Dirk India, a unit of Ambuja Cement brought a new concept to the fly ash users of the country. It started processing fly ash to get a better and uniform quality with much improved properties. Mayur Pathak and Dr Bibekananda Mohapatra are explaining what the product undergoes to make it superior. Dirk India (unit of Ambuja Cement) is in the business of classification of fly ash and manufacturing PPC by the method of intermixing. Fly ash is being classified by centrifugal cyclone separators to various micron size constitutes to different grades of fly ash known as Pozzocrete. This Pozzocrete is sold under various brand names of P100, P63 and P60 depending upon particle size of fly ash. P100 (Microfine Fly Ash) is basically used for high performance concrete starting from M60 and above and Pozzocrete P63 grade of concrete is used for M40 to M60 and P60 is used for normal grade of concrete. Coarse material known as P10 is now used in making bricks and blocks as well as partial replacement of sand. Presently, P100 and P63 had been extensively used by builders and RMC Industries for their large and small projects including infrastructure projects. The possibility of using P10 (less than Blaine 150 m2/Kg) had been studied by grinding the coarse fly ash to the desired Blaine. The same has also been tried in lab ball mill to achieve different fineness and its lime reactivity testing have been checked and lab scale PPC have been prepared by the method of inter-mixing to see the performance in cement and concrete. At the same time, ground fly ash and classified fly ash of identical Blaine have been compared to see their relative properties in their performance both in mortar and concrete. History Over the years, there has been a growing demand for finer, more accurately sized and quality assured particulate-materials to meet the material specifications of industry. To some extent this has been due to a better understanding of the function that the complete particle size distribution of a material and not just some aspect of it contributes to the performance of a powder when applied to a particular process. The ash—produced Pulverised fuel ash (fly ash)/by product quality—varies widely dependant on fuel source and quality variations on plant design, combustion and dust collection processes, plant engineering and plant operational practice International Standards for Cement Replacement Materials dictate that chemical, physical and particle size characteristics should be controlled. These characteristics are not a part of all the Pulverised fuel ash produced by coal fired power stations. Hence it becomes necessary to process the Pulverised fuel ash by using the technology that produces the product that confirms stringent requirements laid out by various international standards. The high performance concrete includes all those types of concrete with special requirements such workability, pump ability, high flow and higher retention in fresh state and high strength and high durability in solid state. Classified fly ash has very high potential to produce all these types of concrete effectively. At the same time, one needs to ensure right particle size as well as particle size distribution, which can only be achieved by state-of-the-art classification systems similar to one of those installed at Dirk India. By choosing right type of fly ash one can precisely produce high volume fly ash mixes (fly ash above 40 per cent) event at low grade and normal grade concrete as well as normal replacement levels for high early strength mixes. Objective As the processed Pulverised fuel ash needs to undergo a process which will not change the geometry of the tiny particles as the shape, which is spherical, plays very important role in the concrete. This ensures that the chemistry of the ash is not changed. Apart from this the ash needs to be processed in large volumes and at minimum cost and also in environmental friendly manner. Our long experience lead us to mechanical air classifier, which met all the requirements of processing the fly ash. Description of the equipment (Classifier): Mechanical air classifier works on the principle that wherever relative motion exists between a particle and a surrounding fluid, the fluid will exert a drag force on the particle. If the individual particle was falling under the influence of gravity in still air, it would accelerate until it reached a constant velocity which is known as the terminal settling velocity. This occurs when the drag force exerted by the air balances the gravitational force exerted on the particle. This velocity rises the air thus the smaller particles of lower terminal settling velocity is entrained and carried upwards. Since the difference in terminal settling velocity between particles in the sub sieve range (fine powders) becomes so small, then a simple gravitational system becomes impractical. To overcome these limitations it is necessary to increase the gravitational force in this way the air velocity necessary for the entrainment of a given particle is increased, the difference between particles magnified and high efficiency classification becomes possible in a relatively small space. In practice this is achieved by causing the carrying air and particles to follow a curved path which spiral inwards to a concentric discharge point. In this way forces of several hundred gravities may be generated and consequently high capacity and precise separation achieved in relatively small machines. The inside view (Figure 1) shows the particle separation process which takes place inside this tiny equipment. As the operating parameters are controlled by PLC, the processed ash particle size range remains precisely within the set parameters thus ensuring the consistency of the product irrespective of the varying feed material particle size range and volume. Nomenclature and properties of different grade of fly ash On the basis on above classification, materials are recovered with different Blaine fineness and residue on 45µ sieve and nomenclature of different grade of flyash mentioned in Table 1.Table 1: Nomenclature of different grade of fly ash Grade P10 P20 P30 P40 P60 P63 P73 P83 P100 Blaine Fineness (m2/kg) 100 225 260 300 320 400 450 500 625 Residue on 45µ sieve (%) 75 38 33 25 18 10 4.5 3 0 Properties of classified fly ash Chemical properties of classified fly ash: Fly ash of different grade are collected and tested for chemical (IS 4031: 1988) and Physical properties (IS 4032:1985). Chemical and physical test results are mentioned in Table 2 and 3 respectively.Table 2: Chemical properties of classified fly ash Name of the Tests IS:3812-2013 (Part-I) Requirement P10 P20 P30 P40 P60 P63 P73 P83 P100 Reactive silica (%) 20 % (Min) 20 23 24 26 29 32 36 40 46 Reactive silica increases with increase in Blaine Fineness from P10 to P100 from 20 per cent to 46 per cent Physical properties of classified fly ash: Physical testing of all the classified fly ash of different grade are carried out at Dirk Plant Laboratory at Nashik as per the IS IS 4031: 1988 and IS-1727:1967 standards. Results are reported in Table 3 with reference to specification in IS-3812 part 1 (2013).Table 3 : Physical properties of classified fly ash of different grade Name of the tests Fineness-specific surface Residues on 45 Micron Moisture content Soundness by autoclave Lime reactivity LRT Compressive strength at 28 days % of plain cement mortar Unit m2/kg % % % (N/mm2) % Min/Max Min Max Max Max - Min IS:3812-2013 (Part-I) Requirement 320 34 2.0 0.8 - 80 P10 100 75 0.5 0.1 0 64 P20 225 38 0.5 0.1 3.20 72 P30 260 33 0.5 0.2 4.00 78 P40 300 25 0.5 0.2 4.83 90 P60 320 18 0.5 0.1 6.54 94 P63 400 10 0.5 0.1 7.55 99 P73 450 4.5 0.5 0.1 7.85 102 P83 500 3 0.5 0.1 9.02 105 P100 625 0 0.5 0.1 9.24 108 Lime reactivity with Blaine fineness and residue of 45µ Reactivity of fly ash is largely determined by – Specific surface area, Fraction passing 45 microns, Glass composition, Glass content and Stress in the glass due to microcrystalline material. Uchikawa (Ref 1) has reported that small fly ash particles are rich in glass ultimately increasing the lime reactivity whereas large particles contain for too much of crystalline material due to its slower cooling rate. Fly ash with more than 50 per cent plus 45 micron fraction needs to be ground to break the agglomerates and large cenospheres. All the grades of flyash with increased Blaine and reduced residue are tested for Lime reactivity testing as per IS: 1727 - 1967 and results are mentioned in Table 3. From graph 1 and 2, it is observed that lime reactivity value has linear correlation coefficient (R2=0.94) with Blaine fineness as well as with Residue on 45 µ. Cement Reactivity Index The classified fly ash samples of different fineness were taken and the cement reactivity test is done as per IS: 1727 - 1967 testing procedures for Pozzolanic materials. The observations are compiled in Table 4. It was observed that in case of classified fly ash as fineness increases water requirement decreases and reactivity or strength factor increases. For P63 at fineness of 400 m2/kg, 28days strength matches with parent OPC and beyond fineness of 400 to 625 m2/kg it surpasses OPC strength by 2 to 8 per cent (graph 3 & 4).Table 4: Testing of pozzolanic materials as per IS: 1727 - 1967 (Cement Reactivity Index) Sr. No. Product Residue + 45 Blaine m2/kg Water in gms Water requirement (%) Compressive strength 28 days (MPa) Cement Reactivity Index (% of strength development) 1 OPC 16.32 310 250 100.0 48.00 100 2 P10 83.44 100 270 108.0 30.72 64 3 P20 42.14 225 262 104.8 34.56 72 4 P30 33.28 260 255 102.0 37.44 78 5 P40 23.16 300 248 99.2 43.20 90 6 P60 17.08 320 240 96.0 45.30 94 7 P63 9.22 400 235 94.0 47.52 99 8 P73 4.80 450 230 92.1 49.00 102 9 P83 3.78 500 228 91.2 50.40 105 10 P100 0 625 220 88.0 52.00 108 The difference in rate of development of strength depends on: Increase in reactive silica, increase in Blaine fineness, decrease in residue, increase in % of spherical shape particles (Cenosphere) Relationship between classified and ground fly ash The classified fly ash samples of equal fineness were tested for lime reactivity (LRT0) was determined and results were compiled in Table 5.Table 5: Comparative lime reactivity values of classified and ground fly ash Grade Blaine m2/kg Lime reactivity (N/mm2) (classified) Lime Reactivity (N/mm2) (ground lab ball mill) P 60 345 6.54 5.8 P 63 400 7.55 6.8 P 73 450 8.51 6.9 P 83 500 9.02 7.0 P100 625 9.24 7.2 It is well observed that at the same level of Blaine fineness, Lime reactivity of classified flyash is on higher side than the laboratory ball mill ground flyash. (Graph No 5). It was also observed to achieve higher lime reactivity it is important to maintain the optimum Blaine fineness for ground ball mill flyash as 450 m2/kg, whereas for classified the Blaine fineness should be optimized to 600 m2/kg. Graph 5: showing the lime reactivity of classified and ground lab ball mill Graph 5: showing the lime reactivity of classified and ground lab ball mill Scanning electro microscopy: The particle shape of classified fly ash is almost spherical as per SEM pictures shown in figure 4. SEM study of P63 shows clear evident that majority of particles were spherical shape with few exceptions. On the contrary, Ground fly ash particle (Figure no 4) has distorted shapes and converted in angular irregular shape particle. Due to this angular particle shape of ground flyash the water requirement is higher than that of classified fly ash (Ref 3). Figure 4: SEM picture of P63 classified fly ash (A) & SEM picture of ground fly ash Fineness 345 m2/kg. (B) Fly ash in concrete: Fly ash can be used as a partial cement replacement (on average 30per cent) in structural concrete to improve the durability and other aspects of concrete. This fact is recognized by well-established researches on the basis of data from construction industry both cement and fly ash industry. Fly Ash can be introduced into the concrete either by adding Portland Pozzolana Cement inter ground with 25 to 30 per cent fly ash or by adding on average 35 per cent classified fly ash directly in concrete mix. (Table No 7).Table 8: Effect on Properties of concrete due to inter ground cement product Property PPC intermixing with classified fly ash PPC inter ground with fly ash Water Demand Reduced due to lower Blaine and low water requirement of classified flyash Increased N.C due to higher Blaine of PPC and higher fineness of OPC component Pozzolanic Action Superior (due to low w/c) Moderate (Higher w/c) Workability Enhanced cohesive and better rheology Slight improvement Permeability Higher durability reducing permeability in concrete Permeability lower than OPC but higher than Interground PPC due to higher w/c   Comparative trials of classified fly ash and ground fly ash on concrete properties The following trials for concrete mix design were conducted with the OPC mix 350, 400, 450 kg cement content and 30 per cent replacement of OPC by both ground and classified flyash having same 345 m2/kg Blaine.Table 9: Comparative Concrete trials of classified and ground fly ashThe following observations were made on the basis of results of above exercise. a) The water demand increased in case of P60G (ground fly ash) mixes as compared to classified fly ash P60 by almost 10 to 12 per cent b) P60 mixes requires lower water demand which exhibited better workability c) Compressive strength of concrete mix using classified fly ash P60 is higher by 18 to 36 per cent at 28 days and nearly 23 per cent higher at seven days as compared to that with use of ground fly ash P60 G d) Classified fly ash P60 exhibited much better durability parameters i.e. water permeability (Din 1048) and RCPT values as compared to ground fly ash Applications for coarse fly ash P10 After classification process, we get 30 to 50 per cent coarse fly ash known as P10 which can be used in following different applications: 1) Coarse fly ash for making of fly ash bricks, concrete blocks (solid as well as hollow) and other precast elements as a partial filler 2) It can also be used for partial sand replacement or partial filler in concrete of lower grades which needs to be pumped 3) Can also be used for low grade self-compacting concrete in which powder content (Fines below 150 micron) is as high as 450 to 500 kg/m3 P10 as partial sand replacement: In case of coarse sand or wash crush sand since the fine content is very low (600 micron passing less than 15 per cent) it is required to add some powder usually ends up in addition of extra quantity of cement or any other powder which is uneconomical. In such cases P10 (coarse fly ash –100 per cent passing through 300 micron) can prove to be affordable alternative. It replenishes the requirement of fines in concrete and also has some pozzolanic activity and better particle packing which results in enhanced various properties of concrete. Concrete trials for M25 grade was conducted. Comparison was done between concrete control mix with sand replaced mix. Mix design and its results are tabulated in Table 10.Table 10: Comparative Concrete trials with P10 as partial sand replacement for concrete M25 Material Control mix Sand replacement mix OPC 53 UT (Kg) 263 263 Fly ash (Kg) 87 87 Water (Kg) 165 165 20 mm (Kg) 697 818 10 mm (Kg) 231 273 N.S (Kg) 741 786 C.S (Kg) 231 0 P10 (Kg) 0 139 Admixture (Kg) 3.5 3.5 Slump at Initial (in mm) 120 180 Slump at 30 min (in mm) 100 150 Comp Strength 7 Days MPa 23.5 30.5 Comp Strength 28 Days MPa 32.5 43.5 It is observed from Table 10 that P10 can be very well used as partial sand replacement to enhance the fresh as well as hardened properties of concrete. It is observed that workability and strength both are improved. Conclusions The following observations are recorded on the basis of above exercise: The classification process of fly ash is most effective in maintaining rheology of fly ash whilst increasing reactivity and overall performance in concrete and cement The particle separation can be done precisely and economically as per requirements in air classification process The water requirement of classified fly ash is much lower than ground fly ash which is a key factor which enhances performance of classified fly ash in Cement as well as concrete. At 400 Blaine classified fly ash performance is at par with given OPC when tested in concrete as well is mortar as per IS: 1727-1967 strength factor or cement reactivity test In case of inter mixing PPC produced with 400 SSA fly ash P63 has lower NC than that of OPC whilst PPC produced by inter grinding has higher consistency than OPCv In case of classification of fly ash different grades of fly ash can be derived as per requirement of various applications in concrete. The fly ash can be derived for extreme applications from (Coarse fly ash P10) filler applications to (Ultrafine fly ash P100) for high performance concrete along with fly ash P60 / P63 for regular grade of concrete and cement applications Classified fly ash has potential to provide effective solution for concrete and cement industry terms of following parameters, a) Workability and slump retention b) Cohesive mix hence pumping for Hi-rise and long distance c) High performance concrete including self compacting concrete d) High early strength concrete with P100 (Ultrafine classified fly ash) e) Concrete with improved durability parameters Thus classification technology can provide most sustainable solution to cement and concrete industry. Footnote: ABOUT THE AUTHORS: Mayur Pathak, Dirk India (unit of Ambuja Cement) and Dr Bibekananda Mohapatra, Director General National Council for Cement and Building Material.