Use of rice husk ash in concrete

The International Journal of Cement Composites and Lightweight Concrete, Volume 6, Number 4 November 1984

Use of rice husk ash in concrete Moayad N. AI-Khalaf* and Hana A. Yousift

SYNOPSIS Rice husk ash was prepared as a pozzolana by a special process such that the final product conformed to engineering requirements in terms of physical and chemical properties, and the silica remained in an amorphous form with a minor amount of unburnt carbon. Results indicated that such a pozzolana can be produced with vary- ing pozzolanic activity index depending on the degree of grinding and the burning temperature. The effect of rice husk ash content as partial replacement of cement on compressive strength and volume changes of different mixes is investigated. Test results showed that up to 40% replacement can be made with no significant change in compressive strength compared with the control mix. However, the effect on volume changes is within the limit specified in the American Standard.

KEYWORDS Concrete materials, pozzolans, rice husk ash, cement replacement, chemical composition, compressive strength shrinkage, construction materials, cements, wastes, strength of materials.

* Assistant Professor in Concrete Technology. t Assistant Lecturer in Concrete Technology, Department of Building and Construction, University of Technology, Baghdad, Iraq

(~) Construction Press 1984

0262-5075/84/06430241/$02.00

INTRODUCTION Several investigations have been carried out to utilise waste materials in construction. Apart from getting rid of these materials, their use in construction protects the environment from contamination. The waste product, rice husk, generated from the accumulation of the outer covering of rice grains during the milling process, con- stitutes about 20% of 300 million metric tons of rice produced annually in the world [1 ]. The chemical analysis of husks [2] indicates that its major constituents are ash (13-39% by weight), lignin, cellulose and moisture content. Attempts to utilise rice husks should therefore take advantage of their high ash content, which consists essentially of SiO 2, derived from the amorphous silica present in the cellular structure [2]. Rice husk ash (RHA) in a highly reactive form has been used as an excellent filler for rubber [3], as a suitable raw material for making hydraulic cement [4], as a good corrective admixture for reducing expansion due to a l k a l i - silicate reaction [5], and also to reduce temperature in high-strength mass concrete [6].

The primary work on (RHA) was started at the Asian Institute of Technology by Columna [7], who found out that 'village burnt' husks were converted to ash at temperatures less than 300°C. Rice husk ash prepared in this way is expected to have a considerable amount of carbon which has an advberse effect upon its pozzolanic activity. A study of the efficiency and effects of burning methods on the quality and properties of RHA was carried out by Mehta [8]. He found that field burning of rice husks produced crystalline silica ash. However, burning the husks in a controlled temperature furnace, the residual silica ash is in a highly reactive form, and when mixed with lime produced a rich black cement. Mehta continued his work by investigating the effect of rice husks burned at an industrial furnace on the compressive strength and durability of lime and portland-RHA cements [4].

241

Use of rice husk ash in concrete AI-Khalaf and Yousif

Cook, et al. [1] used the RHA as a pozzolanic material. The material was prepared by burning the husks at 450°C for 4 hours and grinding the ash by using a ball mill for 1½ hours. They carried out tests to determine the strength and volume change characteristics of concretes and pastes containing different percentages of RHA as partial replacement by weight of cem'ent.

The aim of this work is to utilise the locally available rice husks as a pozzolana such that it meets with the necessary engineering requirements. Some properties of cement pastes and mortars containing different percentages of RHA as partial replacement by weight of cement are also investigated.

E X P E R I M E N T A L P R O G R A M M E

Preparat ion of RHA Burning of rice husks was carried out in a furnace with controlled temperature in order to establish the optimum burning temperature and burning time. In the first part of this study, the temperature was increased by 100°C per hour, after every hour the sample (500 gm) was removed from the furnace, cooled and weighed to an accuracy of 0.01 gm. The results were then expressed as a percentage by weight of dried sample at 105°C. To establish the most suitable burning time, a sample of 200 gm of rice husks, dried at 105°C, was burnt at various temperatures namely 450, 500, 550, 600, 700 and 850°C. For each burning temperature, the sample was removed from the furnace after 0.5, 1.0, 1.5, 2.0, 3.0, 4.0 and 5.0 hours, cooled and weighed. The weight loss was then determined.

Grinding was effected by using a modified Los- Angles machine. Two methods were tried on samples, weighing 1 kg, and burned at 450°C for 2 hours - - the first by using steel balls, and the second, using a steel chain of 2 m length and 450 gm/m weight in addition to

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Characteristics of rice husks burned at 500°C

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the steel balls. It appeared, from Blaine fineness tests, that the second method produced more efficient grinding and therefore was adopted throughout this work. In addition, tests and measurements were also made to find the relationship between time of grinding, burning temperature and fineness of RHA.

Chemica l analysis and x-ray di f fract ion analysis of RHA Chemical analysis of RHA was made on samples burned at various temperatures for 2, 3 and 4 hours and ground for 1½ hours. The results were expressed as a percentage by weight of dried sample at 105°C.

The silica content of the ash, which is derived from the amorphous silica present in the cellular structure of the husks, was checked after the remcval of the carbonaceous matter during the burning process by means of x-ray diffraction.

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Pozzolanic activity of RHA at various fineness and burning temperatures Based on previous tests, and to investigate the relationship between the fineness of RHA and its pozzolanic activity, samples burned at 500°C for 2 hours were ground for different periods. Further, to study the relationship between the pozzolanic activity of RHA and the burning temperature, samples were burned at 450, 500, 550, 600 and 700°C for 2 hours and ground for the same period (6 hours).

The preparation and testing of plain mortar and cement-RHA mortar specimens for pozzolanic activity test were carried out in accordance with ASTM C 618 80.. The materials used throughout the work were distilled water, ordinary portland cement and natural sand graded in accordance with section (4) of ASTM C 109 77. The percentage oxide composition, by weight, of cement was as follows: Loss

on CaQ SiO2 AI203 Fe203 SO 3 M g O K20 Na20 ignition 62.95 22.04 5.84 2.80 2.09 2.56 0.50 0.12 0.94

Strength and volume change tests All specimens of pastes and mortars were prepared using the materials described for pozzolanic activity tests except that the burnt husks at 500°C for 2 hours were ground for 1½ hour (fineness 11,500 cm2/gm).

For the strength tests mortar cubes of 1:2 and 1:3 cement to sand ratio with 0, 10, 20, 30, 40 and 50% RHA replacement by weight of cement and having the same consistency (as determined by the flow test) were used. In addition, other 1:2 mortar mixes with 0, 10, 20, 30, 40 and 50% RHA replacement by weight of cement but with variable W/(C + RHA) ratio for each cement-RHA combination were used to determine the influence of W/(C + RHA) on compressive strength. Three 50 mm cubes were moulded from each mix. After casting, the

specimens were placed immediately in a moist closet maintained at a temperature of 23 + 2°C and RH of about 95% for 24 hours, after which the specimens were removed from the moulds and stored in water at a temperature of 23 + 2°C. The specimens were tested in compression after 1,3, 7, 28 and 60 days of curing.

The volume change tests were made using five types of paste mixes with 0, 10, 20, 30 and 40% RHA replacement by weight of cement. For shrinkage measurement of mortars, five mixes of 1:2 cement to sand ratio were also used with cement replacement up to 40%. Sufficient amount of distilled water was added to each mix to produce a flow of 110%. For each of the shrinkage or swelling test, three 25 x 25 x 280 mm prisms were prepared from each mix. After moulding the specimens were placed in a moist closet at 23 + 2°C and RH of about 95% for 24 hours. The specimens were demoulded and placed immediately in water at a temperature of 23 + 2°C for 28 days (including the period in the mould). The specimens for shrinkage studies were then left in dry air with an average temperature of 30°C and an average humidity of 40% for 32 days, while specimens used for swelling studies were kept in water. Volume change measurements were taken at regular intervals up to 60 days.

TEST RESULTS AND DISCUSSION

Loss on ignition The loss on ignition test results are shown in Figure 1, which indicates that the most convenient and economical temperature required for conversion of rice husks into ash with approximately constant weight is 500°C. However, during ignition the temperature of the samples rose to higher values than the ones stated above for a short period depending on the weight of the sample. Below this temperature, the ignition was

243

Use of rice husk ash in concrete AI Khalaf and Yousif

not fully completed and the considerable amount of the unburnt carbon was expected to have an adverse effect upon the pozzolanic activity of the resulting ash. Cook et al. [1 ] found that the suitable temperature for burning rice husks is 450°C. The results shown in Figure 2 suggest that the most suitable time to produce the ash from rice husks burned at 500°C is at about 2 hours. Beyond this time, any further weight loss is too small and can be neglected. In the tests by Cook etal. [1] the suitable time to produce the RHA was found to be four hours, which appears to be rather high in relation to these tests. Simi- lar results were also obtained for rice husks burned at 450, 550, 600, 700 and 850°C.

Grinding t ime tests The results of the tests grinding time, burning temperature and fineness of RHA are shown in Figure 3. The data indicate that the fineness of RHA (expressed in terms of specific surface)increases with increase in grinding time for all burning temperatures. In general, for a given grinding time, three was a considerable reduction in the specific surface area of RHA as the burning temperature increased. Such reduction in the specific surface area may be due to the decrease in carbon content with the increase in burning temperature as shown in Table 1, indicating that the carbon particles are too fine and have too high specific surface area compared with the silicate particles.

For samples burned at 450°C for 2 hours there was a greater variation in specific surface area between samples. Although the reason for this is not fully known, it is thought that this is due partly to the availability of a considerable amount of unburnt and non-uniformly dis- persed carbon particles. However, samples burned at 450°C have specific areas which are nearly the same as that of samples burned at 500°C, Figure 3. This similarity between the specific surface of the two samples was continued for up to two hours of grinding. Beyond this time, the difference in the measured specific surface areas increased, but not significantly. In order to control the quality and uniformity of the final RHA product, it is suggested that they should not be burnt at a temperature of less than 500°C.

Chemical analysis and x-ray diffraction The chemical analysis of RHA prepared under the specified burning and grinding conditions showed that its main constituent is silica - - typically 85-88%; the ash also contained

some minor oxides such as alkalis, sulphate, calcium and few traces of other elements Table 1. Further, for all burning temperatures, the loss on ignition, as an approx- imation of carbon content, is within the limit specified by most standards [9,10].

The x-ray diffraction patterns (Figure 4) of RHA burned at 450, 500, 550, 600 and 700°C for 2 hours indicated that the silica content of the ash, which is derived from the amorphous silica present in the cellular structure of the husks, remained amorphous after the removal of carbonaceous matter during the burning process, and there were no crystalline phases of silica such as quartz, tridymite and cristobalite. On the other hand, results for husks burned at a temperature of 850°C for 2 hours, indicate the development of silica into a quartz crystalline phase. Results also showed that the silica remained in the amorphous form, when husks were burnt at temperatures between (450-700°C for 3 and 4 hours. Since the crystalline forms of silica have 16wer reactivity than the amorphous one, it is essential that rice husks should not be burnt at a temperature of 850°C and higher.

Based on data presented in Table 1 and on ASTM C618 (section 2) definition, the prepared RHA can be classified as an artificial pozzolan of siliceous material. Further, the RHA used throughout this work, conformed to the chemical and physical requirements of ASTM C618 class N pozzolans. The specific gravity of RHA was found to be 2.14. This value is within that specified for other pozzolans, which is usually between 2.1 and 2.4 [11]. This is also very close to the value reported by Mehta and Polivka [5] for rice hull ash which was 2.08. However, Cook et al. [1 ] reported a value of 3.14 which is rather high.

Pozzolanic activity The relationship between the fineness of RHA and the amount of water required for standard consistency of mortars, having (cement + RHA) to sand ratio of (0.73 + 0.27) : 2.75, expressed in terms of W/(C + RHA) ratio (Figure 5) suggests that the water requirement decreases as the fineness of RHA increases. This is mainly due to the fact that the consistency of mortar mix increases due to the lubricating ability of the (cement-RHA) paste in mortar and to an increase in the cohesion of the mixture as the fineness of RHA increases. Results also showed that for the same mix proportions, the amount of water required for stan-

Table 1 Chemical analysis of rice husk ash

Temperature Loss Percentage oxide composition and time of on burning ignition SiO2 K~O SO3 CaO Na~O MgO AI203 P~Os CI Fe203 MnO

450°C for 2 hrs 3.49 85.88 4.10 1.24 1.12 1.15 0.46 0.47 0.34 0.39 0.18 0.091 500°C for 2 hrs 3.30 86.89 3.84 1.54 1.40 1.15 0.37 0.40 0.35 0.45 0.19 0.087 550°C for 2 hrs 2.89 87.19 4.10 1.54 1.30 1.05 0.43 0.37 0.32 0.33 0.17 0.091 600°C for 2 hrs 2.69 86.02 3.76 1.82 1.12 1.15 0.39 0.36 0.30 0.27 0.16 0.086 700°C for 2 hrs 2.38 85.81 4.10 1.88 1.40 1.22 0.40 0.38 0.30 0.14 0.17 0.091 850°C for 2 hrs 1.89 87.72 3.96 1.25 1.43 1.11 0.36 0.40 0.30 0.16 0.16 0.090

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dard consistency of cement-RHA mortar is always higher than that required for plain mortar of the same consistency even if it contains too fine RHA.

Results obtained from pozzolanic activity index tests (Figure 6) show that the pozzolanic activity increases as the specific surface increases. The minimum pozzolanic activity of RHA required by class N section [4], of ASTM C618-80 can be obtained when RHA has a specific surface of about 11 500 cm2/gm. Considering Figure 3 for samples burned at 500°C for 2 hours, the corresponding grinding t ime required is 1½ hours. Further, Figure 6 also shows that the strength of cement-RHA mortar having (cement + RHA) to sand ratio of (0.73 + 0.27) : 2.75 approaches the strength of plain mortar of the same mix proportions when the specific surface of RHA is about 17 000 cm2/gm. Moreover, a highly active RHA can be

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obtained when its specific surface is about 21,000 cm2/ gm. This can be reached by grinding for 12 hours.

The relationship between the pozzolanic activity of RHA burned at various temperatures and the corresponding burning temperature is shown in Table 2. The results indicate that, in general, pozzolanic activity increases as burning temperature increases. This may be due to the decrease in carbon content which replaces part of the weight of RHA but which has no pozzolanic activity. However, there is no significant difference between the pozzolanic activity of ashes burned at 500 and 700°C. Taking into consideration the additional energy required for burning at a higher temperature, it is obvious that the most suitable burning temperature is 500°C.

S t r e n g t h r e s u l t s Mortar mixes of different mix propor- tion were used to investigate the effect of using RHA as partial replacement by weight of cement, as well as the effect of W/(C + RHA) ratio upon the compressive

Table 2 Pozzolanic activity of rice husk ash at v a r i o u s b u r n i n g t e m p e r a t u r e

Burning Compressive Mix - description temperature strength (C+RHA):sand (°C) W/(C + RHA) N/mm 2

Pozzolanic activity index

(%)

(1.0 + 0.0):2.75 - - 0.68 24.3 - - (0.73 + 0.27);2.75 450 0.77 23.4 96.3 (0.73 + 0.27):2.75 500 0.77 23.5 96.6 (0.73 + 0.27):2.75 550 0.78 22.5 92.6 (0.73 + 0.27):2.75 600 0.78 23.6 97.1 (0.73 + 0.27):2.75 700 0.78 24.2 99.6

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strength at various ages. The results are.shown in Tables 3 and 4. These data show that, in general, the higher the percentage of RHA content, the lower the compressive strength at early ages. On the other hand, at 60 days the 1:2 mortar mix with 30% cement replacement reached nearly the same strength as those of the corresponding plain mortar. For 50% cement replacement mixes, there was a significant reduction in the compressive strength at early ages as well as at 60 days.

The results also show that regardless of the agg/cem ratio of the mix, the rate of increase in the compressive strength of cement-RHA mortars increases relative to those of plain mortars with the progress of hydration. Further, this rate of increase in compressive strength is higher for mixes of high agg/cem ratio than that for the corresponding mixes of low agg/cem ratio. A higher RHA content in the mix also leads to a lower strength. This may be due to the fact that the quantity of RHA present in the mix is higher than that required to combine with the liberated lime during the process of hydration thus lead- ing to excess silica leached out and causing a deficiency in strength as it replaces part of the cementitious material but does not contribute to strength.

The influence of W/(C + RHA) ratio upon the compressive strength of cement-RHA mortars was also studied using different mix proportions [12]. Typical data for such relations is illustrated in Figure 7 for 1:2 cement- RHA mortar with 50% cement replacement. Initially, as the W/(C + RHA) ratio decreases the strength of cement-RHA mortars increases. However, when the W/(C + RHA) ratio decreases below a certain value (0.65 in Figure 7), the compressive strength also decreases. This is mainly due to the reduction in the flowability of the

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Table 3 Compressive strength of 1:2 cement - - RHA mortars of the same consistency

Age at test

Mix-description (C 1 day 3 days 7 days 28 days 60 days + RHA):sand W/(C + RHA) N/mm ~ N/mm 2 N/mm 2 N/mm 2 N/mm ~

(1.0 + 0.0):2 0.62 14.1 20.9 26.7 37.9 43.2 (0.9 + 0.1):2 0.56 12.8 19.7 25.4 37.2 45.4 (0.8 + 0.2):2 0.63 11.9 18.6 23.7 35.2 43.6 (0.7 + 0.3):2 0.71 10.9 17.4 21.9 33.7 42.4 (0.6 + 0.4):2 0.80 7.6 12.2 16.3 27.3 33.7 (0.5 + 0.5):2 0.91 5.5 8.4 11.5 15.9 20.7

Table 4 Compressive strength of 1:3 cement - - RHA mortars of the same consistency

Age at test

Mix-description (C 1 day 3 days 7 days 28 days 60 days + RHA):sand W/(C + RHA) N/ram 2 N/mm 2 N/ram ~ N/ram 2 N/mm 2

(1.0 + 0.0):3 0.71 6.6 8.9 14.2 23.3 26.5 (0.9 + 0.1):3 0.77 6.4 8.8 14.6 25.4 31.7 (0.8 + 0.2):3 0.81 5.9 8.1 13.6 24.1 28.6 (0.7 + 0.3):3 0.87 5.5 7.7 13.6 22.8 26.9 (0.6 + 0.4):3 0.94 3.9 5.8 11.4 20.0 25.7 (0.5 + 0.5):3 1.03 2.9 4.1 7.4 12.3 17.6

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fresh mortars so that full compaction can not be achieved. If fully compacted mortars can be obtained, the W/(C + RHA) ratio strength relation would follow the dotted line in Figure 7.

Swel l ing and shrinkage Swelling and shrinkage characteristics of cement-RHA pastes are presented in Figures 8 and 9 respectively. The results show that the higher the RHA content, the higher are the swelling and shrinkage. At 60 days, the swelling ratio of cement-RHA paste to that of cement paste is 1.4 and 1.51 for 30 and 40% cement replacement respectively, while the corresponding shrinkage ratios are 1.31 and 1.39. These val-

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ues are considerably lower than those obtained Cook et al. [1] at the same age. However the shrinkage characteristics of cement-RHA mortars shown in Figure 10 indicate that the shrinkage ratios of cement-RHA mortar to that of plain mortar are 1.18 and 1.26 for 30 and 40% RHA content respectively. This percentage increase in drying shrinkage of mortar bars seems to be within the limit specified by ASTM C618, i.e. less than 0.03%. Comparing the shrinkage characteristics of cement- RHA mortars with those of cement-RHA pastes, it is apparent that the effect of increasing RHA content for mortar mixes is less than those for paste mixes due to the restraining effect of fine aggregates on shrinkage.

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CONCLUSIONS 1, The most convenient and economical burning conditions required to convert rice husks into a homogenous and well burnt ash, taking into consideration the quality of the produced ash and the energy used in its preparation, are 500°C for 2 hours.

2. The relationship between grinding time and fineness of RHA burned at various temperatures suggested that for a given grinding time, there is a considerable reduction in the specific surface area of RHA as the burning temperature increases.

3. Based on these studies, the RHA produced can be calssified as an artificial pozzolana of siliceous material, the material conforming to the chemical and physical requirements of class N pozzolan (ASTM C618). It has a specific gravity of 2.14.

4. For a mortar mix with constant RHA content, the water requirement decreases as the fineness of the ash increases. The minimum pozzolanic activity of RHA required by class N can be obtained when the ash has a

247 E


specific surface of about 11 500 cm2/gm. The strength of cement-RHA mortar approaches the strength of the corresponding plain mortar of the same consistency when the specific surface of RHA is about 17 000 cm2/ gm.

5. For 1:2 and 1:3 mortar mixes of standard consistency, the optimum percentage of RHA that can be replaced by weight of cement without the 60 day strength being less than that of the corresponding plain mortar was 30 and 40% respectively.

6. The higher the percentage of RHA, the higher are the volume change characteristics compared with those of the corresponding plain mixes. However, the percentage increase in drying shrinkage of mortar bars seems to be within the limit specified by ASTM C618-80.

REFERENCES 1. Cook, D. J., Pama, R. P. and Damer, S. A. 'The

behaviour of concrete and cement paste containing rice husk ash', Proceedings, Conference of Hydraulic Cement Pastes, Their Structure and Properties, Uni- versity of Sheffield. April 1976, pp. 268-283.

2. Mehta, P. K. and Pitt, N. 'Energy and industrial materials from crop residues', Resource Recovery and Conversion, (Amsterdam)Vol. 12, No. 1, 1976, pp. 23-38.

3. Haxo Jr, H. E. and Mehta, P. K. 'Ground r ice-- hull ash as a filler for rubber', American Ceramic Society, October 1974, pp. 273-287.

4. Mehta, P. K. 'Properties of blended cements made from rice husk ash', Journal of American Concrete Institute, Proceedings, Vol. 74, No. 9, September 1977, pp. 440-442.

5. Mehta, P. K. and Polivka, Milos 'Use of highly active pozzolans for reducing expansion in concretes containing reactive aggregates'. Living with marginal aggregates, ASTM STP 597, American Society for testing and materials, 1976, pp. 25--35.

6. Mehta, P. K. and Pirtz, D. 'Use of rice hull ash to reduce temperature in high strength mass concrete'. Journal of American Concrete Institute, Proceed- ings, Vol. 75, No. 2, February 1978, pp. 60-63.

7. Columna, V. B. 'The effect of rice hull ash in cement and concrete mixes', M. Eng. Thesis. Asian Institute of Technology, 1974.

8. Mehta, P. K. 'Rice hull ash cement . . , high quality, acid resisting', Journal of American Concrete Institute, Proceedings, Vol. 72, No. 5, May 1975, pp. 235-236.

9. Annual Book of ASTM Standards 'Concrete and min- eral aggregates: manual of concrete testing', Part 14, 1976.

10. Smith, M. A., 'Review of standard specifications for fly ash for use in concrete', January 1975, Building research station, Watford.

11. Neville, A. M. 'Properties of concrete', Pitman, London, 1981.

12. Yousif, H. A. 'An investigation into the use of rice husk ash in concrete', M.Sc. Thesis, University of Technology, Baghdad, Iraq. 1979.

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Use of rice husk ash in concrete