol (PVA)

Paracetamol

Fig. 12. Effect of the retarders studied on the setting time of the reference Rawabi plasters product samples.

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Fig. 13. Effect of paracetamol on the setting time of Rawabi reference plaster.


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marizes the effect of the different weight percentage of the admixtures on the setting time. The values of setting time in case of using a commercial retarder are shown in Fig. 16.

Influence of admixtures on the dihydrate mor-phology

The morphology of the dihydrate crystals results from the condi tions of plaster hydration proceeding [11]. Fig. 17 shows clearly the changes in the dihydrate gypsum crystals obtained. The images of the plain plaster exhibit significant variations in crystal size, but they maintain their original needle shape and arrangement. The dihydrate crystals are predominantly needle-shaped

Fig. 14. Electrical conductance values in the presence of tartaric acid.

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Setting time [min] = 1020 C[%] + 10R2 = 1

Setting time [min] = 1802 C[%] + 10R2 = 0.9991

Setting time [min]= 391 C[%] + 10R2 = 0.9951

Setting time [min] = 39.1 C[%] + 10R2 = 0.9951

Setting time [min] = 31.733 C[%] + 10R2 = 0.9863

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Tartaric acidCitric acidEDTATripoliParacetamol

Fig. 15. Correlations between the admixture amount and the setting time.

Fig. 16. A correlation between the amount of the com-mercial retarder used and the setting time.

Setting time [min] = 18 C[%] + 10R2 = 1

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Retarder amount, C [%]

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Sample

Aging

time,

days

Bending Breaking,

MPa

Diameter of Burning,

cm

Rawabi 6 1.5 2.5

Jordan 1 6 1.8

Rawabi (6% Dust) 6 2.1

0.012% Tartaric acid 6 2.1 3.0

0.012% Citric acid 6 1.9 2.5

0.012% EDTA 6 1.8 3.0

0.12% Tripoli 6 2.4

0.12% LDH 6 2.7 2.5

Size of < 300 µm 2 1.9

6 4.0

Table 1. Bending breaking and burning diameter results referring to the samples investigated.


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of big length–radius ratio. The results obtained are in accord with literature findings [12, 13].

Additives influence on the bending breaking and burning

Table 1 shows the bending breaking and burning diameter results obtained in case of the plain sample. They are compared to those found for the competitor sample from the Jordanian market (Jordan 1) as well as those containing different additives. In general, the latter improve the bending breaking value of the reference sample. The best values are found for the Tripoli and LDH. Their bending breaking increases from 1.5 MPa to 2.4 and 2.7, respectively. This may be attributed to small particles of the non-soluble Tripoli and LDH as well as to the small size of the dihydrate grown. It should be added as well that the particles size decrease leads to bending breaking increase of up to 4 MPa.

Influence of homogeneity of the material fedThe size of the material fed varies significantly.

40 % of it is in the form of a powder, the needed size (50%) and 10 % are lumps. The powder is burned in the kiln producing anhydrite materials, while the lumps proceed further with almost no calcinations, further acting as accelerators. Thus 50 % of the material fed is responsible the setting time decrease to half of its value. Laboratory and bench scale experiments are performed to investigate the effect of the size of the material fed. Fig. 18 shows the setting time of samples of a size < 300 mm. The setting time increases from 10 min for the reference sample to an average value of 25 min. The diameter and the resistance to the sharp knife cutting are the criteria used in the plate casting experiments. The average diameter of the different samples increases from ca 9 - 10 cm to ca 14 - 16 cm, while the average time increases from 5 - 6 min to ca 11 - 12 min.

Fig. 17. Admixtures influence on the morphology of the dihydrate crystals obtained.


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Fig. 19 shows the Vicat results of the different tri-als in case of studying the size effect. Trials 1 and 2 are materials calcined in the laboratory. Trials 3 and 4 are the 6-ton real plant experiment. Fig. 20 shows the effect of the materials of size < 300 mm on the setting time. The rest is fine, i.e. < 150 mm.

CONCLUSIONS

In the sight of this study, the following points can be concluded:

This project studied the setting time of the plaster produced from Jabal Mulaih, in Tafila, south Jordan. It ranged between 10 min and 20 min.

The effect of the water to plaster ratio, gypsum powder size, water temperature, water quality, additives (retarders or accelerators), and aging time on the set-ting time is followed. It is found that the setting time is delayed with (i) the increase of the water amount, the size of the material, admixture amount, and aging time; (ii) the decrease of the dissolved salts. When admixtures are used, the threshold mechanism is the dominant and tartaric acid was the most powerful retarder among the chemical additives studied.

The morphology of the dihydrate crystals is the result of the condi tions of plaster hydration. The shape appearances of dihydrate gypsum crystal changed with the type and amount of the admixture changing and its inhibiting mechanism.

All admixtures improve the bending breaking value of the plain reference sample.

The homogeneous size approach increases the set-ting time, increases the bending breaking and improve the burning resistance significantly.

AcknowledgementsThe support by Faculty-For-Factory (FFF) program

at the University of Jordan, Deanship of Scientific Re-search at Tafila Technical University (TTU), Ministry of Higher Education and Scientific Research, Jordan Atomic Energy Commission (JAEC), Abdul Hameed Shoman Foundation and Rawabi for Mining is greatly acknowledged. Special thanks to His Excellency Eng. Abdul Rahman Al-Hanaktah, Mr. Najy Al-Hanaktah, Mr. Wahid Al-Shorbajy, Mr. Isam Al-Hanaktah and all the other workers in the factory for their collaboration and understanding. About 6 tons of gypsum were used for

Fig. 18. The setting time of laboratory calcined samples of size < 300 mm.

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Fig. 20. Effect of the materials of size < 300 mm on the setting time.

Fig. 19. Vicat results referring to the size effect.

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Rawabi (Original)Jordan 1New approach (trial 1)New approach (trial 2 i)New approach (trial 2 ii)New approach (trial 2 iii)New approach (trial 3 i)New approach (trial 3 ii)New approach (Trial 4 i)

Setting time [min] = 0.2896 C [%] + 24.761R2 = 0.9791

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lab-scale experiments. The latter were successful and helped us tracking the problem.

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