Fly ash - lime - phosphogypsum cementitious binder: A new trend in bricks

Materials and Structures/Matériaux et Constructions, Vol. 33, January-February 2000, pp 59-64 Fly ash - lime - phosphogypsum cementitious binder: A new trend in bricks S. Kumar Department of Civil Engineering, B.I.E.T., Jhansi, India. TECHNICAL REPORTS Paper received: March 31, 1999; Paper accepted: June 22, 1999 A B S T R A C T Fly ash-lime-phosphogypsum (FaL-G) bricks are one of the best alternatives to the conventional burnt clay bricks. This paper gives the results of an experimental investigation in which the compressive strength, water absorption, density and durability of Fly ash-lime-phosphogypsum (FaL-G) bricks were investigated by using varying quantities of fly ash, lime and calcined phosphogypsum. The properties of Fly ash-lime-phosphogypsum (FaL-G) bricks are compared with those of the ordinary burnt clay bricks. The results indicate that these bricks are lighter in weight, durable in aggressive environments and have sufficient strength for their use in building construction. R É S U M É Les briques à base de cendres volantes-chaux-phosphogypse (FaL-G) sont une des meilleures alternatives aux briques conventionnelles en terre cuite. Cet article présente les résultats d une étude expérimentale dans laquelle la résistance à la compression, l absorption d eau, la densité et la durabilité des briques en FaL-G ont été examinées en utilisant différentes quantités de cendres volantes, de chaux et de phosphogypse calciné. Les propriétés des briques en FaL-G sont comparées avec celles des briques ordinaires en terre cuite. Les résultats obtenus indiquent que ces briques sont plus légères, durables dans des environnement agressifs et assez résistantes pour être utilisées dans la construction. 1. INTRODUCTION The bricks used for construction of buildings are mostly burnt clay bricks. The clay available in river basins, water ponds and coastal areas is plastic in nature and suitable for proper moulding, drying and burning of bricks. But, soil that is less plastic or expansive, gritty or sandy can not be used for brick manufacturing. In most of the central and south India, good quality soil for production of burnt clay brick is not available which result in higher cost of bricks due to transportation from long distances or use of low quality clay bricks manufactured with sub-standard clays. In such regions a substitute to burnt clay bricks is essential. FaL-G bricks act as one of the best option available to substitute burnt clay bricks in the regions where suitable soil is not available for burnt clay bricks manufacturing. FaL-G bricks can also be used to replace burnt clay bricks for conservation of energy and pollution control. The basic constituents of FaL-G are fly ash, lime and phosphogypsum. Fly ash is available in large quantities in India, which is around 50 million tons per year [1], as a waste product from thermal power plants and industrial plants using pulverized coal or lignite as fuel for boilers. Lime can be availed from mineral sources or may be procured from industrial wastes. Phosphogypsum is an important by-product of phosphoric acid fertilizer industry. Approximately 5 million tons of phosphogypsum is produced each year in India and causes serious storage and environmental problem [2]. Several studies have been reported on utilization of FaL-G in bricks, blocks, and plain and reinforced concrete [2-10]. Bhanumathidas and Kalidas [5] were amongst the first who have reported the application of FaL-G in brick manufacturing. Ambalavanan and Roja [9] reported the feasibility study of utilizing waste lime and gypsum with fly ash in FaL-G brick manufacturing. They have observed that in most of the cases the use of waste lime did not give promising results and some benefication is needed to increase the strength of FaL-G bricks. The process of benefication increases the cost of bricks considerably compared to the burnt clay bricks 1359-5997/00 RILEM 59

Materials and Structures/Matériaux et Constructions, Vol. 33, January-February 2000 [9]. In earlier studies on FaL-G bricks [5, 9], limited information was reported and the durability aspect of this new building material was not experimentally examined. For wider application of FaL-G bricks in construction industry, extensive research is needed. With this perspective, FaL-G bricks were manufactured in the laboratory and their properties were investigated. This paper gives the results of an experimental investigation in which the compressive strength, water absorption, density and durability of FaL-G bricks manufactured with different proportion of flyash, lime and calcined phosphogypsum were studied. The use of FaL-G bricks in building construction will solve several other problems, such as pollution, energy conservation, land usage and other associated social problems. 2. EXPERIMENTAL PROGRAMME 2.1 Materials The following materials were used for the manufacturing of FaL-G bricks. 2.1.1 Fly ash The fly ash was collected in dry state from the electrostatic precipitators. The chemical characteristics of the fly ash are given in Table 1. 2.1.2 Lime Hydrated lime of commercial grade was used in all mixtures. The results of chemical analysis of the lime are given in Table 2. 2.1.3 Gypsum Gypsum has been obtained as phosphogypsum, a waste by-product from a plant manufacturing fertilizers Table 1 Chemical analysis of fly ash Constituents Percentage Loss on ignition 5.90 SiO 2 57.01 Al 2 O 3 23.83 Fe 2 O 3 6.66 CaO 3.34 MgO 1.77 SO 3 0.56 Table 2 Chemical analysis of lime Constituents Percentage Loss on ignition 5.65 CaO 63.25 SiO 2 +Al 2 O 3 25.00 MgO 4.70 Table 3 Chemical analysis of calcined phosphogypsum Constituents Percentage CaSO 4. 2H 2 O 91.12 and chemicals. Chemical analysis of the used calcined phosphogypsum is presented in Table 3. 2.1.4 Water Ordinary drinking water from the piped water supply was used. 2.2 Mix proportions SiO 2 1.05 Fe 2 O 3 0.30 Table 4 Mix proportions of FaL-G bricks Constituent material (percentage) Mix designation Fly ash Lime Calcined phosphogypsum M-1 20 60 20 M-2 20 50 30 M-3 20 40 40 M-4 25 55 20 M-5 25 45 30 M-6 30 50 20 M-7 30 40 30 M-8 30 35 35 M-9 35 45 20 M-10 35 35 30 M-11 40 40 20 M-12 40 30 30 M-13 50 30 20 M-14 50 20 30 M-15 50 25 25 M-16 60 20 20 M-17 60 10 30 M-18 60 30 10 M-19 70 20 10 M-20 80 10 10 In order to establish the feasibility of producing a binder that can impart adequate strength, bricks were cast with the mix proportions given in Table 4. Shrinkage cracking is a major weakness in gypsum based blocks, especially when they are restrained in a constructed wall. Shrinkage cracking can be minimized by keeping the water content per unit volume of binder as low as possible. Hence, in the present study, a low slump mix was used to limit the shrinkage. 60

Kumar 2.3 Process operations 2.3.1 Mixing of raw materials All specimens for a given mix proportion were cast in a single batch. The FaL-G bricks were manufactured by a team of technicians in the presence of author to maintain uniformity in casting of bricks. The weighed quantity of fly ash and calcined phosphogypsum were mixed in dry condition. The hydrated lime was prepared in the laboratory by adding water to the weighed quantity of unslaked lime. Then complete slaking of lime for 6 to 8 hours was allowed. The slaked lime slurry was sieved through 1.18 mm sieve. The sieved slurry of hydrated lime was added to the mixture of fly ash and calcined phosphogypsum to obtain a paste of semi-dry consistency to facilitate casting of bricks in standard moulds. The ingredients were mixed thoroughly by kneading until the mass attained a uniform consistency. Standard consistency test was performed with each mix proportion to calculate the water content of the mix. The cementitious binder was mixed with waterbinder ratio equivalent to 0.9 times the water required to produce cementitious paste of standard consistency. The standard consistency denotes the quantity of water required for 100 gm of binder (fly ash + lime + phosphogypsum) for the desired workability. The standard consistency test was performed using Vicat apparatus in accordance with the standard test procedure prescribed for cement testing. 2.3.2 Preparation of bricks The wooden moulds of internal dimension 220 mm 100 mm 75 mm were used for bricks. The size of bricks was kept approximately the same as those of the normal burnt clay bricks available in northern India. The FaL-G mix was placed in the moulds in two layers, each layer being compacted using a vibrating table. The bricks were left for a day or two for natural drying and curing in atmosphere at 27 ± 3 C and relative humidity (RH) > 80%. 2.3.4.2 Water absorption After 96 days of curing, bricks were tested for water absorption. These bricks were taken out from water curing tanks and were allowed to drain water by placing them on a 10 mm wire mesh. Visible surface water was removed with a damp cloth from the bricks, which were immediately weighed. After obtaining the saturated weight these bricks were dried in an oven at 105 C for more than 24 hours. These bricks were taken out from the oven and were weighed at room temperature. From the wet and dry mass of these bricks, water absorption was obtained. 2.3.4.3 Density After 96 days of casting, these bricks were dried to a constant mass in an oven at 105 C. They were cooled to room temperature and their density was obtained by dividing the mass of a brick by its overall volume. The shrinkage of the bricks was not measured. It may be relevant to mention that the cracks were not visible on these bricks with naked eye after 96 days of casting. 3. EXPERIMENTAL RESULTS AND DISCUSSION The experimental results are presented in Figs. 1 to 7. Figs. 1 to 3 show the compressive strength of FaL-G bricks with a constant phosphogypsum content of 30, 20 and 10 percent, respectively. Fig. 4 shows the compressive strength of those FaL-G bricks that were manufactured with equal proportions of lime and phosphogypsum. Figs. 5 and 6 show the water absorption and density of FaL-G bricks, respectively. The loss in compressive strength of FaL-G bricks cured in sulfate solution for 72 days is shown in Fig. 7. 2.3.3 Method of curing The bricks were taken out from the moulds and were covered with wet gunny bags. After sufficient strength was obtained, these bricks were transferred to the water filled curing tanks at 23 ± 2 C. The durability of FaL-G bricks was investigated by curing them in the aggressive environment of sulfate solution. The sulfate solution having sulfate (SO 4 -- ) concentration equal to 10000 ppm was prepared in the laboratory by mixing 14.79 gm of Na 2 SO 4 in one liter of water. 2.3.4 Method of testing 2.3.4.1 Compressive strength The FaL-G bricks were taken out from water one day prior to the testing and were tested for compressive strength after 24, 72 and 96 days of casting. The bricks cured under sulfate solution were tested for their compressive strength after 72 days of casting. Fig. 1 - Compressive strength of FaL-G bricks (Gypsum = 30%). 61

Materials and Structures/Matériaux et Constructions, Vol. 33, January-February 2000 Fig. 2 - Compressive strength of FaL-G bricks (Gypsum = 20%). Fig. 4 - Compressive strength of FaL-G bricks (Gypsum : Lime = 1:1). Fig. 3 - Compressive strength of FaL-G bricks (Gypsum = 10%) Figs. 1 and 2 show that the compressive strength of FaL-G bricks initially increases with the increase in fly ash content but after attaining a maximum value it start decreasing with the further addition of fly ash. It can be observed from Fig. 1 that FaL-G bricks attained their maximum strength with fly ash content equal to 40 percent. By reducing phosphogypsum content of FaL-G bricks from 30 to 20 percent, the strength of bricks reduces for the same fly ash content. The maximum compressive strength of FaL-G bricks manufactured with 20 percent phosphogypsum is observed with 30 to 35 percent fly ash, as shown in Fig. 2. Fig. 5 - Water absorption of FaL-G bricks. Similar to Figs.1 and 2, Fig. 3 shows that the compressive strength of FaL-G bricks, for the constant value of 10 percent phosphogypsum, decreases with the increase in fly ash percentage. Fig. 4 shows the compressive strength of FaL-G bricks having phsophogypsum and lime in equal proportions. Fig. 4 indicates that with the increase in fly ash content above 40 percent, compressive strength of FaL-G bricks decreases. The compressive strength of FaL-G bricks with high proportion of fly ash (80 percent) was 5.9 MPa after 96 62

Kumar Fig. 7 - Reduction in compressive strength of FaL-G bricks in sulfate solution. Fig. 6 - Density of FaL-G bricks. days of casting. Therefore, these bricks can easily replace the burnt clay bricks. Fig. 5 shows the water absorption of FaL-G bricks. It is observed that in general, the water absorption of FaL- G bricks increases with the increase in fly ash content. For the same fly ash content, the addition of more phosphogypsum reduces the water absorption of these bricks. On comparing FaL-G bricks with ordinary burnt clay bricks it is observed that water absorption of FaL-G bricks in the present investigation was obtained in between 18.8 to 36.3 percent whereas the water absorption of ordinary bricks shall not be more than 20 percent by weight [11]. Density of FaL-G bricks is shown in Fig. 6. It is observed that the water absorption and density are closely related to each other. The density of FaL-G bricks were found to be 20 to 40 percent lower than that of the ordinary burnt clay bricks and thus its use reduces dead weight of the structures and thereby there is an overall economy in the construction. Fig. 7 shows the loss in compressive strength of FaL-G bricks exposed to short term accelerated test for 72 days in aggressive environment of sulfate solution. The results indicate that the bricks with high proportion of fly ash have less resistance to sulfate attack. It is also observed that increase in phosphogypsum content results in the increase of sulfate resistance of these bricks. The loss in compressive strength of FaL-G bricks is very substantial up to fly ash content of 60 percent. FaL-G bricks with high proportion of flyash have shown greater water absorption and hence they are more susceptible to sulfate attack. Therefore, bricks with high proportion of fly ash should not be used in aggressive environmental conditions. In areas where good burnt clay bricks are not available and the ingredients of FaL-G are available in mutual proximity, the cost of production is comparable with the conventional products. This is due to cheap availability of ingredients and no fuel consumption. 4. CONCLUSIONS Based on the experimental investigation reported in this paper, following conclusions are drawn: 1. The strength of FaL-G bricks increases with age. 2. Phosphogypsum has more pronounced binding action than lime. 3. FaL-G bricks with cheaply available fly ash in large proportion have sufficient strength for their use in low cost housing, non-load bearing construction and in regions where good quality burnt clay bricks are not available. 4. Water absorption of FaL-G bricks is found to be in the range of 18.8 to 36.3 percent, whereas the water absorption for ordinary burnt clay bricks should not be more than 20 percent. The water absorption of FaL-G bricks increases with the increased flyash content. 5. The densities of various designated FaL-G bricks are found to be 20 to 40 percent lower than the ordinary burnt clay bricks. This will reduce the weight of multistoried buildings considerably and thus there is saving in the construction. 6. The curing conditions such as ordinary water curing or sulfate curing do not make any significant difference for good quality bricks. Hence, FaL-G bricks can be 63

Materials and Structures/Matériaux et Constructions, Vol. 33, January-February 2000 considered durable in aggressive environments. FaL-G bricks require no skilled labour and can be moulded in any shape and size depending on the requirements. These bricks have better tolerances and no efflorescence as compared to conventional bricks. A number of other benefits also be ascribed for the prospect of FaL-G bricks which includes low consumption of mortars, better efficiency in laying and low cost of finishing. It is further needed to develop the awareness among users, professionals and financial supporters for using these waste materials for solving the housing problems in addition to balance economy and achieve energy conservation. 5. ACKNOWLEDGEMENT The author wish to thank Mr. L. P. Singh a post graduate student of Harcourt Butler Technological Institute, Kanpur, for providing valuable information needed for the present investigation. REFERENCES [1] Verma, C. L., Handa, S. K., Jain, S. K. and Yadav, R. K., Techno-commercial perspective study for sintered fly ash light weight aggregates in India, Construction and Building Materials 12 (1988) 341-346. [2] Singh, M. and Garg, M., Durability of cementitious binder derived from industrial wastes, Mater. Struct. 30 (December 1997) 607-612. [3] Ouyang, C., Nanni, A. and Wen F. Cheng, Internal and external sources of sulfate ions in Portland cement mortar: two types of chemical attack, Cement and Concrete Research 18 (5) (1988) 699-709. [4] Valenti, G. L., Cioffi, R., Santoro, L. and Ranchetti, S., Influence of chemical and physical properties of Italian fly ashes on reactivity towards lime, phosphogypsum and water, Cement and Concrete Research 18 (1) (1988) 91-102. [5] Bhanumathidas, N. and Kalidas, N., New trend in bricks and blocks: the role of FaL-G, The Indian Concrete Journal 66 (1992) 389-392. [6] Singh, M. and Garg, M., Phosphogypsum, fly ash, cementitious binder - its hydration and strength development, Cement and Concrete Research 25 (4) (1995) 752-758. [7] Garg, M., Singh, M. and Kumar, R., Some aspects of the durability of a phosphogypsum - lime fly ash binder, Construction and Building Materials 10 (1996) 273-279. [8] Singh, L. P., Investigation of physical properties of bricks utilizing fly ash, lime and gypsum, M. Tech Dissertation, Kanpur University, Kanpur, India, 1994. [9] Ambalavanan, R. and Roja, A., Feasibility study on utilization of waste lime and gypsum with fly ash, The Indian Concrete Journal 70 (11) (1996) 611-616. [10] Kumar, S. and Singh, L. P., Utilization of FaL-G bricks in low cost housing, in Environmental Aspects of Engineering Practices, Proceedings of a National Seminar, Jhansi, India, April 1998. [11] Indian Standard, IS:1077, Specification for common burnt clay building bricks, Bureau of Indian Standards, New Delhi, India, 1986. 64