Advantages of using Fly Ash in Concrete PPC or site mixing of OPC and fly ash site

Transcription

1 Advantages of using Fly Ash in Concrete PPC or site mixing of OPC and fly ash site By Anil Banchhor, S. Krishnan The Associated Cement Companies Limited, Cement House, 121, M. K. Road, Mumbai, 1 The paper presents the results of an experimental investigation that was carried out to evaluate the effectiveness of key performance characteristics of concrete incorporating fly ash by means of, inter-ground PPC and site mixing OPC and fly ash. A host of properties of concrete were studied. Both drum mixer and pan mixer were used in the stud to evaluate the mixing efficiency. The results of investigations indicate that the performance of concrete using Fly Ash is better than the OPC concrete, especially with respect to durability indicator due to pozzolanic action of fly leading to pore refinement and denser concrete matrix The beneficial effects of fly ash are seen more pronounced in case of factory-ground Portland Pozzolona Cement (PPC) than site mixing of fly ash with OPC under normal Indian construction site conditions. The inter-grinding of fly ash with clinker and gypsum maximizes the pozzolanic potential of fly ash with more consistent product with good control on variability, leading to a better performance of PPC concrete. 1. INTRODUCTION Concrete has become an indispensable construction material and it is now used in greater quantities than any other material. In the present context durability and sustainable development are key issues for development. Ordinary Portland cement has a high calcium base affecting the microclimate of concrete and mortar. The interface bond between the cement paste and aggregates can be improved with better pore structure and minimised micro cracks using mineral admixtures like fly ash granulated blast furnace slag rice husk, silica fume etc. Out of the above, the use of fly ash has gained prominence due to growing awareness about the benefits and easy availability of the good quality fly ash, RMC plants and bigger construction sites are using fly ash with OPC in batching plants An experimental study was conducted to measure the effectiveness of key performance characteristics of concrete made with OPC, inter-ground PPC and site mixed OPC & Fly Ash. The comparative performance has been studied for the fresh and hardened properties of concrete like slump, slump retention, water demand,

2 compressive strength,, water permeability, drying shrinkage etc. for concrete of grade M and M-25. The microstructure of the concrete was also studied. In this study, OPC and the fly ash used were from the same source, as that of underground PPC. The effect of efficiency of mixing on the properties of concrete was understood by carrying out mixing in a drum mixer and in a pan mixer The study will continue for one-year duration. It has been found that the properties of fresh and hardened concrete of interground PPC is better than the site blend of OPC and fly ash. One of the major factors contributing to this appears to be the homogeneity of the blend of fines particles namely cement and fly ash. The normal drum mixer will not be able to produce a highly homogenous mix of fine powders whereas, inter-grinding of fly ash with clinker and gypsum produces a highly homogenous mix with a high level of inter-particle contact between cement and fly ash grains. This process also maximises the pozzolanic potential of the fly ash with more consistent product with good control on variability, which results in to better performance characteristics of concrete. The intergrinding makes fly ash particles finer, which imparts additional reactive surfaces for hydration and improves the overall particle size distribution of the resultant cement In this respect, The ordinary drum mixer is found to be less efficient as compared to the Pan mixer with respect to homogeneity. 2. EXPERIMENTAL PROGRAMME In this study, normal grades of concrete viz. M & M25, which are being used, at most of the construction sites, have been taken, and the findings can be used effectively at construction sites. Another important parameter is the use of Pan mixer and drum Mixer, so that the construction practice of using batching plant at big projects/ RMC and rotating drum mixer of smaller sites can be co-related with the performance of the concrete based on mixing efficiency of mixers. 2.1 Materials for Study All the cementitious materials from the same source were used to compare the performance characteristics of concrete made by separate blending and interground PPC. OPC and PPC from ACC s Wadi Cement Plant were used for the study purpose. Fly ash from Raichur Thermal Power Plant confirming to IS was used for as a separate mineral additive; because fly ash from the same source and

3 the same quality is being used at Wadi to manufacture inter-ground PPC. The fly ash having surface area of 289 sq. m. was sieved through 9micron sieve and coarse particles were removed to bring the fly ash particle to required specific surface area of 329 sq. m. as per IS In making concrete for the study, crushed granite from New Mumbai area was used as coarse aggregates along with fine aggregates from river Vaitarna Cement & Fly ash Source ACC - Wadi Cement Works. Test results given in Table 1. Fly Ash from Raichur Thermal Plant was tested And results are given in Table Aggregates: Coarse aggregate of Crushed Granite from New Mumbai Maximum size Metal 1 1 mm ; Metal 2 mm Fine aggregate Type: River sand Zone: II Retention on 6 micron: 48.% Source: Vaitrana Riverbed

4 Table 1 : Properties of cement Table 2: Properties of fly ash as per IS Property PPC OPC- 53 Std. Consistency (%) Specific Surface (m2/kg) Setting Time (minutes) Initial Final Soundness test Le Chatelier (mm) Autoclave (%) Comp. Strength (M Pa) 3 Days 7 Days 28 Days.5 Nil Nil Physical Analysis of Fly Ash Specific Gravity 2.14 Residue on 45 micron 23 percent Specific Surface in m 2 / kg 289 S. Surface after sieving 329 with 9 micron sieve & before use Lime Reactivity (M Pa) 4.53 Cement Reactivity ( %) 85% Chemical Analysis of Fly ash Constituent Percentage SiO Al 2 O Fe 2 O CaO 2.7 MgO 1.4 IR 86.3 LOI.6 Na 2 O.16 K 2 O 1.54 Chloride Testing of Fresh concrete Fresh concrete was tested for Slump, slump retention at minutes, bleeding, and yield. Slump was maintained around and not less than 1mm to represent the typical conditions prevalent at most small/medium construction sites in the country. 2.3 Testing of Hardened concrete Hardened concrete testing includes compressive strength at ages 1D, 3D, 7D, 14D, 28D, 56D, 9D, 18D and 36D.. Rapid Chloride penetration Test (RCPT) as per ASTM C-12-97, Initial Surface Absorption Test as per (ISAT) as per BS , Water Permeability as per DIN , UPV and hammer test, ph of concrete, and Modulus of elasticity. were carried out for 28D, 2.4 Testing of mortar

5 The mortar separated from concrete by sieving the same through a 4.75 mm sieve as taken for evaluation. These includes compressive strength and water absorption of mortar cubes at 1,3, 7 and 28D, 56D, 9D, 18D and 36D, 2.5 Mix Proportions Two series of mixes, one for M- and the other for M-25 grade of concrete were used. In each grade, concrete with only OPC, only factory ground PPC and site mixed OPC-fly ash blend were made. Total Cementitious material; content per cubic metre was kept constant for all mixes in each grade. Corrections were made only for specific gravity differences. Each of the mixes was made in a typical tilting drum one bag drum mixer and a lab mixer of 8-litre capacity. The study is on the performance of factory ground PPC (containing 25% fly ash) and site blended OPC and fly ash in concrete. OPC was included in the study as a reference. Table -3 Table 3: Mix Proportions Grade M - Grade M 25 Type of concrete OPC PPC OPC + Fly ash OPC PPC OPC + Fly ash Material kg/m kg/m 3 kg/m 3 kg/m 3 kg/m 3 kg/m 3 3 Cement Fly ash -- (25% (25% 89 ) ) Sand Coarse aggregate mm Coarse aggregate mm Water Water to binder ratio 3. OBSERVATIONS: 3.1 Fresh Concrete Properties: 3.11 Slump: The slump of concrete was measured immediately after discharge from the mixer and also at minutes. All the mixes were cohesive and the initial slump of PPC concrete was found to be higher as compared to OPC+FA and OPC

6 concrete. The slump loss after minutes also found to be lower in case of PPC concrete. Figures 1 to 4 Slump mm Figure 1. Slump and Retention M - Drum Mixer INITIAL 5 4 Slump mm Figure 2: Slump and Retension M 25 -Drum Mixer INITIAL 4 MIN MIN OPC+FA PPC OPC OPC+FA PPC OPC Type of Concrete Type of Concrete Figure 3. Slump and Reent ion M- Pan Mixer INIT 4 Slump mm Figure 4: Slump and Retention M25- Pan Mixer INITIAL 4 MIN OPC+FA PPC OPC MIN OPC+FA PPC OPC Type of Concret Type of Concrete 3.12 Bleeding: In concrete, after compaction in place bleed water comes on the top, indicating formation of channels, which would affect durability. The bleeding of PPC concrete was found to be lower than the OPC and the mix of OPC and FA, in drum mixed concrete. Bleeding of OPC-Fly ash concrete was lower for pan mixer as compared to the drum mixer. However, with both the mixer types, PPC had the lowest bleeding. ( see table 4) Allowing the fresh concrete to stand in a measuring cylinder and removing the bleed water by a pipette after 1hr bleeding was determined Table 4: Bleeding in ml after 1 hour. M Drum Mixer M Pan Mixer M 25 Drum Mixer M 25 Pan Mixer OPC+F PP OP OPC+F PP OPC OPC+ PP OP OPC+F PP OPC A C C A C FA C C A C

7 3.2 Hardened Concrete Properties: 3.21 Compressive strength: Compressive strength of concrete of both the grades is given in Figures 5-8. Strength development of OPC + Fly ash concrete lags at all ages, compared to factory made inter-ground PPC. However, these differences are much less in pan mixed concrete. Compressive strength mpa Figure 5: Comcrete Compressive Strength M Drum Mixer OPC +FA PPC OPC Compressive Strength mpa Figure 6: Comcrete Compressive Strength M 25 Drum Mixer OPC +FA PPC OPC Type of Concrete Type of Concrete Figure 7: Concrete Compressive Strength M Pan Mixer Compressive Strength mpa OPC +FA PPC OPC Type of Concrete Figure 8: Comcrete Compressive Strength M 25 Pan M ixer OPC +FA PPC OPC Type of Concret e Modulus of elasticity: Modulus of elasticity values of the drum mixed concrete are given in Table. 5 The values were determined on a 15mm dia x m length cylinder in wet condition. Higher E values of PPC was observed because of pore filling effect of fly ash.

8 3.23 Water permeability: Water permeability tests were carried out as per DIN 148 specification on 15 mm cube specimens. The specimens were air dried in laboratory conditions, before being subjected to the tests, as this will be closer to field conditions of normal concrete. The values of permeability are given in Table 6. Table 5: Modulus of elasticity and Water Permeability- Drum Mixer 28 day Type of concrete Modulus of elasticity in Gpa at 28 days M- OPC + FA M- PPC M- OPC M25 OPC + FA M 25 PPC M25 OPC Water Permeability in mm at 28 days 3.24 Initial Surface Absorption (ISAT): This test was carried out as per BS 1881 on 15 mm cube specimens. The specimens were kept for open-air sun drying for 48 hrs, before being subjected to the tests, as this will be closer to field conditions of normal concrete. The test was carried out on a side surface as the top surface in a laboratory specimen tends to be rich in paste up to a few mm of depth, due to trowelling action, while finishing the specimen in the mould. The values of permeability are given in Table 7 Table 7: Initial surface absorption - Drum Mixer 28 day Initial surface absorption in ml/ m 2 / sec Type of Concrete Time of measurement from start in minutes M- OPC + FA M- PPC M- OPC M25 OPC + FA M 25 PPC M25 OPC Optical Microscopy: Optical microscopic examination of polished sections of concrete brings out the following.

9 In all the samples hard and partly hydrated, un hydrated cement grains are seen In the case of OPC + Fly ash site mixed concrete, fly ash grains are not well distributed, whereas in the case of PPC concrete, Fly ash grains are well distributed in the mass of concrete. PPC based concrete is showing better distribution of fly ash particles and compactness of binder phase Concrete samples which are made in a pan mixer appears to be comparatively more homogeneously distributed than that of drum mixer, in reference to aggregate and binder phases 3.26 ph of concrete: The concrete samples was ground and mixed together before drawing a representative sample. Water was boiled to remove dissolved air and cooled before mixing with the sample. PH meter was used to measure the ph of the solution Table for M-25 grade concrete. Grade of concrete M M M Cementitious mix Age of hydration ph of 5% solution OPC+ Fly ash 7 days days 12.1 PPC 7 days days 12.2 OPC 7 days days 12.5 Table for M-25 grade concrete (( or we can also mention that similar results have been obtained for M-25 grade concrete) 3.3 Mortar Properties: Compressive Strength: The compressive strength values of mortar separated from concrete is given in Table 8. The trends are similar to those of concrete. Water absorption: Mortar cubes made from concrete were dried at 11 deg C for 24 hrs. to constant weight. Then these were immersed in water for minutes and at the end to this period, they were removed, surface dried and weighed. The

10 percentage increase in weight is recorded as percentage water absorption. These were given on Table 9. The results are in line with the trend observed on concrete, with factory made ppc having a lower level of absorption. Table 8: Mortar Compressive strength MPa Drum Mixer Pan Mixer M M Type 1 day 3 day 7 day 28 day 1 day 3 day 7 day 28 day OPC + FA PPC OPC M 25 M 25 OPC + FA PPC OPC Table 9. Percentage water absorption of mortar Drum Mixer 4. DISCUSSIONS: Type of Concrete Percentage water absorption M- OPC + FA 6.8 M- PPC 5.6 M- OPC 6.5 M- OPC + FA 6.1 M- PPC 5.6 M- OPC Fresh Concrete Properties 1. Workability: the concrete made with Fly ash show improved workability and this aspect can be used for making concrete of higher strength by reducing the water content. The workability of interground PCC is found to be better than OPC+FA mix because the fly ash particles of PPC become much finer due to intergrinding. The slump retention of PPC is also found to be better than OPC+FA mix and OPC. This is due to retention of water within the finer mass of concrete and improved water holding capacity of the PPC because of improved particle size distribution. 2. Bleeding: The addition of fly ash reduces the rate of bleeding. Since the fly ash particles are finer than cement particles, they modify the minute space in wet concrete and block the flow of water in the channels and

11 thus reduce the bleeding. It was also observed that the control of internal bleeding plays an important part in determining the strength of the transition zone between aggregates and the cement paste and therefore the mechanical properties of the concrete. 4.2 Hardened Concrete Properties 1. Compressive Strength: Compressive strength of factory ground blended cement values were higher than the site mixed concrete and this appears to be due to the finer and desirable particle size distribution of the fly ash particles due to grinding as well as the optimum level of gypsum presence. 2. Elastic Modulus: Values of fly ash blended concrete were higher probably due to the pore filling effect of the fly ash particles, compared to OPC and this is more pronounced in the case of factory ground PPC due to the enhanced effect of this parameter 3. Water permeability: The values of water permeability are lower for fly ash blended concrete due to pore refinement and this is more pronounced in factory produced PPC concrete, due to greater secondary reaction products, because of more number of reactive fly ash particles and their desirable particle size distribution. 4. ISAT: The values of initial surface absorption test are lower for fly ash blended cement concrete due to pore refinement and this is more pronounced in factory produced PPC concrete, due to greater secondary reaction products, because of more number of reactive fly ash particles and their desirable particle size distribution. 4.3 Effect of Intergrinding of Fly Ash with clinker for manufacturing PPC: 1. Improved Sphericity: The intergrinding creates new reactive surfaces for the fly ash and improves the sphericity of the coarse angular fly ash particle. In short it can be attributed to the rounding of corners of fly ash and also breaking of larger particles to smaller sizes having near spherical shapes. This also creates larger surface for reaction as compared to unground fly ash. Spherical fly ash particles Grinding to smaller rounded particles

12 2. Improved Particle size distribution: The intergrinding of clinker with Fly ash delivers much better particle size distribution as compared to separately mixed fly ash with OPC. Crow & Dungstan concluded that: fineness appeared moré critical for the reactivity of low calcium ash than to those with high calcium fly ash. However apart from than fineness, many other fly ash related variables influence strength development and there in an optimum level of fineness (45 to 48 Blaine) up to which the strength increases. Beyond this the water demand increases and thereby a drop in the strength. 3. Gypsum Content: Gypsum is known to activate the alumnus phase of fly ash thereby improving pozzolanicity and strength/ durability. During intergrinding, gypsum is added as percentage of total mass of clinker and fly ash. But in the case of site mixed fly ash, extra gypsum is not added towards fly ash and thereby the aluminous phase in not utilised to fullest extent. The site mixing of gypsum is possible at RMC plants, where good quality control is possible which is not available at most of the construction sites, where ordinary drum mixers are used Mixing efficiency of the concrete mixers and the prevalent construction practice 1 Mixing efficiency: The mixing efficiency of the ordinary drum mixers at site is poor as compared to Pan mixer / twin shaft mixers of batching plant, particularly at low water content. The mixing in the ordinary drum mixer is due to free falling action of the ingredient and the shearing action of baffle plates are not very effective. Pan mixers and high RPM twin shaft mixers of batching plant are much better in terms of high efficiency. 2 Quantity of water and mixing time: The quantity of water is not controlled in most of the small construction sites where drum mixers are being used and the operator tend to add more water in the mix than recommended to reduce the mixing time and quickly unload the concrete for the waiting workers gang to transport it. Masons also prefer concrete mix of large slump so that their efforts for compaction and finishing are reduced. Hence the mixer operator is constantly under pressure from the concreting gang for early unloading of the mix from mixer and the mason s demand for higher slump. Hence presence of a supervisor is a must for control in the quantity of water. In many sites, it has been observed that as soon as the supervisor leaves mixer location for some other work, the control on the quantity of water is lost. 3 Addition of required Quantity of fly ash: Strict control on the quantity of the fly ash also needs to be exercised in case of ordinary drum mixer.

13 This points to the fact that at smaller construction sites fly ash should not be added in making concrete with ordinary drum mixer, if a strict quality control is not possible. 4 Variation in quality of fly ash: Moreover the quality of fly ash will not be uniform if it is the collected from in different fields and variations may also be due to variations in quality of coal, grinding & burning conditions etc.. Therefore testing facility of fly ash as well as expertise in testing is required before using fly ash at construction sites for proper quality control. 5. CONCLUSIONS: It has been found that addition of fly ash to OPC in concrete improves the properties of fresh concrete and enhance parameters of which indicate durability. Here also, the properties of fresh and hardened concrete of interground PCC is better than the site blend of OPC and fly ash. The inter-grinding of fly ash with clinker and gypsum maximise the pozzolanic potential of the fly ash with more consistent product with good control on variability, which results in to better performance characteristics of concrete. The variability in type & quality of fly ash, control on doses and mixing plays a vital role in imparting strength and durability to the concrete. The intergrinding makes fly ash particles finer, which imparts additional reactive surfaces for hydration and improves the particle size distribution of the resultant cement. The site mix fly ash concrete also do not have adequate gypsum, as compared to inter-ground PCC in which gypsum is added based on the overall quantity of clinker and fly ash while grinding. The gypsum (SO3) is know to activate the aluminous phase of the fly ash and it is absent in the site mixed fly, whether it is a batching plant or a smaller construction site. The fly ash should be mixed at site in the batching plant only since mixing efficiency of the ordinary drum mixers is poor at low water content. Improper distribution of fly ash particles may result in to some catastrophe.

14 Proper precautions on quantity of water and mixing time needs to be taken care along with strict control on the quantity and quality of the fly ash also needs to be exercised. The testing facility of fly ash as well as expertise in testing is required before using fly ash at construction sites apart from quality control at site in batching. Factory interground PPC takes care of all the problems mentioned above, wherein fly quality is checked regularly. The performance of the cement is also checked at regular interval, which takes care of all the ingredients and the product performance is guaranteed as per the BIS code 1489 part 1. Fly ash should be added only in the batching plant, where good control on quantity and quality can be ensured. Ordinary drum mixer should not be used for site mixed fly ash concrete. References 1. Mehta P. K. & Moteiro Paulo J. M.- Concrete Microstructure, Properties & Materials 2. IS 456, Plain & Reinforced Concrete Code of Practice 3. IS Pulverised Fuel Ash- Specification 4. Mullick, A. K. Use fo Fly Ash in Structural Concrete: Part II- How much?, The Indian Concrete Journal, June 5 Vol.. 79, No. 6, PP Malhotra V.M. and Mehta P. K. - High-Performance, High Volume Fly Ash Concrete: Materials, Mixture Proportioning, Properties, Construction Practice and Case Histories 6. Banchhor Anil & S. Krishnan Performance Evaluation of interground PCC and site Mixed Fly Ash Concrete, ICI-Asian conference, Mumbai ACECON- 5 Sept.22-25, 5, 7. Banchhor Anil, Krishnan S. Khadilkar S. A. and Karandikar M. V. Study on concrete made with PPC, OPC & site mixed fly ash with OPC 9 th NCB international seminar at Delhi 11-13

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