India produces large quantity of waste by-products from various agro industrial processes. Among these waste, utilization of phosphogypsum, fluorogypsum, slag, fly ash, lime sludges, etc. is paramount due to their enormous availability and the pollution of the ground water likely to be caused by their unplanned dumping on the fertile land. The utilization of waste gypsum particularly phosphogypsum (PG) and the fluorogypsum (FG) is significant. Unlike PG, the utilization of FG, a waste of hydrofluoric acid industry is also important as it too creates pollution of the atmosphere and the ground water. Investigations accomplished at the CBRI showed that useful cementitious binder can be produced by blending small quantity of Ca(OH)2 and chemical additives with the FG followed by fine attrition. Data showed that a plaster/binder of low water demand, high compressive strength and low water absorption can be produced. The hydration of binder as evaluated by differential thermal analysis (DTA) show rapid conversion of anhydrite phase into dihydrate gypsum. The addition of 15-20% waste lime sludge may add economy to the new binder. The FG binder is suitable for making building bricks, flooring tiles and plastering which may be considered a new concept to the effective use and disposal of FG waste to accrue economy to the HF industry as well as to the Nation. Cost wise the FG binder is cheaper than the lime and cement binders. The technology of high strength plaster from FG and its use in making flooring tiles has been commercialized.
Dr. Manjit Singh,Former Scientist ‘G” (Director Grade Scientist) & Head, Environmental S&T & Clay Products Divisions, Central Building Research Institute, Roorkee (India) Advisor/Consultant to Gypsum & Cement Industries
Huge quantity of wastes are being produced from the industries like metallurgy, petrochemicals, fertilizers, paper and pulp making per annum in India. From the building materials angle, the utilization of fly ash, slags, phosphogypsum, fluorogypsum, press mud, red mud etc. is paramount. Over 6.5 million tonnes of by-product gypsum are produced annually from several phosphatic and hydro-fluoric acid industries in India, Singh1. The waste of phosphoric acid industry popularly called as phosphogypsum is available to great extent. Fluorogypsum a waste of hydrofluoric acid manufacture has been studied for use in supersulphated cement or binding agents as reported by Taneja, et.al 2
, Swanski 3
In China Republic, similar cementitious binders based on fluorogypsum were developed by Yan et.al.4-5
. The use of fluorogypsum which is available to the extent of slightly over 1.0 million tonne per annum is significant. Fluorogypsum contains impurities of fluoride and free acidity. These impurities particularly free acidity may interfere with the setting and strength development of plaster/building components. Since the utilization of fluorogypsum is limited, thus, there is a disposal problem of this waste and it has become essential to suitably utilize the material in value added building materials.
|Figure 1: DTA of fluorogypsum anhydrite
||Figure 2: SEM of unpurified fluorogypsum
have shown that water-resistant binding materials can be produced from phosphogypsum, slag, cement, fly ash and lime sludge. In this paper, investigations undertaken to characterize and to beneficiate fluorogypsum to make high strength binding/cementitious plaster have been presented. Effect of various chemical activators/additives on setting, strength, water absorption, porosity, etc. of the fluoro plaster/binder has been studied. The hydration and microstructure properties of the binder are reported. The suitability of fluorogypsum plaster for making building bricks and blocks containing various admixtures and its use in plastering work has been discussed. The economy of making the plaster is also indicated.
Materials & Methods
The sample of fluorogypsum was collected from M/s Navin Fluorine International, Bhestan, Gujarat, India. It was analyzed for chemical composition as per IS: 128810 and as per standard test procedures cited by Scott, et.al.11-12. The fluorogypsum contained Fluoride 1.32%, SiO2 + insoluble in HCl 0.65%, Al2O3 + Fe2O3 0.65%, CaO 41.19%, MgO Tr., SO3 56.10 % and Loss on ignition 0.61% and pH 5.0. Data showed that fluorogypsum possess high purity i.e. CaSO4.2H2O besides fluoride as the major impurity. The low pH value shows presence of free acidity. Minor earthly impurities of SiO2 and Al2O3 + Fe2O3 have also been identified.
The chemical activators of laboratory grades ranging from sulphates to chloride of alkali and alkaline earth hydroxides were used to activate hydration of fluoroanhydrite. Differential thermal analysis (DTA) (Stanton Red croft, UK) and Scanning electron microscopy (SEM LEO 438VP) of the raw gypsum and hydrated plaster were studied.
DTA (Fig. 1) shows endotherm and exotherm at 140° and 250°C due to the inversion of poorly weathered anhydrite into hemihydrate (CaSO4.½ H2O) and anhydrite (CaSO4 (II)) peaks. The SEM of fluorogypsum sample (Fig. 2) shows majority of crystals in fluorogypsum are anhedral to subhedral platy prismatic interspersed with lath in the jumble form. Twinning of some of the crystals may also be recorded.
Purification of Fluorogypsum
As the fluorogypsum contains free acidity which may corrode the grinding media (balls etc.) and the lining of the ball mill and sometimes the plaster/ binder may become hygroscopic, it is therefore, essential to neutralize the acidity to get the suitable product. With this objective, effect of addition of dry hydrated lime (Ca (OH)2) was studied on the pH of fluorogypsum. It was found that at 0.8 -1.0% addition of (Ca(OH)2) to the fluorogypsum, a neutral pH value of 7.0 was obtained. The SEM of purified fluorogypsum is shown in Figure 3. It can be seen that gypsum crystals are euhedral platy prismatic and lath shaped in nature without agglomeration/jumbling showing absence of any foreign impurity.
Preparation of Gypsum Binder / Plaster
|Figure 3: SEM of purified fluorogypsum
The fluorogypsum dried at 42 ± 2°C, was ground in the ball mill to a fineness of 98-99% passing through 90 micron (I.S. 9 sieve) sieve. The ground material was then blended with different chemical activators for 1 hour in the blender / powder mixer to get a uniform product. The binder was tested and evaluated as per references13-14. The water absorption and porosity of fluorogypsum plaster was examined by immersing the 2.5 cm x 2.5 cm x 2.5 cm cubes of the binder/plaster (28 days cured) in water for a period of 2h, 8h and 24 hrs. The porosity of cubes was evaluated by multiplying the water absorption with bulk density of the hydrated plaster.
Effect of addition of lime sludge obtained from M/s Rashtriya Chemicals & Fertilizers, Mumbai, (chemical composition (by wt.%) - P2
2.01, F 0.12, Na2
O + K2
O 0.026, organic matter 0.06, SiO2
+insoluble in HCl 1.50, CaO 50.50, Al2
0.026, MgO 0.64, SO3
0.98, LOI 44.50) was examined on the properties like compressive strength, bulk density, water absorption and porosity of fluorogypsum plaster/binder on the basis of 2.5cm x 2.5cm x 2.5cm cubes for the period of 3, 7 and 28 days.
Preparation of Bricks and Plastering Studies
The effect of various materials such as saw dust, rice husk, exfoliated vermiculite etc. on the properties of fluorogypsum binder was studied to arrive at optimum mix composition for casting bricks. 5cm x 5cm x 5cm cubes were cast at the workable consistency for the properties like compressive strength, bulk density, water absorption and the porosity. Based on cube data, large size bricks (19cm x 9cm x 9cm) were cast by hand molding.
Preparation of Building Blocks & Flooring Tiles
Based on 5 cm cube data (previously optimized on 5 cm x 5 cm x 5cm cube strength), 40 x 20 x 10 cm blocks were cast using anhydrite plaster previously produced plus optimized chemical activator (such as sod. Sulphate or sod. sulphate + ferrous sulphate) with washed saw dust and foamed slag separately. The foamed slag (a by-product of steel plants) of size 6 mm and down having density 300 - 400 kg/m3 may be used to develop insulative blocks.
The flooring tiles of sizes 200 mm x 200 mm x 20 mm and 300 mm x 300 mm x 20 mm or larger size are cast by vibration moulding of the mix containing fluoroanhydrite powder with different pigments, catalyzed (0.5 - 1.5%)), a small quantity of glass fibre and coloured stone chips at normal consistency. The fly ash and red mud industrial solid wastes are also added to the anhydrite mixes for moulding these tiles. These tiles after demoulding are cured in high humidity (over 90%), dried at 42 ± 2°C and tested for properties such as flexural strength, compressive strength, water absorption, wear resistance and porosity as per IS:1237-1980, specification for cement and concrete tiles.
Plastering of Brick Wall
To find out suitability of fluorogypsum binder for use in internal plastering, mortars of mix proportions 1:1, 1:2 an 1:3, by volume were prepared at mason consistency to plaster the burnt clay brick wall. Mortar mixes 1:1, !:2 and 1:3, binder-sand in 12mm thickness were applied over the internal brick wall. The fineness modulus of the sand was kept at 1.91 (50:50 Badarpur and Ranipur river sand). The finish coat of 3 mm of neat binder was applied over 9 mm of 1:2 , binder-sand under coat. Before applying binder-sand plaster, the brick wall was well watered so that mortar water may not get evaporated before the mortar was set. The plastered patches were examined for their various characteristics after 24 hours and onward.
Properties of Fluorogypsum Binder/Plaster
|Figure 4: DTA of hardened fluorogypsum plaste
The physical properties of fluorogypsum plaster activated by the chemical activators are reported in Table 1. Data show that with the use of (NH4
activator, the setting time of fluorogypsum plaster was beyond the maximum specified limit of 6.0 hrs as per ASTM C 61-50. At the same time, the compressive strength was also much less. However, with the use of combined chemical activators i.e. Ca(OH)2
, the setting time was accelerated and was much within the limit and the rate of strength development was even quite high at 3 days of hydration. The hydration of fluorogypsum plaster/binder was supplemented by DTA. The thermo grams are shown in Fig. 4. It can be seen that intensity of endotherms at 140-150°C,190-200°C and exotherms at 360-370°C were increased due to dehydration of gypsum and inversion of CaSO4
(III) in to β-CaSO4
. The intensity of endotherms and exotherms found to be increased with curing period indicating increase in gypsum formation. The fluorogypsum has been found sound in nature.
The hardening of fluorogypsum is due to increase in its solubility and the rate of dissolution of the anhydrite and due to increase in the rate of nuclei formation. In fact, the chemical activators added to the anhydrite plaster react with the CaSO4
ions and form transient double salt. The colloidal particles of the activator concentrate on the surface of CaSO4
molecules and establish the potential centers around which crystallization sets in when solution becomes supersaturated.
. Activatorions —> Ca2+
Water Absorption and Porosity of Fluorogypsum Binder/Plaster
The results of water absorption and the porosity of fluorogypsum binder is shown in Table 2. Data show that fluorogypsum binders produced with the chemical activators Ca(OH)2 – CaCl2 – Na2SO4 possess lower water absorption and the porosity values than the (NH4)2SO4 activator. On the basis of strength development, water absorption and porosity properties of the fluorogypsum binder, the addition of chemical activators i.e. Ca(OH)2 – CaCl2 – Na2 SO4 (3.0 – 0.5 – 0.5 by wt.%) were selected for further studies.
Effect of Lime Sludge on the Properties of Fluorogypsum Binder
The addition of lime sludge from the paper industry on the properties of flourogypsum binder containing activators (Ca(OH)2:CaCl2:Na2SO4) is shown in Table 3. The trend of results show an increase in consistency and decrease in strength values with the addition of lime sludge.
However, the attainment of strength is quite high. Data showed that with increase in lime sludge addition, the water absorption and the porosity values were increased with the enhancement of immersion period. At 20.0% addition of the lime sludge, maximum water absorption (21.90%, 23.60%, 26.40% at 2hr, 8hr, 24 hr) and the porosity values (40.60, 43.70, 48.90 at 2hr., 8hr., 24 hr.) were attained. These studies suggest that fluorogypsum plaster may be partly replaced with the lime sludge to economize the use of such binder.
Development of Bricks from Fluorogypsum Binder/Plaster
Burnt clay bricks are essential ingredients for providing shelter to the millions, and is most popular because it can be adopted for any size or shape of construction. Brick constitutes about 13% of the total cost of the building materials required for construction of moderate house. The burnt clay brick industry as it exists today, is not able to meet the demand of a the modern construction agencies which require bricks of higher strength, better shape and of lower water absorption. The escalating cost of energy and unprecedented pressure on activities have undoubtedly caused great setback to the production and quality of the bricks. It is, therefore, imperative that an alternative material is required to bridge the gap which is environment–friendly and is acceptable to the construction agencies. It would therefore be useful to consider certain features of technological development in this country with particular reference to the current thinking on the need to use waste materials. Thus, utilization of fluorogypsum binder for making building bricks can be considered a new and useful preposition in building sector.
The efforts were therefore, made to use fluorogypsum waste for making building bricks. The effect of various materials such as saw dust, rice husk, exfoliated vermiculite etc. on the properties of fluorogypsum binder was studied to arrive at optimum mix composition for casting bricks. 5cm x 5cm x 5cm cubes were cast at the workable consistency for the compressive strength, bulk density, water absorption, and the porosity.
Effect of Saw Dust and Rice Husk
|Figure 5: Building bricks cast from fluorogypsum binder/plaster using saw dust, rice husk and exfoliated vermiculite
The effect of saw dust and rice husk on the compressive strength and bulk density of the fluorogypsum binder are listed in Table 4. It can be seen that with the addition of saw dust and the rice husk to the fluorogypsum binder, the compressive strength and the bulk density values were reduced. In case of saw dust, the fall in strength was comparatively less than the addition of rice husk. However, the decrease in the bulk density was much more with the addition of rice husk than the saw dust. In view of reduction in the density values, the manufacture of lightweight bricks can be contemplated.
The effect of saw dust and rice husk on the water absorption and the porosity of the fluorogypsum binder was studied. Data showed that the strength values are higher in case of addition of rice husk than the addition of saw dust. This may be attributed to the organic impurities particularly sugars present in the saw dust. However, there is decrease in bulk density of the plaster with increase in saw dust and rice husk. With the addition of 10% saw dust to the plaster, the water absorption was found to be 8.75%, 9.42% and 9.98%, while the porosity was 15.58, 16.78 and 17.78 at 3, 7 and 28 days of curing. In case of addition of 5% rice husk, the water absorption of the plaster was 9.38%, 10.35% and 11.46% and the porosity was 17.40, 19.20 and 19.40 at 3, 7 and 28 days respectively.
Effect of Addition of Exfoliated Vermiculite on the Properties of Fluorogypsum Binder
The effect of addition of exfoliated vermiculite on the compressive strength and the bulk density of fluorogypsum are reported in Table 5. Data show that with the increase in vermiculite content, the compressive strength and the bulk density are reduced. However, there is an increase in the strength and density values with the increase in curing period. It can be noted that the density can be further reduced by increasing the vermiculite content but the cost of the composition may also be increased. At 10% addition of vermiculite, the water absorption was 17.64%, 18.18% and 20.45% while the porosity was 30.51, 32.54 and 36.61. However, at 10.0% addition of vermiculite, adequate strength and the density values are achieved.
Preparation of Full Size Bricks
On the basis of properties obtained by the addition of an optimum quantities of saw dust (10%), rice husk (5%) and the exfoliated vermiculite addition (10%) to the fluorogypsum binder, the full size bricks (19 x 9 x 9 cm) were cast at normal consistency. These bricks were tested for physical appearance, compressive strength, water absorption and efflorescence as per IS: 3495 (Part 1)15. The properties of bricks are listed in Table 6. The strength and water absorption values complied with the properties of IS:1289416 except those bricks prepared with rice husk. Typical photograph of binder–saw dust, binder–rice husk, binder–exfoliated vermiculite and binder-lime sludge bricks are shown in Fig. 5.
Suitability of Fluorogypsum Binder in Plastering
|Figure 1: DTA of fluorogypsum anhydrite
||Figure 2: SEM of unpurified fluorogypsum
The suitability studies never carried out following the proceed explained in this paper earlier. It was found that plaster patches developed adequate strength and hardness after 24 hours of application and further continued. The texture of the plaster was smooth and hard and showed good adhesion with the bricks.
Building Blocks & Flooring Tiles from FG Properties of Building Blocks
The properties of building blocks produced from FG are listed in Table 7. It can be seen that on using 10% saw dust and 40% of foamed slag, the density is lowered and the strength data comply with the minimum strength of 2.0 MPa specified in IS:2849-1983, a specification for non-load bearing gypsum partition blocks (solid and hollow type). These blocks are mainly suitable for internal non-load bearing partition walls or for inner leaf of cavity wall construction. The blocks should not be used under damp conditions as they are liable to suffer deterioration as their strength is seriously reduced.
According to Mydall et.al.17
, the vitreous materials have far lower thermal conductivity than crystalline material. Since foamed slag used in making of blocks is a vitreous material, therefore, 300 x 300 x 30 mm size boards may be cast using the above composition. The boards after curing for 28 days are dried and subjected to the thermal conductivity test as per heat flow method specified in ASTM (518-1971), specification for method of thermal conductivity of building insulating materials by heating flow method. The results are given in Table 8. The data show that the blocks produced using foamed slag have lower thermal conductivity than the conventional building materials like light weight concrete and common burnt clay bricks.
The strength of blocks is much higher than the minimum specified value of 50 kg/cm2
(5 MPa) IS:3590-1976, specification for load-bearing light weight concrete blocks. Hence, the blocks produced with neat plaster at normal consistency are rightly suitable for load-bearing internal partition walls. The typical photographs of blocks made using (1) 10% saw dust, (b) 40% foamed slag and (c) neat plaster is shown in Fig. 6.
Properties of Flooring Tiles
The properties of flooring tiles moulded from polymerized and activated FG are listed in Table 9. It can be seen that the phosphogyp-sum flooring tiles complied with the requirements of flexural strength, water absorption and wear resistance as given in IS: 1237-1980.
The tiles produced using fly ash or red mud in place of pigments also complied with the standard requirements. These tiles are suitable for use in flooring for general purposes for places where light loads are taken up by the floor such as office buildings, schools, colleges, hospitals and residential buildings. A typical photograph of flooring tiles produced out of anhydrite binder is shown in Fig. 7.
Cost Economics of the Flooring Tiles
The cost of a plant of capacity 100 m2/day FG tile may cost the capital investment of 30-35 lakhs. The major equipment required for the plant may be blender, ball mill/pulverizer, vibro press, demoulding plates, casting moulds, powder mixers, driers, curing chamber, etc. These plant & machinery are easily available in Indian market. Some machineries are the proprietary items. An Indian Patent entiled ‘A Novel High Strength Plaster Composition and Flooring Tiles Made Therefrom’, Patent No. 696/Del/2000 by Dr.Manjit Singh & Dr. Mridul Garg has been claimed.
High strength plaster can be developed from fluorogypsum waste. A mixture of Chemical activators like Ca(OH)2, CaCl2 and Na2SO4 has been finalized as the strengthening compound to invert waste anhydrite into strong gypsum matrix at faster pace. The addition of 15-20% lime sludge to the binder can economize the production of fluorogypsum plaster without sacrificing the strength value. Building bricks can be produced by adding an optimum quantity of saw dust (10%), rice husk (5%) or exfoliated vermiculite (15-20%) to the fluorogypsum binder. The fluorogypsum on admixing with river sand, lime sludge and the exfoliated vermiculite (in optimum proportion) is suitable for plastering (Finish & undercoat) over the internal brick wall. The fluorogypsum plaster/binder may replace different type of cement/lime based putties available in the market. The fluorogypsum plaster is also suitable for making building blocks when mixed with washed saw dust and foamed slag for insulative purpose and light weight partitions in normal as well as multistoried buildings.
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