EVALUATION OF SINTERED FLY ASH AGGREGATE PRODUCED IN INDIA

Transcription

1 EVALUATION OF SINTERED FLY ASH AGGREGATE PRODUCED IN INDIA Ashok Kumar Behera 1, P N Ojha 2, V P Chatterjee 2 & Mantu Gupta 2 1 Indian Metals Ferro Alloys Limited, India 2 National Council for Cement and Building Materials, India ABSTRACT: The paper presents the study conducted on sintered fly ash coarse aggregate produced indigenously by M/s IMFA (Indian Metals Ferro Alloys Limited, India). The sintered fly ash coarse aggregate (Low Density aggregate, LDA /Lightweight Aggregate, LWA) is lower in density in comparison to normal weight aggregate. The two fractions of sintered fly ash coarse aggregates (8-16mm & 4-8mm) are evaluated. Experimental programme evaluated the Microstructural, Physical & Chemical properties of sintered fly ash coarse aggregates for its suitability as coarse aggregate for concrete Masonry units, Hollow & Solid Lightweight Concrete Blocks and Structural Lightweight Concrete (SLC). LDA is found suitable for making concrete for Masonry units, Hollow & Solid Lightweight Concrete Blocks and Structural Lightweight Concrete (SLC) for non-wearing surfaces. KEYWORDS: Low Density Aggregate (LDA), Lightweight Aggregate Concrete (LWAC), Structural Lightweight Aggregate (SLA), Structural Lightweight Aggregate Concrete (SLC), Bulk Density, Micro Pores and Micro Cracks. 1.0 INTRODUCTION The use of lightweight aggregate concrete (LWAC) can be traced to as early as 3000BC, when the famous towns of Mohenjo-Daro and Harappa were built during the Indus Valley Civilization. In Europe, earlier use of LWAC occurred about 2000 years ago when Romans built the Roman temple (Pantheon), the Prestigious Aqueduct and the Colosseum in Rome. Earlier lightweight aggregates (LWAs) were of natural origin, mostly volcanic origin like pumice and scoria by mechanical treatment [1]. With increasing demand, non-availability of natural LWAs and scarcity of natural aggregates worldwide, techniques have been developed to produce them in factories. These are produced from the natural raw materials like expanded clay, shale, slate, etc. as well as from industrial byproducts such as fly ash, clay, blast furnace slag etc. The properties of the aggregates depend upon the raw materials and the process used for producing them. Large quantity of fly ash is still disposed of in landfills and storage lagoons leading to environmental damage by causing air and water pollution on a large scale. As large quantities of the fly ash remain unutilized in most countries of the world, the manufacture of good quality lightweight fly ash aggregates seems to be an appropriate step to utilize a large quantity of fly ash and also yield significant environmental benefits. ASTM C331 [2] refers to Standard Specification for Lightweight Aggregates for Concrete Masonry Units. ACI 213R [3] and ASTM C330 [4] are widely used two standards for the use of lightweight aggregates (LWA) in structural lightweight concrete (SLC). In existing literatures following criteria for physical and chemical properties of LWA for production of structural lightweight concrete is discussed:

2 Physical and Chemical properties of LWA for production of Structural Lightweight Aggregate Concrete i) Grading As per ASTM C330, the grading requirements for structural lightweight aggregate of normal size designation 19.00mm-4.75mm and 9.5mm-2.36mm is given in Table below. Sieve Size Percentage Passing (mm) For Coarse Aggregate (19.00mm-4.75mm) For Coarse Aggregate (9.5mm-2.36mm) 19 mm mm mm mm mm ii) Relative Density/Bulk Density (Loose) Due to their cellular structure, the relative density & bulk density of lightweight aggregate are lower than that of normal weight aggregate. As per ACI 213R-03, the practical range of relative densities of lightweight aggregate varies from almost 1/3 to 2/3 that of normal weight aggregate. As per ASTM C330 and ASTM C331, the maximum dry loose bulk density of the lightweight coarse aggregate is 880 Kg/m 3. iii) Moisture content & Absorption: Lightweight aggregates due to their cellular structure are capable of absorbing more water than normal weight aggregates. LWA generally absorb from 5 to 25% by mass of dry aggregate, depending on the aggregate pore system. A. M. Neville [5] indicates that water absorption of good quality lightweight aggregate for use in Structural concrete usually not more than 15%. Sintered fly ash aggregates, Lightweight aggregates have shown water absorption in the range of 13-14% according to Swamy & Lambert [6] and Teychene [7]. The rate of absorption in lightweight aggregates is a factor that also has a bearing on mix proportioning, handling, and control of concrete, and depends on the aggregate pore characteristics. The water, which is internally absorbed in the lightweight aggregate, is not immediately available to the cement and should not be counted as mixing water. iv) Loss on Ignition: The maximum loss on ignition of lightweight aggregate is 5% as per ASTM C330 and ASTM C331. In India, IS: 9142 [8] refers to Specification for Artificial Lightweight Aggregates for Concrete Masonry Units and IS: 2185 (Part-II) [9] refers to production of Hollow & Solid Lightweight concrete blocks. These codes are not specific to the use of sintered fly ash aggregates. Further, there is no standard available for use of sintered fly ash lightweight coarse aggregate as concrete marking material for production of structural lightweight aggregate concrete (SLC). Presently IS: 9142 is under revision. The new code will be in two parts. The 1 st part deal with Specification for Artificial Lightweight Aggregates for Concrete Masonry Units and 2 nd part deals with specification for Sintered fly ash Lightweight Aggregate for Structural Concrete. NCB has carried out evaluation of LWA/LDA obtained from sintering of fly ash produced indigenously by IMFA (Indian Metals Ferro Alloys Limited, India). The property of this LWA has been investigated in light of its use in Plain Cement Concrete and Structural Lightweight Concrete.

3 2.0 PRODUCTION PROCESS OF SINTERED FLY ASH AGGREGATE The production process of Sintered Fly Ash Aggregate or LDA is continuous. It can be divided into four stages of operation-raw Material Handling, Pelletization, Sintering & Finished product segregation and storage. Raw material handling: Fly Ash and bottom ash, grinded coal with binder is fed to continuous mixers. Fluidizing and blending in silos is required for homogeneous mixing and free flow. Water is added to dry mix in the mixer to get the wet mix. Pelletization: Schematic flow diagram for production of sintered fly ash aggregate The wet mix is fed to pelletizers. Pelletizers make green pellets of various sizes. The pellets are formed by agglomeration. [10] Agglomeration is the term used for entire field of consolidation of solid particles into larger shapes. The size of pellets is regulated with tilt and speed of the pelletizers. The green pellets are carried over conveyors for feeding to sinter strand. Sintering: The sinter strand has three sections, a) The firing Section, b) The sintering or soaking Section, c) The cooling section. Green pellets are spread over the moving sintering strand. The pellets travel to firing zone with temperature range of 1100 to 1300 degree centigrade. The sintering happens in the intermediary stage of sinter strand and on the later part they cool naturally. The burning of the carbon in the pellets and loss of moisture creates a cellular structure bonded together by the fusion of fine ash particles. Finish Product handling: The cooled down pellets are passed through breaker to detach fused aggregates. The aggregates are then collected in different storage silos as per size, ready for onward dispatch to finish product yard. The final product sizes can be from 2 mm to 16 mm in size.

4 3.0 EXPERIMENTAL INVESTIGATION, TESTING, TEST RESULTS & DISCUSSION The representative samples of LWA (two fractions 8-16mm & 4-8mm) were collected and they were used for relevant Micro-structural analysis, Physical testing and chemical testing. The testing was conducted as per relevant Indian Standards. 3.1 Micro-Structural Investigation & Discussion on Properties of LDA: Petrographic analysis using optical microscope Petrographic analysis of both the fractions of LDA (8-16mm & 4-8mm) are conducted under polarising type Optical Microscope. The minerals present in the LDA sample are quartz, orthoclase-feldspar, glass, semi glass and opaque minerals. Subhedral to anhedral quartz grains with sharp angular grain margins are uniformly distributed in the sample. Grain size of quartz varies from 2µm to 206 µm with an average of 102 µm. Majority of glass grains are in the size range of 70µm to 100 µm. Perforated and layered quartz grains are also observed. Subhedral orthoclase grains with rounded grain margins are partially shattered. The glass grains are of various shapes and sizes like needle, rod, elongated, lath, triangular and rectangular. Glass grains are also uniformly distributed in the sample. Grain size of glass varies from 2µm to 161 µm with an average of 93 µm. Majority of glass grains are in the size range of 60µm to 90 µm. Few glass grains are in segregated forms. Semi glass grains with sharp grain margins are randomly distributed in the sample. Subhedral opaque minerals with corroded grain margins are also uniformly distributed in the sample. Grain size variation in opaque minerals is too large. The major micro-structural characteristics are as under- a) Shape & Colour: The pelletizing action provides a rounded, spherical, semi-spherical shape to the finished aggregate. The external grey colour and the internal black core are related to the carbon content and oxidation state of the iron present. b) Texture: The external appearance and surface texture seems smooth, but on the micro-scale it is relatively rough with numerous micro-pores. These open pores structure allow moisture to enter and exit the aggregate. When mixed in concrete some of these pores are filled with cementitious products. In the mature specimen it is difficult to define the aggregate to cement matrix interfacial zone. [11] c) Internal voids: The burning of the carbon in the pellets and loss of moisture creates a cellular structure bonded together by the fusion of fine ash particles. The internal structure is a honeycomb one of generally interconnected voids of varying size and shape. These air voids act as insulant, enhancing the thermal properties of resulting concrete d) The morphological, micro-structural & mineralogical characters of the nodules give a clear picture of its pozzolonic characters which may enhance the bond strength between cementpaste and aggregate in concrete. The other microscopic features such as air voids & discontinuous micro-cracks may get sealed at ambient temperature when it comes in contacts with water. Figure1: External Appearance of LDA

5 3.1.2 Microscopic Analysis using Stereoscopic Microscope The studies are conducted in stereoscopic microscope for the in-situ nodules for the sintered fly ash aggregate for both fractions (8-16mm & 4-8mm). 1. For fraction (4-8) mm: These LDA nodules are of various shape and sizes with different morphological characters. Two types of nodules commonly present in this aggregate are as follows: a. Aggregate has rounded to semi rounded nodules with corroded surface having numerous micro-cracks and voids. Outer surface of the nodule is light cream to grey in color. Assemblage of grains of various shapes and sizes are clearly visible on the surface of the nodules. Majority of these grains are undigested material used for nodulization. Several micro-cracks of varying thickness which are normally discontinuous in nature are developed on the surface of the nodules of this variety. Voids of various shapes and sizes are clearly marked on the surface of these types of nodules. (See figure 2). b. Aggregate has rounded to semi rounded nodules with very smooth surface are tightly packed with sintered grains of the material used. Even the walls of the pores formed during the process are very smooth in nature. Few discontinuous micro cracks are present. Dark brown color glass grains of various shapes and sizes are developed on the surface and inner portion of the nodule. These glass grains of the various shape and sizes are well sintered in the nodules. Outer margins of the glass clusters are tightly cemented which developed cohesive margins between the glass clusters and the matrix. Undigested grains of various sizes are also formally cemented in the matrix of the nodule. In these types of nodules only surfacial voids of lower depth are observed. (See figure 3). Figure 2: LDA size (4-8) mm (Plate 1): A close-up of Sintered Fly ash aggregate surface. Nodule with corroded surface and open voids. Micro cracks developed on the surface. Grains are distributed in agglomerated form. (1.5x) Figure 3: LDA size (4-8) mm (Plate 2): A close-up of Sintered Fly ash aggregate surface. Nodule with smooth surface and few micro cracks. Cluster of sintered material is present. Grains of various shapes and sizes are distributed in the agglomerated form. (1.5x) 2. For fraction (8-16) mm: These LDA nodules are of various shape & sizes with different morphological characters. Two types of nodules commonly present in this aggregate are as follows:

6 a. Numerous continuous micro cracks of various thicknesses are present on the surface of nodule. Outer surface are well cemented on the sintered matrix. Very fine voids of shallow depth are present on the surface of the nodules. (See figure 4) b. In this nodule rounded to semi-rounded clusters of melted molten glassy phase are cemented in the matrix. Numerous pores of shallow depth are uniformly developed on the surface of the module. Few discontinuous cracks of shallow depth are observed on the surface of the nodule. Surface of these nodules are very smooth in nature. (See figure 5). Figure 4: LDA size (8-16) mm (Plate 1): A close-up of Sintered Fly ash aggregate surface. Nodule with smooth surface and numerous micro cracks. Cluster of sintered material is present. Grains of different sizes are distributed in agglomerated form (2.5x) Figure 5: LDA size (8-16) mm (Plate 2): A close-up of Sintered Fly ash aggregate surface. Nodule with smooth surface and few micro cracks. Cluster of sintered materials with numerous micro pores are present. (2.5x) 3.2 Testing, Test Results and Discussion on Physical & Chemical Properties The representative samples of LWA (two fractions 8-16mm & 4-8mm) were collected and they were used for relevant physical testing & chemical testing. The physical and chemical test results as obtained are presented in Table 1, Table 2 and Table 3. Sieve Size (mm) Table 1: Results of Sieve Analysis Percentage Passing Fraction-I (8-16 mm) Fraction-II (4-8mm) Table 2: Test Results of Physical & Chemical Properties Sl. No. Test Carried out For fraction 4-8mm For fraction 8-16mm 1 Specific gravity Water absorption, % Abrasion Value, % Crushing Value, % Impact Value, % - 32

7 Table 2 (Contd..) Sl. No. Test Carried out For fraction 4-8mm For fraction 8-16mm 6 Loose Bulk Density, kg/l Deleterious Material (Except Coal & Lignite ) Nil Nil 8 Loss on Ignition (LOI) Silica (SiO 2) Iron oxide (Fe 2O 3) Aluminium oxide (Al 2O 3) Calcium oxide (CaO) Magnesium oxide (MgO) Sulphate (SO 3) Alkalies: Na 2O K 2O Chlorides Table 3: Accelerated Mortar Bar Test (As Per ASTM C 1260) S.No. Sample Type 1N NaOH 80 0 C Remarks 14 Day Expansion % 1 Low density aggregate LDA-I Innocuous 2 Low density aggregate LDA-II Innocuous One of the important aspects of LDA is water absorption. The water absorption of both the fractions of LDA at different time interval i.e. 5:00 Minute, 10:00 Minute, 15 Minute, 30 Minute, 45 Minute, 60 Minute, 1:15 Hours, 1:30 Hours, 1:45 Hours, 2:00 Hours, 3:00 Hours, 4:00 Hours, 5:00 Hours, 6:00 Hours & 6:30 Hours was determined to analyse rate of water absorption. The test results as obtained are presented in Fig 6 & 7. Fig 6: Water absorption at different interval for fraction 4-8mm LDA Fig 7: Water absorption at different interval for fraction 8-16mm LDA On analysis of test results and its comparison with relevant International/National standards following are the point wise discussion:

8 The abrasion value of the LDA fraction (16mm-8mm) is 32% which is not satisfactory as per IS: 383 [12] [13] for the coarse aggregate to be used in concrete for wearing surfaces. However, the abrasion value is satisfactory for the coarse aggregate to be used in other concrete. The crushing value of the LDA fraction (16mm-8mm) is 44% which is not satisfactory as per IS: 383 for the coarse aggregate to be used in concrete for wearing surfaces. However, the crushing value is Satisfactory for the coarse aggregate to be used in other concrete. The impact value of the LDA fraction (16mm-8mm) is 32% which is not satisfactory as per IS: 383 for the coarse aggregate to be used in concrete for wearing surfaces. However, the crushing value is Satisfactory for the coarse aggregate to be used in other concrete. The test results of deleterious material for both fractions of LDA are Satisfactory as per IS: 383. The sieve analysis result of LDA fraction-i ( 16 mm 8mm) and fraction II ( 8 mm - 4 mm) conforms to the grading requirements for Lightweight aggregate for structural concrete for coarse aggregate nominal size designation 19.00mm-4.75mm and 9.5mm-2.36mm as per Table -1 of ASTM C 330. Dry Loose bulk density of LDA fraction-i ( 16 mm 8mm) and fraction II ( 8 mm - 4 mm) are 845 Kg/m 3 and 810 Kg/m 3 respectively and conforms to the ASTM C330, ASTM C331 and IS: 2185 (Part-II), as per which the maximum dry loose bulk density of the lightweight coarse aggregate is 880 Kg/m 3. Water Absorption of LDA fraction-i (16 mm 8mm) and fraction II (8 mm to 4 mm) are % and % respectively which are in agreement with the existing literature for light weight aggregates used for structural concrete. As per existing literature lightweight aggregates generally absorb from 5 to 25% by mass of dry aggregate. A. M. Neville indicates that water absorption of good quality lightweight aggregate for use in Structural concrete usually not more than 15%. The initial rate of water absorption of both the fraction of LDA is quite high. In first hour, LDA fraction-i (16mm 8mm) and fraction II (8 mm - 4 mm) water absorption are 9.09% and 9.01% respectively i.e. the LDA absorbs almost 70% of total water absorption in first hour (See figure 6 & 7). Loss on Ignition (LOI) of LDA fraction (16mm-8mm) is 3.55% which conforms to the ASTM C330 & ASTM C331 requirement of maximum loss on ignition of 5%. The LOI value also conforms to the IS: 9142 requirement of maximum loss on ignition of 4%. Accelerated mortar bar test as per ASTM C-1260 [14] is carried out on the LDA. The expansion at 14 days on LDA is 0.02% which is less than 0.1%. Therefore, the LDA sample can be classified as innocuous as per ASTM C CONCLUSIONS Based on the test results & discussions of Micro-structural, Physical and Chemical properties of sintered fly ash lightweight coarse aggregate following major conclusions can be drawn: i. Both the fractions of lightweight aggregate conforms the criteria of IS: 9142 & ASTM C331 and therefore it can be used for production of concrete masonry units. ii. iii. iv. Both the fractions of lightweight aggregate also conforms the IS: 2185 (Part-II) and therefore it can be used for production of Hollow & Solid Lightweight concrete blocks. Results of Abrasion, Crushing & Impact values indicate that lightweight aggregate shall not be used for concrete to be used in wearing surfaces. However, the corresponding Abrasion, Crushing & Impact values is Satisfactory for the coarse aggregate to be used in other concrete. Both the fractions of lightweight aggregate conforms the criteria of ASTM C330 and therefore it can be used for production of structural lightweight concrete.

9 v. Based on Micro-structural property and external texture of aggregate, distinct refinement of interfacial transition zone and stronger bond between aggregate-cement pastes in concrete is expected. 5.0 FUTURE SCOPE OF WORK The study of properties of sintered fly ash aggregate reveals about its suitability as a coarse aggregate in production of concrete Masonry units, Hollow & Solid Lightweight concrete blocks as well as Structural lightweight concrete. However, performance of concrete in structure using LWA needs to be further evaluated in terms of fresh concrete properties, mechanical properties and durability s properties of hardened concrete. Design parameters needs to be established based on experimental study. 6.0 ACKNOWLEDGMENTS This paper pertains to the R & D work carried out by Construction, Development and Research centre at National Council for Cement and Building Materials. The authors acknowledge the sponsor M/s IMFA (Indian Metals Ferro Alloys) for carrying out this research work at National Council for Cement and Building Materials, Ballabgarh. References 1. Chandra Satish, Berntsson Leif, for Lightweight Aggregate concrete (Science, Technology and Applications) copyright 2003 by Noyes Publications. 2. ASTM C331-05: Standard Specification for Lightweight Aggregates for Concrete Masonry Units. 3. ACI 213R-03: Guide for Structural Lightweight-Aggregate concrete. 4. ASTM C330-00: Standard Specification for Lightweight-Aggregates for Structural concrete. 5. A.M. Neville, Properties of Concrete.(Fourth and Final Edition, Chapter Lightweight Aggregate p-697) 6. Swamy, R.N. and Lambert, G.H., the Microstructure of Lytag Aggregates, Int.j. camp. Lightweight aggregate, 3(4): (1981). 7. Teychene, D.C., Lightweight Aggregates-their properties and uses in Concrete in the U.K., Proc.1 st Int. Congress on Lightweight Concrete, 1:23-37(May 1968) 8. IS: Specification for Artificial Lightweight Aggregate for concrete Masonry units. 9. IS: 2185 (Part-II)-1989: Specification for concrete Masonry units Part II: Hollow and Solid Lightweight concrete Blocks. 10. Bijen, J.M.J.M., Manufacturing process of Artificial Lightweight Aggregates from Fly ash, The Int. J. Cement and Lightweight Concrete..,8(3) Zhang, M.H. and Gjorv, O.E., Microstructure of Interfacial Transition Zone between lightweight Aggregate and Cement, Cem. Conc. Res. 20: (1990) 12. IS: Specification for Coarse and Fine aggregate from natural sources of aggregates. 13. IS: (Different parts) Methods of test for aggregates for concrete. 14. ASTM C : Standard Test Method for Potential Alkali Reactivity of Aggregates (Mortar-Bar Method)