EFFECT OF RICE HUSK ASH ON PROPERTIES OF HIGH STRENGTH CONCRETE

Transcription

1 A.44 EFFECT OF RICE HUSK ASH ON PROPERTIES OF HIGH STRENGTH CONCRETE Dao Van Dong- Doctor, Pham Duy Huu- Professor, Nguyen Ngoc Lan- Engineer University of Transportation and Communication, Vietnam ABSTRACT: The paper presents several key properties of high strength concrete using rice husk ashes (RHAs). RHAs obtained from two sources: India and Vietnam were used with various contents to partially replace for cement binder in high strength concrete. Key properties of concrete, including: slump, density, compressive strength, water and chloride permeability resistances, were investigated in comparison between samples without using RHA and samples using two types of RHAs. Experimental results showed reasonable improvements in compressive strength, water and chloride permeability resistances of concrete using the RHAs. The results also presented that the improvements of samples composed the India RHA were much better than that of the Vietnam RHA. The utilisation of RHA in concrete can obtain several benefits. On the one hand, it contributes to reduce of agricultural waste that is the main cause of environmental problems in agricultural countries. On the other hand, it is an approach to improve the quality of concrete without using costly additives such as silicafume. KEYWORDS: Concrete, rice husk ash, density, compressive strength, water permeability resistance, chloride permeability resistance. 1. INTRODUCTION High strength concretes (HSC) have been used widely in construction applications since they have consisted reasonable high properties, such as mechanical and durable properties. The most effective applications of HSC are in high rise building and bridge constructions as it allows reducing the cross-section area of the structural elements. Investigations in HSC as well as high performance concrete (HPC) to improve their properties have been attempted. According to CEB-FIP State-of-the-Art Report [1] HSC is defined as concrete which has a minimum compressive strength of 6 MPa at 28-days age. However, the manufacture of HSCs requires the use of a high amount of cement binder compared to the traditional concretes. Cement binder proportion for more than 6 MPa compressive strength concretes is at least 45 kg/m 3 [2]. This figure is up to a thousand kilogram of cement for ultra high strength concrete generations [3]. The use of high cement binder contents causes of many disadvantages for properties of HSCs. Firstly, high cement binder content for a unit of concrete volume releases substantial heat due to hydration reactions between cement and water. Each kilogram of cement added into the concrete mix results in approximately 15 kj of heat [4]. Consequently, temperature rises significantly when more cement is mixed for HSC, especially for mass concrete structures. This problem leads to high temperatures inside of the concrete structures and is main causes of cracks in concretes, especially, during early age. To reduce the cracks the concrete must be cured carefully. As a result, expenditure for curing works is significant and contributes to raise the total cost of concretes. Secondly, high cement binder content used for concrete creates more freecalcium hydroxide compounds in cement paste. This leads to unstable volume as well as low 442

2 water resistance of concrete structure. For those reasons, costly additives, such as silicafume, normally should be used to limit disadvantages of the use of high cement binder content [2]. Although HSCs obtain substantial compressive strengths, their durable properties have still been considered. This is because the improvement in strength of HSCs is not the same rate of other durable properties, such as chloride and sulfate resistances [5]. Rice husk is a waste from agricultural product and obtained during the dehusking of paddy rice. It is abundant in many parts of the world, especially in agriculture countries. Each tonne of paddy rice can produce approximately 2 kg of husk, which on combustion produces about 4 kg of ash [6]. According to a recent review by Calpe [7], the Global rice paddy production in 27 was as high as 64 million tones that results in approximately 13 million tones of rice husks as a waste product from the milling. Vietnam is an agriculture country with yearly paddy rice production is 36 million tones, the fifth highest paddy rice producing and the third highest exported rice production over the world [7]. This figure will grow up to as high as 4 million tones in 22 [8]. This produces approximately 8 million tones of rice husk annually. Rice husk is mostly disposed as a waste. This results in environmental problems, especially after it is disintegrated in wet conditions. In Vietnam rice husk from paddy rice mills is eliminated directly out the environment or sometime is dumped or burnt in open piles on the fields. In recent reports rice husk which eliminated into rivers and canals by rice mills in Mekong Delta provinces leads to serious pollutions for environment [9]. The utilization of ashes converted from rice husks (RHAs) was considered as early as prior 197s [1]. Since then an amount of investigations has been carried out in manufacturing processes, including combustion and grind technologies, to obtain high performances of RHAs. RHAs have been used effectively in steel industry to produce high quality flat steels and insulators [11]. It has been also shown from several studies that rice husks when burnt in controlled conditions between temperatures of 5 o C to 7 o C and ground to particle sizes of less than 1 µm will perform acceptable pozzolanic properties to apply in cement and concrete industries [12, 13, 14, 15]. Uncontrolled combustion results in poor quality of RHAs as at lower than 5 o C the ashes contain high carbon content and high amount of loss in ignition [13, 14], and at temperatures greater than 7 o C crystalline silica ash is formed [11, 13]. Thus, pozzolanic reactivity is a key characteristic which controls the quality of RHAs when using for cement and concrete. This characteristic is affected by amorphous silica content as well as particle size. RHAs have been trialed manufacturing in Vietnam in recent years [16]. However, there is only one manufacturer who can supply the ash in commercial scale. Unfortunately, the supplier is located in Hanoi, whereas the major source of rice husk is available in the South of Vietnam, especially in the Mekong Delta provinces. Also, the quality of the ash should be considered. Rice husk ashes have been used effectively in blended cement binders and cementitous concretes. Up to 4% of RHAs was blended in cement to manufacture mortar with high strength and good resistance of chloride penetration [12, 17]. Similarly, normal concretes using RHAs to replace partly for cement have gained substantial good properties, such as strength and durability [14, 18, 19]. This is due to RHAs act as a highly reactive pozzolanic material to enhance quality of cement paste. It is also a fine filler to improve the microstructure of the interface transition zone between the cement paste and the aggregate surface. These mechanisms of RHAs are fully applicable to HPCs. However, research in the use of RHAs in HPCs has been limited. The objectives of this paper are to present an enormous potentiality of utilizing of rice husk in manufacturing RHAs for industrial applications, especially as a supper reactive pozzolanic material in cement and concrete productions; and to compare key properties of a HSC using local materials composed two RHAs, one is sold in Vietnam and other is a high quality product from India. 443

3 2. MATERIALS AND EXPERIMENTS 2.1. Materials Cement Cement used in this investigation was a blended Portland cement PCB4 which is in accordance with Vietnamse standard TCVN Properties of this binder are given in Table Rice husk ashes Two types of RHAs were used to replace partly for cement. The first one is a commercial ash manufactured in Hanoi, Vietnam (RHA1). The second ash is a high quality product from India (RHA2). Colour, chemical compositions and physical characteristics of those ashes are shown in Plate 1 and Table 1. a) Plate 1. Colour of RHAs: a) RHA1; b) RHA2 b) Table 1. Properties of cement and RHAs Properties Cement PCB 4 RHA1 RHA2 Physical properties: Specific gravity Loose bulk density (g/cm 3 ) Particle size distribution (%) Passing 45 µm Passing 2 µm Passing 1 µm Passing 5 µm Initial setting time (min) Final setting time (min) LOI (%) Colour grey black white-grey Chemical composition (%): SiO 2 - Reactive silica Al 2 O 3 Fe 2 O 3 CaO K 2 O MgO

4 Aggregates Crushed granite with maximum nominal particle size of 19 mm was used as coarse aggregate. Silicate sand with 1% passing sieve size of 5 mm and fineness modulus of 3.28 was acted as fine aggregate. Their properties and gradation are listed in Table 2 and Figure 1. Table 2. Properties of aggregates Bulk density (g/cm 3 ) Specific gravity Crushing strength (% passing 1.25 mm sieve) Fineness modulus Sand Coarse Agg Passing (%) 1 Sand Coarse Agg Grain size (mm) Figure 1. Gradations of aggregates Superplasticizer A polymer polycarboxylate- based superplasticizer with the trademark of Viscocrete 3-1 was used. This admixture was sold by Sika Ltd. in Vietnam. Recommended content from the manufacturer is from.8 to 1.1 litre per 1 kg cement Mix proportion The purpose of this investigation was to make a high strength concrete with targets of 28-day compressive strength of at least 7 MPa and slump of 15 mm. Proportion of mixtures was selected basing on these targets. The RHA1 was trialed to replace for cement with various ratios, namely, 5, 1, and 15 % by mass of cement. The RHA2 was used to replace 1% of cement. Ratio of water per total cement binder (cement plus RHAs) was fixed at.32. Mixture proportions were presented in Table 3. Table 3. Proportion of investigated mixtures Mix code Materials Control 5%RHA1 1%RHA1 15%RHA1 1%RHA2 Cement (kg/m 3 ) RHA/cement (%) Water/(cement + RHA) Sand/(sand + coarse agg.) Superplaticizer (ml/1 kg (cement + RHA)) Sample preparations The mixtures after mixing followed a controlled procedure were immediately tested to determine slump. The mixes then were cast samples by using cylinder moulds of 15 by 3 mm in dimensions, excepted samples of 1 by 2 mm to test chloride permeability. After 24 hours in moulds the samples were demoulded and immerged in a water batch in laboratory condition until

5 the day of testing. Each test was carried out with a set of three samples and the reported results below are mean values. 3. RESULTS AND DIDCUSSIONS 3.1. Workability All mixtures after mixing were immediately tested to determine slump. The test was carried out by a standard cone [2]. Experimental results are shown in Figure 2. Slump (mm) Density (%) Control 5%RHA1 1%RHA1 15%RHA1 1%RHA2 Control 5%RHA1 1%RHA1 15%RHA1 1%RHA2 Figure 2. Effect of RHA types and their contents to Slump of the fresh concrete mixtures. Figure 3. Reduction in density of various concretes contained RHAs in comparison with the control sample. As can be seen in Figure 2 the use of 5% RHA1 results in a slightly reduction in slump. This could be due to the absorption of porous structure of RHA. However, when the content of the RHA1 increases slump gradually improves. This is possibly because of the dominant effect of the finer particle size of the RHA1 in comparison to the cement. This effect is more effective in the case of RHA2 (slump of 165 mm compared to 14 mm of the control mix). Again, this is due to the RHA2 is much finer than the RHA1 (see Table 1) Density of concrete The replacement of RHAs for cement results in reductions of density of concretes. This is due to specific density of the RHAs is much lower than that of cement. Concretes used 1% of RHAs can be reduced approximately 2% in density in comparison with the control concrete (Fig 3.) Compressive strength Compressive strength of the samples is shown in Figure 4. The replacement of RHA1 for cement results in decreases in compressive strength compared to the control samples. At age of 28 days there is no big different between compressive strength of 5%RHA1 and 1%RHA1 samples. However, the use of 15%RHA1 leads to a significant reduction of compressive strength. Additionally, the rate of development of compressive strength of the RHA1 concrete samples tends to decrease following the age of curing. These could be due to that RHA1 does not act as a cement replacement because of its coarse particle size and low reactivity. From these data it is designed that 1%RHA1 is an acceptable percentage of RHA1 as cement replacement. In contrast, Figure 4 also shows improvements in compressive strength of samples used 1%RHA2. At the age of 7 days there is no different between compressive strength of control samples and that of samples used 1%RHA2. More interestingly, the replacement of 1% cement 446

6 by RHA2 indicates remarkable improvements in compressive strength at 28 and 9 days of age in comparison with the control samples. These improvements are 6.5% and 7.5% increases at 28 and 9 days, respectively. In other words, the rate of increase of compressive strength tends to rise up continuously after the designed age (e.g. 28 days). This trend is opposite to the samples used RHA1 that the rate is gradually reduced. Further investigation for various RHA2 contents should be extended to get an optimum content. From the results obtained in these tests it is possible to reveal that RHA1 does not present good reactive pozzolanic properties. This material can be considered as a filler rather than a pozzolanic additive. Meanwhile, RHA2 indicates much better pozzolanic properties, both in terms of compressive strength and the rate of hydration. This could be due to the results of fine particle size as well as reactive dioxide silicate content in the form of amorphous structure. 1 Comp. strength (MPa) Control 5%RHA1 1%RHA1 15%RHA1 1%RHA Time (days) Figure 4. Compressive strength of concrete samples used various RHAs types and contents at different ages Water permeability Samples were cut into cylinders of 15 mm by 15 mm and coated surrounding by epoxy to determine water permeability. Six half of the samples were tested in accordance with BS EN [21]. Water pressure was risen for each 2 atm interval up to 26 atm and kept for 16 hours for each level. Unfortunately, testing machine used in this research can be only withstood a maximum water pressure of 26 atm. Therefore, it was designed that water pressure was stopped at 26 atm and maintained for 16 hours. Observation during the tests showed that water did not penetrate through the sample to the upper face at 26 atm. Tested samples therefore were removed from apparatus and immediately split down the centre. The split half the samples were observed to determine the maximum depth of water penetration. The results are plotted in Figure 5. In general, HSC samples in this investigation have very low water permeability. Control samples are penetrated approximately 4 mm. Samples used 1% RHA1 to replace for cement obtain a little bit lower in water penetration, that is 35 mm depth of penetration. Interestingly, samples that contained 1% RHA2 to substitute for cement obtain a negligible value of water penetration, from zero to 2 mm in depth. The improvements in water resistance of samples used RHAs can be explained as following mechanisms: RHA1 reacts mainly as a filler to enhance packing of aggregate structure of concrete but it does not react as a puzzolanicity because of its chemical compositions. Whereas, RHA2, on the one hand, is a filler liked RHA1, and on the other hand it 447

7 is a supper pozzolanic material so it reacts with hydration products and reduces Ca(OH) 2 content. These result in a denser matrix and therefore improving water permeable resistance Chloride permeability Chloride permeability was tested by Rapid chloride permeability Test [22]. Three sets of samples were carried out. These were control samples; 1% RHA1 samples and 1% RHA2 samples. Results in Figure 6 clearly show improvements of chloride resistance when concrete samples were consisted of RHAs. However, samples used RHA1 results in a negligible reduction, 1165 coulombs compared to 138 coulombs for pure cement concrete. Like control concrete, it is still in low chloride ion penetrability group. In contrast, the use of RHA2 improves significantly the chloride resistance, to 557 coulombs. Samples composed RHA2 is classified into very low chloride ion penetrability [22]. Mechanisms to improve the chloride permeable resistance of the concretes used of RHAs are similarly as the explanations in Section Depth of water penetration (mm) Control 1%RHA1 1%RHA2 Chloride permeability (Coulombs) Control 1%RHA1 1%RHA2 Figure 5. Effect of RHA types to water penetration of harden concretes Figure 6. Effect of RHA types to chloride permeability of harden concretes 4. CONCLUSIONS On the basis of the analyses and results presented, the following conclusions can be drawn: Rice husk is an abundant waste generated from agriculture product in Vietnam. This is a potential source to produce RHAs for construction applications in Vietnam. Low quality RHA can be used as filler for concrete. The acceptable content is 1% to replace for cement with an acceptance of reduction in compressive strength. High quality RHA can be used as a super pozzolanic additive for HSC. The concrete product using RHA2 in this investigation can be compared with HPC. HSC used 1% RHA2 to replace for cement obtains substantial improvements in properties, especially, compressive strength, water and chloride resistances. Further research to obtain an optimum RHA2 content should be continued. Investigations in manufacturing high quality RHA in Vietnam is necessary. ACKNOWLEDGEMENTS The authors would express thanks to Section of Construction Materials and Laboratory of the Institute of Science and Technology for Transport Construction, University of Transport and Communications, Vietnam for kind helps in this research. 448

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