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Development of alpha plaster from phosphogypsum for cementitious binders
Construction and Building Materials 23 (2009) 3138–3143
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Development of alpha plaster from phosphogypsum for cementitious binders Mridul Garg *, Neeraj Jain, Manjit Singh Environmental Science & Technology Division, Central Building Research Institute, Roorkee 247 667, India
a r t i c l e
i n f o
Article history: Received 11 December 2008 Received in revised form 21 May 2009 Accepted 18 June 2009 Available online 23 July 2009 Keywords: Microstructure Compressive strength Hydration Temperature Waste management
a b s t r a c t Efforts have been made to make high strength alpha plaster from phosphogypsum, a by-product of phosphoric acid industry. Phosphogypsum was autoclaved in slurry form (phosphogypsum 50% + water 50%, by wt.) in the laboratory at different steam pressures for different durations in presence of chemical admixtures. It was found that with small quantity of chemical admixture (sodium succinate/potassium citrate/sodium sulphate), alpha plaster of high strength can be produced. The optimum pressure and duration of autoclaving was found to be as 35 psi and 2.0 h, respectively. The alpha plaster was examined for making cementitious binders by admixing hydrated lime, fly ash, granulated blast furnace slag, marble dust and chemical additives with alpha plaster. Data showed that cementitious binder of compressive strength of 22.0 and 30 MPa (at 28 days of curing at 40° and 50 °C) and low water absorption was produced. DTA and SEM studies of the binder showed formation of CSH, ettringite and C4AH13 as main cementitious products to give strength. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Gypsum is an important building materials not only used in cement manufacture, agriculture or as filler in various commodities but is a starting material for making plaster. Plaster is produced from the calcination of gypsum (CaSO42H2O) which partially dehydrates to produce a hemi hydrate (CaSO41/2H2O) (CaSO42H2O ? CaSO41/2H2O + 3/2H2O). The oldest traces of plaster renders are 9000 years old, and were found in Anatolia and Syria. By the end of 19th Century, plaster became popular and was used in the industry in a very massive way, particularly in Paris and many other parts of Europe [1]. During the 20th Century, plaster was found to be of immense use outside of the construction industry. For example in the ceramic industries (sanitary ware, tableware, giftware), in dentistry, in metal casting, jewelry, in medical applications, in cosmetics, animal food, and many more applications. It continued to bring new uses for the material. There are three important crystalline forms of gypsum plaster viz. alpha, beta and gamma. The most popular form is beta form which is made at about 120–180 °C in open pan, kettle or in a rotary kiln. This variety of plaster has high consistency, medium strength and plasticity. For structural applications and in ceramic and dentistry, high strength plaster is essential which is not possible to get with beta plaster. High strength plaster can be
* Corresponding author. Tel.: +91 1332 283298; fax: +91 1332 272272. E-mail address: [email protected] (M. Garg). 0950-0618/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2009.06.024
achieved by reduction in water requirement of plaster during or after calcination by the addition of special type of additives [2,3]. Production of plaster of low consistency (alpha plaster) was reported by Randel and Dailey [4] by heating gypsum lumps in saturated steam pressure at 1.05 kg/cm2 pressure for 6–7 h followed by drying and grinding. Singh and Rai [5] confirmed the findings by calcining gypsum at slightly higher pressure (1.75 kg/cm2). A plaster of consistency 40–45 ml was produced. Several other attempts were made by different researchers to make high strength plaster by adding gum arabic, lime, soda ash or resins like carbamide–formaldehyde to reduce water demand of plaster to as low as theoretical values [6]. On industrial scale, low consistency plaster is achieved by autoclaving gypsum in autoclaves of gypsum which takes long time (6– 8 h) to form alpha plaster, it was thought to reduce autoclaving time to few hours by the addition of chemical additives also called crystal modifiers or nucleating agents so that production cycle of alpha plaster may be increased. Investigations were, therefore, undertaken to dehydrate gypsum powder collected from fertilizer industry called phosphogypsum in slurry form in autoclave in presence of different chemical additives in small quantity. DTA and SEM studies were used to evaluate alpha plasters. Gypsum plaster is not preferred for use in external moist or damp situations. Making of weather resistant gypsum binders has been reported by many researchers. Suitability of alpha plaster was studied for making water-resistant cementitious binders by addition of materials like fly ash, hydrated lime, granulated slag or marble dust. The physical and hydration properties of binders are reported.
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M. Garg et al. / Construction and Building Materials 23 (2009) 3138–3143 Table 3 Composition of binders.
2. Experimental 2.1. Raw materials
Composition (% by wt.)
The phosphogypsum (procured from M/s Rashtriya Chemicals & Fertilizers, Mumbai), fly ash, granulated blast furnace slag, hydrated lime and marble dust were used to prepare alpha plaster and water-resistant cementitious binders. The chemical composition of phosphogypsum (unprocessed and beneficiated) and other raw materials are shown in Tables 1 and 2, respectively. 2.2. Preparation of alpha plaster from phosphogypsum Phosphogypsum, a waste of phosphoric acid industry contains impurities of phosphates, fluorides, organic matter, alkalies, etc [7,8]. These impurities are considered harmful for the quality of plasters produced from phosphogypsum. Hence, removal or reduction of impurities is essential [9,10]. Phosphogypsum was beneficiated in the pilot plant (installed in Central Building Research Institute, Roorkee) by mixing it with water to solubilise the impurities. It was followed by wet sieving through vibrating screen and rejecting the coarse fraction (rich in impurities <10%) retained over sieve and then centrifuge the gypsum slurry to remove water soluble impurities and dried in the rotary drier at 110–120 °C [11]. After beneficiation the reduction of impurities in phosphogypsum are given in Table 1 which complied the requirements as given in Indian Standard IS: 12679 (1989), specification for by product gypsum for using plaster, block and boards. The beneficiated phosphogypsum was mixed with equal volume of water to form a uniform slurry. A small quantity of crystal modifier such as sodium succinate, potassium citrate and sodium sulphate were mixed separately with gypsum water slurry to activate dehydration of phosphogypsum. The gypsum slurry was subjected to predetermined steam pressure of 1.75 kg/cm2 for half an hour to two hours. The dehydration of gypsum was avoided at higher steam pressures to stop formation of anhydrite. The autoclaved gypsum was filtered and dried at 130 °C immediately to the fineness of more than 300 m2/kg (Blaine’s) surface area in a ball mill. The alpha plaster was examined for properties like consistency, setting time, compressive strength and bulk density. The DTA (Stanton Red croft (UK)) and SEM (LEO 438 VP, UK) were performed on the plaster samples to ascertain conversion of gypsum into alpha plaster. 2.3. Preparation of cementitious binders The cementitious (water-resistant gypsum) binders (A and B) were produced by blending the ground alpha plaster with fly ash (lime reactivity 4.5 N/mm2)/granulated blast furnace slag, marble dust and hydrated lime followed by grinding in a ball mill to a fineness of 330–450 m2/kg (Blaine) in two compositions (Table 3)
Table 1 Chemical composition of phosphogypsum. Constituents (%)
Unprocessed phosphogypsum
Beneficiated phosphogypsum
IS: 12679 (1989) limits for phosphogupsum
P2O5 F Na2O + K2O Organic matter SiO2 + in HCl insoluble CaO MgO SO3 LOI pH
0.56 0.35 0.40 0.059 0.90 31.50 0.053 45.10 19.80 4.0
0.24 0.20 0.03 0.02 0.85 32.0 0.051 45.90
Max. Max. Max. Max.
6.5
Min. 5.0
0.40 0.40 0.30 0.40
P2O5 F Na2O SiO2 + insoluble inHCl CaO R2O3 (Al2O3 + Fe2O3) MgO SO3 LOI Sulphide MnO
Fly ash
Granulated slag
Lime sludge
Marble dust
3.60 1.0 0.76 62.90 1.50 28.30 0.80 0.20 1.50
30.0 31.50 27.5 7.32
0.62 2.29
Fly ash Hydrated lime Alpha plaster Granulated slag Marble dust Chemical activator
A
B
40 30 30 – – –
– 12.5 22.5 50.0 15.0 1.0
and tested as per IS:4031 (1991)-Methods of physical tests for hydraulic cement and IS: 6909-1973, specification for supersulphated cement for different properties. To determine the compressive strength and bulk density of the cementitious binders, cubes of size 2.5 cm were cast at normal consistency and after 24 h of curing, the samples were exposed to different temperatures (27–50 °C) in a sealed desiccator over water for a period up to 28 days.
3. Results and discussion 3.1. Properties of alpha plaster The physical properties of alpha plaster are given in Table 4. The results show an increase in compressive strength and bulk density with increase in salt concentration. At 0.20% concentration of sodium succinate (Na2C4H4O56H2O) alpha plaster gives maximum attainment of strength. The enhancement in strength value can be attributed to the decrease in normal consistency of the plaster. The fall in consistency may be correlated with the formation of columnar, hexagonal and large size dense gypsum crystals with minimum fissures and voids. While the beta plaster is composed of small porous crystals need higher quantity of water to produce workable paste. The setting time of alpha plaster is much higher than beta plaster. It is due to large crystals of plaster with smaller surface area. These findings are in agreement with the studies carried out by Cushman and Cushman [12] that the addition of crystal modifier changes the physicochemical properties such as lowering the water partial pressure or equivalently increasing the boiling point of the solution, catalyzing the dehydration of gypsum, modifying crystal habit and promoting the rate of growth of the crystal. The more stable crystals of alpha hemihydrate require less water to make a flowable slurry. Due to low water/plaster ratio, the density of set gypsum is increased which is responsible for enhancement of
Table 4 Physical properties of alpha plaster.
Table 2 Chemical composition of raw materials. Constituents (%)
Binders
3.10 52.0 0.50 0.31 0.16 41.0
3.01 30.80 2.70 19.27 43.40
Salts
Consistency (%)
Setting time (Min)
Bulk density (kg/m3)
Compressive strength (MPa)
Alpha plaster (without salt)
58.0
10.0
1200
15.0
Sodium succinate 0.05 0.10 0.20 0.25
46.0 38.0 35.0 36.5
34.0 36.0 34.0 33.0
1250 1270 1310 1280
19.00 22.40 28.58 26.46
Potassium citrate 0.05 0.10 0.20 0.25
48.0 40.0 38.0 38.5
32.0 30.0 32.0 31.0
1220 1240 1280 1260
17.20 20.20 23.70 22.46
Sodium sulphate 0.05 0.10 0.20 0.25
43.0 41.0 40.0 40.5
24.0 22.0 20.5 21.0
1220 1230 1270 1265
15.20 18.40 20.60 19.20
Beta plaster
66.0
8.0
1180
10.50
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270 °C due to conversion of hemihydrate into soluble anhydrite. The intensity of endotherm peak increases with the increase in concentration of sodium succinate. It can be seen that at 0.20% of
Fig. 1. DTA of alpha plaster produced in presence of different concentration of sodium succinate admixture.
compressive strength. In beta plaster, the setting time is shorter which can be ascribed to the large number of effective nuclei to cause crystallization of dehydrate gypsum. DTA has been used to explain formation of alpha plaster from phosphogypsum. Fig. 1 shows appearance of a long endotherm at 180–190 °C due to formation of hemihydrate plaster. This endotherm is followed by formation of a small exotherm at 250–
Fig. 3. Microstructure of alpha plaster produced in presence of sodium succinate admixture (0.20%).
Table 5 Properties of cementitious binders. Properties
Consistency (%) Setting time (min) Water absorption Porosity
Cementitious binders
Beta plaster
A
B
44 24 19.6 27.1
41 17 4.0 7.2
61–65 8–10 30–35 20–25
Table 6 Properties of cementitious binder A. Duration (d)
Curing temperature (°C) 27
40
50
Compressive strength (MPa) 1 3 7 28
5.6 5.9 6.8 7.2
7.0 9.2 22.3
7.8 13.2 26.4
Bulk density (g/cc) 1 3 7 28
1.27 1.29 1.30 1.34
1.28 1.31 1.33 1.39
1.29 1.31 1.33 1.40
Table 7 Compressive strength and bulk density of cementitious binder B. Duration (d)
Compressive strength (MPa) 1 3 7 28
Fig. 2. DTA of alpha plaster produced in presence of different concentration of additives: (a) potassium citrate and (b) sodium sulphate.
Bulk density (g/cc) 1 3 7 28
Curing temperature (°C) 27
40
50
1.6 16.2 20.8 20.6
– 16.8 20.7 22.4
– 27.2 29.7 32.2
1.60 1.63 1.64 1.67
1.62 1.65 1.67 1.70
1.68 1.70 1.71 1.76
M. Garg et al. / Construction and Building Materials 23 (2009) 3138–3143
sodium succinate concentration, endotherm of maximum intensity is obtained confirming maximum formation of the alpha plaster in phosphogypsum. However, the appearance of exotherm does not show marked increase in intensity with variation in concentration of additive. The occurrence of such small exotherm following the hemihydrate
3141
endotherm confirms the formation of alpha hemihydrate in accordance with the findings of Holdridge [13]. In case of addition of potassium citrate and sodium sulphate (Fig. 2) additives, the intensity of hemihydrate endotherms and soluble anhydrite exotherms appear at similar temperatures but with decreased intensity of the peaks showing formation of lower level of alpha plaster compared
Fig. 4. DTA of cementitious binders: (a) binder A and (b) binder B hydrated for different periods.
Fig. 5. Microstructure of cementitious binder A hydrated for: (a) 3 days (b) 7 days and (c) 28 days (FA: fly ash; G: gypsum; HL: hydrated lime).
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to addition of sodium succinate. The intensity of these thermal changes can be correlated with the compressive strength of the alpha plaster. Due to maximum intensity of hemihydrate endotherm in sodium succinate, alpha plaster gives highest strength and density values. 3.1.1. Microscopic structure of plaster Fig. 3a and b represents the crystallization of alpha hemihydrtae plaster produced at 35 psi steam pressure in presence of sodium succinate (0.20%). Most of crystals are prismatic columnar, hexagonal and tabular shaped and interspersed with euhedral needles and lath. Occasionally twinned prisms are also seen. The formation of such type of crystals can be ascribed to its process of preparation in which recrystallization takes place. Perederii [14] has also shown similar findings. X-ray diffraction pattern of alpha hemihydrate earlier studied by Singh and Rai [5] has shown that hemihydrate reflections of alpha plaster obtained from phosphogypsum were more pronounced than that of the beta hemihydrate, confirming better crystallization in alpha hemihydrate plaster. 3.2. Cementitious binders from alpha plaster Normally, gypsum plaster or gypsum building products show poor performance in wet or damp situations. Due to this reason, gypsum products are not preferred in these conditions. Water proofing or stabilization of gypsum products with suitable treatments is required. Addition of fly ash, slag, or resinous materials to gypsum plaster improves its water resistance to great extent. Considerable work has been done in China and other places [15,16] to improve water-resistance of binders based on fly ash,
fluorogypsum/phosphogypsum and cement for use in construction. To enhance scope of alpha plaster in construction for exposed situations, it was thought to produce cementitious binders from high strength alpha plaster. The results of these binders are depicted in Tables 5–7. Table 5 shows that binder ‘A’ possesses higher value of consistency, setting time, water absorption and porosity as compared to the binder ‘B’. However, the beta plaster shows much higher values of consistency, water absorption, porosity and short setting time. Data confirm that alpha based binders have better stability towards water than beta plaster generally used in construction. It can be further seen that the compressive strength and bulk density of binders A and B are increased with curing period and temperatures (Tables 6 and 7). Maximum strength values are obtained in both binders A and B at 50 °C. The increase in strength in binders with curing period at 50 °C has been correlated with the DTA (Fig. 4a and b). The Fig. 4 clearly indicated that endothermic peaks at 90–110 °C and 120–130 °C appear due to dehydration of ettringite and gypsum, respectively. These ettringite endotherms are increased with curing and the gypsum endotherms are reduced probably due to consumption of gypsum by the increased formation of ettringite endotherms. Further, formation of small endotherms at 430–450 °C at 3 and 7 days of curing are due to Ca(OH)2 which disappear at 28 days curing. The appearance of endotherms at 690–730 °C may be ascribed to the decomposition of CaCO3. The formation of calcium silicate hydrate (CSH) peak is not visible possibly due to overlapping with the ettringite endotherms. DTA information, thus, confirms that strength in both the binders are due to ettringite, gypsum and probably due to CSH phases.
Fig. 6. Microstructure of cementitious binder B hydrated for: (a) 3 days (b) 7 days and (c) 28 days (G: gypsum; HL: hydrated lime; M: marble dust; S: slag).
M. Garg et al. / Construction and Building Materials 23 (2009) 3138–3143
The microstructure of cementitious binders A and B are shown in Figs. 5 and 6. In Fig. 5a it can be seen that at 3 days hydration of binder, large quantity of agglomerated irregular euhedral to subhedral bodies of rounded fly ash particles coated with unhydrated & hydrated lime are formed. The fly ash spheres can be seen partially smeared with lime and gypsum crystals of prismatic and tabular shaped. At 7 days of curing the binder (Fig. 5b), size of agglomeration is increased. The fly ash spheres and gypsum crystals are coated with hydrated lime particles in abundance. It can be further seen that at 28 days (Fig. 5c), the fly ash spheres are partially or fully hydrated to form calcium silicate agglomerate intermingled with prismatic & tabular shaped crystals of gypsum. Probably these agglomerates of different crystals yield strength development in the cementitious binder. From Fig. 6 it can be seen that at 3 days of hydration of the binder (Fig. 6a), agglomerated irregular bodies are formed. Occasionally gypsum crystals of prismatic habit are conjugated with rounded bodies of slag and hydrated lime. At 7 days of hydration subhedral to anhedral prismatic crystals of gypsum of broadened surface are seen along with agglomerated mass of partially hydrated dendritic slag particles (Fig. 6b). At some places unreacted rounded bodies of slag are also visible. Unhydrated and hydrated lime particles interspersed with unidentified bodies may be of marble dust are present over the hydrated mass. It can be further seen that at 28 days of hydration (Fig. 6c), euhedral prismatic crystals of gypsum and hydrated slag bodies coated with unreacted slag and marble particles are formed. 4. Conclusions 1. The alpha plaster can be produced from beneficiated phosphogypsum by autoclaving the gypsum at an optimum pressure of 35 psi in presence of 0.20% (by wt.) of sodium succinate additive. 2. The alpha plaster gives high strength and low consistency than the traditional beta hemihydrate plaster 3. The alpha plaster can be blended with other wastes like fly ash, slag, lime sludge, etc. to produce water resistant binder for construction work.
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4. The binder shows low water absorption and high strength than the beta plaster. 5. The manufacture of alpha plaster and cementitious binder from waste phosphogypsum is recommended as these products have immense scope in masonry work, prefabricated building products and structural elements.
Acknowledgement Authors are thankful to the Director, Central Building Research Institute, Roorkee for his permission to publish the paper.
References [1] Grooves AW. Gypsum & anhydrite. London: Overseas Geological Surveys, Her Majesty’s Office; 1958. [2] Brookby HE. US patent 1,370,581, March 8; 1931. [3] Hoggart GA. US patent 1,960,538, May 29; 1934. [4] Randel WS, Dailey MC. US patent 1,901,051, March 14; 1933. 1931, 240, October 17; 1933. [5] Singh Manjit, Rai Mohan. Autoclaved gypsum plaster from selenite and byproduct phosphogypsum. J ChemTech Biotechnol 1988;43:1–12. [6] Gardener HF. US patent 1,996,372, April 2; 1935. [7] Singh Manjit. Role of phophogypsum impurities on strength and microstructure of selenite plaster. Constr Build Mater 2005;19:480–6. [8] Singh Manjit, Garg Mridul. Cementitious binders from waste phosphogypsum. J Ind Build Cong 2001;8:157–63. [9] Singh Manjit, Garg Mridul, Rehsi SS. Purifying phosphogypsum for cement manufacture. Constr Build Mater 1993;17:3–7. [10] Singh Manjit. Prospects of phosphogypsum based industry in India. Ind Ceram 1985;28:51–7. [11] Singh Manjit, Garg Mridul, Verma CL, Handa SK, Kumar R. An improved process for the purification of phosphogypsum. Constr Build Mater 1996;10:597–600. [12] Cushman Darby, Cushman LLP. Process for the production of alpha hemihydrate calcium sulfate from gas sludge. US patent 5562892; 2004. [13] Holdridge DA. The characterization of plaster. Trans Br Ceram Soc 1965;64:211–31. [14] Perederii IA. Microstructure of normal and high strength gypsum and products made of them. Chem Abstr 1960;54:106268c. [15] Yan P, You Y. Studies on the binder of fly ash–fluorogypsum–cement. Cem Concr Res 1998;28:135–40. [16] Yan P, Yang W, You Y. Microstructure and properties of the binder of fly ash fluorogypsum – Portland cement. Cem Concr Res 1999;29:349–54.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Development of alpha plaster from phosphogypsum for cementitious binders Mridul Garg *, Neeraj Jain, Manjit Singh Environmental Science & Technology Division, Central Building Research Institute, Roorkee 247 667, India
a r t i c l e
i n f o
Article history: Received 11 December 2008 Received in revised form 21 May 2009 Accepted 18 June 2009 Available online 23 July 2009 Keywords: Microstructure Compressive strength Hydration Temperature Waste management
a b s t r a c t Efforts have been made to make high strength alpha plaster from phosphogypsum, a by-product of phosphoric acid industry. Phosphogypsum was autoclaved in slurry form (phosphogypsum 50% + water 50%, by wt.) in the laboratory at different steam pressures for different durations in presence of chemical admixtures. It was found that with small quantity of chemical admixture (sodium succinate/potassium citrate/sodium sulphate), alpha plaster of high strength can be produced. The optimum pressure and duration of autoclaving was found to be as 35 psi and 2.0 h, respectively. The alpha plaster was examined for making cementitious binders by admixing hydrated lime, fly ash, granulated blast furnace slag, marble dust and chemical additives with alpha plaster. Data showed that cementitious binder of compressive strength of 22.0 and 30 MPa (at 28 days of curing at 40° and 50 °C) and low water absorption was produced. DTA and SEM studies of the binder showed formation of CSH, ettringite and C4AH13 as main cementitious products to give strength. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Gypsum is an important building materials not only used in cement manufacture, agriculture or as filler in various commodities but is a starting material for making plaster. Plaster is produced from the calcination of gypsum (CaSO42H2O) which partially dehydrates to produce a hemi hydrate (CaSO41/2H2O) (CaSO42H2O ? CaSO41/2H2O + 3/2H2O). The oldest traces of plaster renders are 9000 years old, and were found in Anatolia and Syria. By the end of 19th Century, plaster became popular and was used in the industry in a very massive way, particularly in Paris and many other parts of Europe [1]. During the 20th Century, plaster was found to be of immense use outside of the construction industry. For example in the ceramic industries (sanitary ware, tableware, giftware), in dentistry, in metal casting, jewelry, in medical applications, in cosmetics, animal food, and many more applications. It continued to bring new uses for the material. There are three important crystalline forms of gypsum plaster viz. alpha, beta and gamma. The most popular form is beta form which is made at about 120–180 °C in open pan, kettle or in a rotary kiln. This variety of plaster has high consistency, medium strength and plasticity. For structural applications and in ceramic and dentistry, high strength plaster is essential which is not possible to get with beta plaster. High strength plaster can be
* Corresponding author. Tel.: +91 1332 283298; fax: +91 1332 272272. E-mail address: [email protected] (M. Garg). 0950-0618/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2009.06.024
achieved by reduction in water requirement of plaster during or after calcination by the addition of special type of additives [2,3]. Production of plaster of low consistency (alpha plaster) was reported by Randel and Dailey [4] by heating gypsum lumps in saturated steam pressure at 1.05 kg/cm2 pressure for 6–7 h followed by drying and grinding. Singh and Rai [5] confirmed the findings by calcining gypsum at slightly higher pressure (1.75 kg/cm2). A plaster of consistency 40–45 ml was produced. Several other attempts were made by different researchers to make high strength plaster by adding gum arabic, lime, soda ash or resins like carbamide–formaldehyde to reduce water demand of plaster to as low as theoretical values [6]. On industrial scale, low consistency plaster is achieved by autoclaving gypsum in autoclaves of gypsum which takes long time (6– 8 h) to form alpha plaster, it was thought to reduce autoclaving time to few hours by the addition of chemical additives also called crystal modifiers or nucleating agents so that production cycle of alpha plaster may be increased. Investigations were, therefore, undertaken to dehydrate gypsum powder collected from fertilizer industry called phosphogypsum in slurry form in autoclave in presence of different chemical additives in small quantity. DTA and SEM studies were used to evaluate alpha plasters. Gypsum plaster is not preferred for use in external moist or damp situations. Making of weather resistant gypsum binders has been reported by many researchers. Suitability of alpha plaster was studied for making water-resistant cementitious binders by addition of materials like fly ash, hydrated lime, granulated slag or marble dust. The physical and hydration properties of binders are reported.
3139
M. Garg et al. / Construction and Building Materials 23 (2009) 3138–3143 Table 3 Composition of binders.
2. Experimental 2.1. Raw materials
Composition (% by wt.)
The phosphogypsum (procured from M/s Rashtriya Chemicals & Fertilizers, Mumbai), fly ash, granulated blast furnace slag, hydrated lime and marble dust were used to prepare alpha plaster and water-resistant cementitious binders. The chemical composition of phosphogypsum (unprocessed and beneficiated) and other raw materials are shown in Tables 1 and 2, respectively. 2.2. Preparation of alpha plaster from phosphogypsum Phosphogypsum, a waste of phosphoric acid industry contains impurities of phosphates, fluorides, organic matter, alkalies, etc [7,8]. These impurities are considered harmful for the quality of plasters produced from phosphogypsum. Hence, removal or reduction of impurities is essential [9,10]. Phosphogypsum was beneficiated in the pilot plant (installed in Central Building Research Institute, Roorkee) by mixing it with water to solubilise the impurities. It was followed by wet sieving through vibrating screen and rejecting the coarse fraction (rich in impurities <10%) retained over sieve and then centrifuge the gypsum slurry to remove water soluble impurities and dried in the rotary drier at 110–120 °C [11]. After beneficiation the reduction of impurities in phosphogypsum are given in Table 1 which complied the requirements as given in Indian Standard IS: 12679 (1989), specification for by product gypsum for using plaster, block and boards. The beneficiated phosphogypsum was mixed with equal volume of water to form a uniform slurry. A small quantity of crystal modifier such as sodium succinate, potassium citrate and sodium sulphate were mixed separately with gypsum water slurry to activate dehydration of phosphogypsum. The gypsum slurry was subjected to predetermined steam pressure of 1.75 kg/cm2 for half an hour to two hours. The dehydration of gypsum was avoided at higher steam pressures to stop formation of anhydrite. The autoclaved gypsum was filtered and dried at 130 °C immediately to the fineness of more than 300 m2/kg (Blaine’s) surface area in a ball mill. The alpha plaster was examined for properties like consistency, setting time, compressive strength and bulk density. The DTA (Stanton Red croft (UK)) and SEM (LEO 438 VP, UK) were performed on the plaster samples to ascertain conversion of gypsum into alpha plaster. 2.3. Preparation of cementitious binders The cementitious (water-resistant gypsum) binders (A and B) were produced by blending the ground alpha plaster with fly ash (lime reactivity 4.5 N/mm2)/granulated blast furnace slag, marble dust and hydrated lime followed by grinding in a ball mill to a fineness of 330–450 m2/kg (Blaine) in two compositions (Table 3)
Table 1 Chemical composition of phosphogypsum. Constituents (%)
Unprocessed phosphogypsum
Beneficiated phosphogypsum
IS: 12679 (1989) limits for phosphogupsum
P2O5 F Na2O + K2O Organic matter SiO2 + in HCl insoluble CaO MgO SO3 LOI pH
0.56 0.35 0.40 0.059 0.90 31.50 0.053 45.10 19.80 4.0
0.24 0.20 0.03 0.02 0.85 32.0 0.051 45.90
Max. Max. Max. Max.
6.5
Min. 5.0
0.40 0.40 0.30 0.40
P2O5 F Na2O SiO2 + insoluble inHCl CaO R2O3 (Al2O3 + Fe2O3) MgO SO3 LOI Sulphide MnO
Fly ash
Granulated slag
Lime sludge
Marble dust
3.60 1.0 0.76 62.90 1.50 28.30 0.80 0.20 1.50
30.0 31.50 27.5 7.32
0.62 2.29
Fly ash Hydrated lime Alpha plaster Granulated slag Marble dust Chemical activator
A
B
40 30 30 – – –
– 12.5 22.5 50.0 15.0 1.0
and tested as per IS:4031 (1991)-Methods of physical tests for hydraulic cement and IS: 6909-1973, specification for supersulphated cement for different properties. To determine the compressive strength and bulk density of the cementitious binders, cubes of size 2.5 cm were cast at normal consistency and after 24 h of curing, the samples were exposed to different temperatures (27–50 °C) in a sealed desiccator over water for a period up to 28 days.
3. Results and discussion 3.1. Properties of alpha plaster The physical properties of alpha plaster are given in Table 4. The results show an increase in compressive strength and bulk density with increase in salt concentration. At 0.20% concentration of sodium succinate (Na2C4H4O56H2O) alpha plaster gives maximum attainment of strength. The enhancement in strength value can be attributed to the decrease in normal consistency of the plaster. The fall in consistency may be correlated with the formation of columnar, hexagonal and large size dense gypsum crystals with minimum fissures and voids. While the beta plaster is composed of small porous crystals need higher quantity of water to produce workable paste. The setting time of alpha plaster is much higher than beta plaster. It is due to large crystals of plaster with smaller surface area. These findings are in agreement with the studies carried out by Cushman and Cushman [12] that the addition of crystal modifier changes the physicochemical properties such as lowering the water partial pressure or equivalently increasing the boiling point of the solution, catalyzing the dehydration of gypsum, modifying crystal habit and promoting the rate of growth of the crystal. The more stable crystals of alpha hemihydrate require less water to make a flowable slurry. Due to low water/plaster ratio, the density of set gypsum is increased which is responsible for enhancement of
Table 4 Physical properties of alpha plaster.
Table 2 Chemical composition of raw materials. Constituents (%)
Binders
3.10 52.0 0.50 0.31 0.16 41.0
3.01 30.80 2.70 19.27 43.40
Salts
Consistency (%)
Setting time (Min)
Bulk density (kg/m3)
Compressive strength (MPa)
Alpha plaster (without salt)
58.0
10.0
1200
15.0
Sodium succinate 0.05 0.10 0.20 0.25
46.0 38.0 35.0 36.5
34.0 36.0 34.0 33.0
1250 1270 1310 1280
19.00 22.40 28.58 26.46
Potassium citrate 0.05 0.10 0.20 0.25
48.0 40.0 38.0 38.5
32.0 30.0 32.0 31.0
1220 1240 1280 1260
17.20 20.20 23.70 22.46
Sodium sulphate 0.05 0.10 0.20 0.25
43.0 41.0 40.0 40.5
24.0 22.0 20.5 21.0
1220 1230 1270 1265
15.20 18.40 20.60 19.20
Beta plaster
66.0
8.0
1180
10.50
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270 °C due to conversion of hemihydrate into soluble anhydrite. The intensity of endotherm peak increases with the increase in concentration of sodium succinate. It can be seen that at 0.20% of
Fig. 1. DTA of alpha plaster produced in presence of different concentration of sodium succinate admixture.
compressive strength. In beta plaster, the setting time is shorter which can be ascribed to the large number of effective nuclei to cause crystallization of dehydrate gypsum. DTA has been used to explain formation of alpha plaster from phosphogypsum. Fig. 1 shows appearance of a long endotherm at 180–190 °C due to formation of hemihydrate plaster. This endotherm is followed by formation of a small exotherm at 250–
Fig. 3. Microstructure of alpha plaster produced in presence of sodium succinate admixture (0.20%).
Table 5 Properties of cementitious binders. Properties
Consistency (%) Setting time (min) Water absorption Porosity
Cementitious binders
Beta plaster
A
B
44 24 19.6 27.1
41 17 4.0 7.2
61–65 8–10 30–35 20–25
Table 6 Properties of cementitious binder A. Duration (d)
Curing temperature (°C) 27
40
50
Compressive strength (MPa) 1 3 7 28
5.6 5.9 6.8 7.2
7.0 9.2 22.3
7.8 13.2 26.4
Bulk density (g/cc) 1 3 7 28
1.27 1.29 1.30 1.34
1.28 1.31 1.33 1.39
1.29 1.31 1.33 1.40
Table 7 Compressive strength and bulk density of cementitious binder B. Duration (d)
Compressive strength (MPa) 1 3 7 28
Fig. 2. DTA of alpha plaster produced in presence of different concentration of additives: (a) potassium citrate and (b) sodium sulphate.
Bulk density (g/cc) 1 3 7 28
Curing temperature (°C) 27
40
50
1.6 16.2 20.8 20.6
– 16.8 20.7 22.4
– 27.2 29.7 32.2
1.60 1.63 1.64 1.67
1.62 1.65 1.67 1.70
1.68 1.70 1.71 1.76
M. Garg et al. / Construction and Building Materials 23 (2009) 3138–3143
sodium succinate concentration, endotherm of maximum intensity is obtained confirming maximum formation of the alpha plaster in phosphogypsum. However, the appearance of exotherm does not show marked increase in intensity with variation in concentration of additive. The occurrence of such small exotherm following the hemihydrate
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endotherm confirms the formation of alpha hemihydrate in accordance with the findings of Holdridge [13]. In case of addition of potassium citrate and sodium sulphate (Fig. 2) additives, the intensity of hemihydrate endotherms and soluble anhydrite exotherms appear at similar temperatures but with decreased intensity of the peaks showing formation of lower level of alpha plaster compared
Fig. 4. DTA of cementitious binders: (a) binder A and (b) binder B hydrated for different periods.
Fig. 5. Microstructure of cementitious binder A hydrated for: (a) 3 days (b) 7 days and (c) 28 days (FA: fly ash; G: gypsum; HL: hydrated lime).
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to addition of sodium succinate. The intensity of these thermal changes can be correlated with the compressive strength of the alpha plaster. Due to maximum intensity of hemihydrate endotherm in sodium succinate, alpha plaster gives highest strength and density values. 3.1.1. Microscopic structure of plaster Fig. 3a and b represents the crystallization of alpha hemihydrtae plaster produced at 35 psi steam pressure in presence of sodium succinate (0.20%). Most of crystals are prismatic columnar, hexagonal and tabular shaped and interspersed with euhedral needles and lath. Occasionally twinned prisms are also seen. The formation of such type of crystals can be ascribed to its process of preparation in which recrystallization takes place. Perederii [14] has also shown similar findings. X-ray diffraction pattern of alpha hemihydrate earlier studied by Singh and Rai [5] has shown that hemihydrate reflections of alpha plaster obtained from phosphogypsum were more pronounced than that of the beta hemihydrate, confirming better crystallization in alpha hemihydrate plaster. 3.2. Cementitious binders from alpha plaster Normally, gypsum plaster or gypsum building products show poor performance in wet or damp situations. Due to this reason, gypsum products are not preferred in these conditions. Water proofing or stabilization of gypsum products with suitable treatments is required. Addition of fly ash, slag, or resinous materials to gypsum plaster improves its water resistance to great extent. Considerable work has been done in China and other places [15,16] to improve water-resistance of binders based on fly ash,
fluorogypsum/phosphogypsum and cement for use in construction. To enhance scope of alpha plaster in construction for exposed situations, it was thought to produce cementitious binders from high strength alpha plaster. The results of these binders are depicted in Tables 5–7. Table 5 shows that binder ‘A’ possesses higher value of consistency, setting time, water absorption and porosity as compared to the binder ‘B’. However, the beta plaster shows much higher values of consistency, water absorption, porosity and short setting time. Data confirm that alpha based binders have better stability towards water than beta plaster generally used in construction. It can be further seen that the compressive strength and bulk density of binders A and B are increased with curing period and temperatures (Tables 6 and 7). Maximum strength values are obtained in both binders A and B at 50 °C. The increase in strength in binders with curing period at 50 °C has been correlated with the DTA (Fig. 4a and b). The Fig. 4 clearly indicated that endothermic peaks at 90–110 °C and 120–130 °C appear due to dehydration of ettringite and gypsum, respectively. These ettringite endotherms are increased with curing and the gypsum endotherms are reduced probably due to consumption of gypsum by the increased formation of ettringite endotherms. Further, formation of small endotherms at 430–450 °C at 3 and 7 days of curing are due to Ca(OH)2 which disappear at 28 days curing. The appearance of endotherms at 690–730 °C may be ascribed to the decomposition of CaCO3. The formation of calcium silicate hydrate (CSH) peak is not visible possibly due to overlapping with the ettringite endotherms. DTA information, thus, confirms that strength in both the binders are due to ettringite, gypsum and probably due to CSH phases.
Fig. 6. Microstructure of cementitious binder B hydrated for: (a) 3 days (b) 7 days and (c) 28 days (G: gypsum; HL: hydrated lime; M: marble dust; S: slag).
M. Garg et al. / Construction and Building Materials 23 (2009) 3138–3143
The microstructure of cementitious binders A and B are shown in Figs. 5 and 6. In Fig. 5a it can be seen that at 3 days hydration of binder, large quantity of agglomerated irregular euhedral to subhedral bodies of rounded fly ash particles coated with unhydrated & hydrated lime are formed. The fly ash spheres can be seen partially smeared with lime and gypsum crystals of prismatic and tabular shaped. At 7 days of curing the binder (Fig. 5b), size of agglomeration is increased. The fly ash spheres and gypsum crystals are coated with hydrated lime particles in abundance. It can be further seen that at 28 days (Fig. 5c), the fly ash spheres are partially or fully hydrated to form calcium silicate agglomerate intermingled with prismatic & tabular shaped crystals of gypsum. Probably these agglomerates of different crystals yield strength development in the cementitious binder. From Fig. 6 it can be seen that at 3 days of hydration of the binder (Fig. 6a), agglomerated irregular bodies are formed. Occasionally gypsum crystals of prismatic habit are conjugated with rounded bodies of slag and hydrated lime. At 7 days of hydration subhedral to anhedral prismatic crystals of gypsum of broadened surface are seen along with agglomerated mass of partially hydrated dendritic slag particles (Fig. 6b). At some places unreacted rounded bodies of slag are also visible. Unhydrated and hydrated lime particles interspersed with unidentified bodies may be of marble dust are present over the hydrated mass. It can be further seen that at 28 days of hydration (Fig. 6c), euhedral prismatic crystals of gypsum and hydrated slag bodies coated with unreacted slag and marble particles are formed. 4. Conclusions 1. The alpha plaster can be produced from beneficiated phosphogypsum by autoclaving the gypsum at an optimum pressure of 35 psi in presence of 0.20% (by wt.) of sodium succinate additive. 2. The alpha plaster gives high strength and low consistency than the traditional beta hemihydrate plaster 3. The alpha plaster can be blended with other wastes like fly ash, slag, lime sludge, etc. to produce water resistant binder for construction work.
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4. The binder shows low water absorption and high strength than the beta plaster. 5. The manufacture of alpha plaster and cementitious binder from waste phosphogypsum is recommended as these products have immense scope in masonry work, prefabricated building products and structural elements.
Acknowledgement Authors are thankful to the Director, Central Building Research Institute, Roorkee for his permission to publish the paper.
References [1] Grooves AW. Gypsum & anhydrite. London: Overseas Geological Surveys, Her Majesty’s Office; 1958. [2] Brookby HE. US patent 1,370,581, March 8; 1931. [3] Hoggart GA. US patent 1,960,538, May 29; 1934. [4] Randel WS, Dailey MC. US patent 1,901,051, March 14; 1933. 1931, 240, October 17; 1933. [5] Singh Manjit, Rai Mohan. Autoclaved gypsum plaster from selenite and byproduct phosphogypsum. J ChemTech Biotechnol 1988;43:1–12. [6] Gardener HF. US patent 1,996,372, April 2; 1935. [7] Singh Manjit. Role of phophogypsum impurities on strength and microstructure of selenite plaster. Constr Build Mater 2005;19:480–6. [8] Singh Manjit, Garg Mridul. Cementitious binders from waste phosphogypsum. J Ind Build Cong 2001;8:157–63. [9] Singh Manjit, Garg Mridul, Rehsi SS. Purifying phosphogypsum for cement manufacture. Constr Build Mater 1993;17:3–7. [10] Singh Manjit. Prospects of phosphogypsum based industry in India. Ind Ceram 1985;28:51–7. [11] Singh Manjit, Garg Mridul, Verma CL, Handa SK, Kumar R. An improved process for the purification of phosphogypsum. Constr Build Mater 1996;10:597–600. [12] Cushman Darby, Cushman LLP. Process for the production of alpha hemihydrate calcium sulfate from gas sludge. US patent 5562892; 2004. [13] Holdridge DA. The characterization of plaster. Trans Br Ceram Soc 1965;64:211–31. [14] Perederii IA. Microstructure of normal and high strength gypsum and products made of them. Chem Abstr 1960;54:106268c. [15] Yan P, You Y. Studies on the binder of fly ash–fluorogypsum–cement. Cem Concr Res 1998;28:135–40. [16] Yan P, Yang W, You Y. Microstructure and properties of the binder of fly ash fluorogypsum – Portland cement. Cem Concr Res 1999;29:349–54.