Construction and Building Materials, Vol. 10, No. I, pp. 521-526, 1996 Copyright 8 1996 Elsevier Science Ltd

Printed in Great Britain. All rights reserved 0950-0618/96 51S.OO+O.O0

Effect of rice husk ash on high strength concrete

Muhammad Shoaib Ismail* and A. M. WaliuddintS

*Department of Civil Engineering, NED University of Engineering & Technology, Karachi, Pakistan tNational Building Research Institute, F-40, S.I.T.E, Hub River Road, Karachi, Pakistan

Received 31 October 1995; revised 20 February 1996; accepted 26 April 1996

High strength concrete (HSC) was produced using locally available materials. The effect of rice husk ash (RHA) passing #200 and #325 sieves as a lO-30% replacement of cement on the strength of HSC

was also studied. The RHA was obtained by burning rice husk, an agro-waste material which is abundantly available in the developing countries. A total of 200 test specimens were cast and tested at 3,7,28 and 150 days. Compressive and split tensile strengths of the test specimens were determined. Cube strength over 70 MPa was obtained without any replacement of cement by RHA. Test results indicated that strength of HSC decreased when cement was partially replaced by RHA for maintaining same level of workability. Copyright Q 1996 Elsevier Science Ltd.

Keywords: rice husk ash; strength; high strength concrete

Unlike steel and stone, concrete is a comparatively new construction material. Use of this material in building construction is relatively recent and may have begun less than a century ago. This century has seen very wide and effective research on this material and the effective- ness of this material has increased from decade to decade. The definition of HSC has been changing from time to time. Until the late 1960s 35 MPa and 42 MPa were considered as HSC while in the mid 1980s 55 MPa concrete was considered as HSC. Perhaps by the end of this century, 150 MPa will be branded as HSC.

Production of HSC is a challenge and depends upon so many factors. In this study an attempt has been made to prove that using local materials, it is possible to obtain HSC up to 70 MPa with slight increase in cost.

HSC is very effective in multistorey buildings as it reduces the cross-sectional area of the structural ele- ments. It is also effective in pavements because of less abrasion and longer durability.

In this study an effort was also made to evaluate the usefulness of using an agro-waste, known as rice husk ash (RHA) (where an appreciable amount of silica is pre- sent) as part re-placement of cement with locally avail- able ingredients. Studies at the University of California at Berkeley indicate that the silica of soil migrates in the plant in shape of monosilicic acid which concentrates there by evaporation. Electron microscope studies have shown dispersion of silica throughout the cellular struc- ture of the husk. The unburnt rice husk contains about

$orrespondence to A. M. Waliuddin

50% cellulose, 25-30% lignin and 15-20% of silica. The former two components are removed by burning, leav- ing behind silica ash. Completely burnt husk is grey or whitish in colour, while partially burnt husk is blackish. Studies conducted by the authors, while investigating the chemical properties of RHA, indicate 80-90% silica with impurities of KzO and Na,O from 14% in addi- tion to oxides of Ca, Mg etc. These results favourably compare with the studies conducted by Metha’.

Literature review

Seng and Rangan’ obtained 60 MPa concrete while Peter and Marios3, using microsilica and a good blend of coarse aggregate, obtained 124 MPa concrete. Jiafen4 using zeolite powder obtained 80 MPa concrete. Carrasquillo’ obtained upto 100 MPa concrete. Burge6 obtained 96.5 MPa concrete using silica fume and cement with a high concentration of C,S. He further observed that the mixture of cement and silica fume may be used with water reducing plasticizers and accel- erators. Aitcin and Metha’ in their study concluded that, for HSC, diabase and limestone aggregates give better results compared to granite and river gravel. They fur- ther indicated that the aggregate properties influ- enced the strength. Hanne et ~1.~ obtained concrete up to 15 000 psi with pozzolanic additives as part replace- ment of cement. Aitcin studied the behaviour of HSC

after four years of placement and concluded that there was no variation in strength and the microstructural study of the inside as well as the skin indicated no detri- mental features. Malhotra’ studied various additives as

521

522 Effect of rice husk ash on high strength concrete: M. S. lsmail and A. M. Waliuddin

F -- \ Mix-Ab-30

II I I I 0 3 7 28 150

Days

W

Figure 1

80 -

I I 0 3 7 28 150

Days

(a) Effect of RHA (passing #32S) on HSC; (b) effect of RHA _^_

(passing #ZOO) on Hsc

part replacement of cement. In his study, fly ash, silica fume, slag and RHA were introduced as cementing mate- rial by adding them as 941, 10% and 20”/ by weight of cement. His observations were that introduction of silica fume and RHA demanded more water for worka- bility. He, however, concluded that high performance HSC was not possible unless supplementary cementing materials like silica fume, RHA etc. are used. It was fur- ther inferred’ that HSC can be obtained with crushed limestone aggregate of 13 mm to 20 mm and down size with sand of fineness modulus ranging from 2.7 to 3.0. In each study*-’ it was concluded that super plasticizers have to be used and the w/c ratio should be in the range of 0.25 to 0.3. Metha” observed that when the burning temperature, to get RHA, is high, the RHA goes into the crystalline stage while low burning yields RHA in amor- phous form which is highly pozzolanic in character as compared to the crystalline form of RHA.

Chopra” used RHA obtained from burnt rice husks (used as fuel for boilers resulting in crystalline RHA) to develop masonry cement by intergrinding this ash with freshly burnt quick lime and slaked lime in the ratio of 60:30:10 respectively with desired results but with qual- ity control problems. This was achieved by grinding the mixture in a ball mill for 7 h.

Experimental programme

The objective of this research was to produce HSC above 70 MPa using locally available materials and also to study the effect of partial replacement of cement by

Table 1 Physical properties of fine and coarse aggregates

Fine aggregate Coarse aggregate

Specific gravity 2.7 2.65 Water absorption (“XI) 0.8 0.5 D.R. unit weight (kg/cu m) I705 1450 Sodium soundness loss (%) 2.1 1.2 L.A. abrasion (%) _ 21 Fineness modulus 2.9-3.0 Type Natural 20 mm down

river sand crushed limestone

Table 2 Physical and chemical properties of cement

Physical properties

Fineness Soundness (autoclave exp) Initial setting time Final setting time Compressive strength at 3 days

at 7 days at 28 days

Chemical composition (%)

33 I5 cm’ig I mm

IlOmin I70 min 22.9 MPa 28.7 MPa 50.0 MPa

Silica (SiO,) 22 Alumina (AllO?) 4.9 Iron oxide (Fe,O,) 4.4 Calcium oxide (CaO) 62.0 Magnesium oxide (MgO) 2.25 Sulfur trioxide (SO,) 1.4 Insoluble residue 0.4 Loss on ignition 2.2 Tricalcium aluminate (C,A) 5.7 Lime saturate factor (LsF) 0.85

varied percentages of RHA with two different finenesses of the compressive strength. The physical properties of coarse and fine aggregates are shown in Tuble I, while Tubles 2 and 3 show the chemical and physical proper- ties of the cement and RHA. The results of mix design are indicated in Tubie 4 and Figure 1, while the proper- ties of fresh and hardened concrete are tabulated in Tub/es 5 and 6 and the effects of fineness of RHA at

Table 3 Properties of RHA

Physical properties i Microscopic investigation Burning temperature Grinding time Fineness

Specific gravity

Crystalline structure 400-700”c 90 min Passing #200 and #325 sieves 2.11

Chemical composition ((%I)

Silica (SiO,) 80 Alumina (AJO,) 3.93 Iron oxide (Fe20,) 0.41 Calcium oxide (CaO) 3.82 Magnesium oxide (MgO) 0.25 Sodium oxide (Na,O) 0.67 Potassium oxide (K:O) I .45 Sulfur trioxide (SO,) 0.78 Loss on ignition at 850°C 8.56

Effect of rice husk ash on high strength concrete: M. S. lsmail and A. M. Waliuddin 523

Table 4 Mix proportion

Materials (kg/cu m) A Aa-lO Aa-

Mix number Aa- Ab-IO Ab-20 Ab-30

Cement RHA #200

RHA #325 Water Fine aggregate Coarse aggregate Plasticizer (ml/kg of cement) W/(C+RHA)

571 514 457 400 514 457 400 _ 57 114 171 -. _ _ _ _ 57 114 171

138 178 189 207 173 184 196 612 591 585 578 592 585 579 1088 1050 1040 1027 1052 1041 1029 25 25 25 25 25 25 25

0.24 0.31 0.33 0.36 0.30 0.32 0.34

Table 5 Characteristics of fresh concrete

Characteristics A Aa-lO Aa-

Mix number Aa- Ab-IO Ab-20 Ab-30

Slump (mm) 70 30 60 30 30 45 32 Density (kg/m’) 2425 2405 2400 2398 2403 2396 2390 Air temp (“C) 30 31 33 32 30 28 29 Cont. temp (“C) 30.5 32.0 33.0 32.0 30.5 29.0 29.5 Workability Good Harsh Good Harsh Harsh Poor Harsh

Table 6 Characteristics of hardened concrete

(a) Compressive strength (MPa) of cubes, f,.

Age (days) A Aa-lO

(w/c: 0.24 0.31

3 54.3 46.2 7 62.3 56.0 28 72.4 68.1 150 85.0 71.1

% increase over 28 days 17.4 4.4

(b) Compressive strength (MPa) of cylinders, f,’ Age (days)

A Aa- (w/c: 0.24 0.31

7 36.6 32.9 28 46.3 43.3 150 56.0 _

(c) Split tensile strength (MPa) of cylinders, j;,, at 28 days

A Aa- (w/c: 0.24 0.31

4.17 3.45

(d) Ratio of cylinder to cube strength, f,‘.& Age (days)

A Aa-lO (w/c: 0.24 0.31

7 0.59 0.59 28 0.64 0.63

Mix number Aa- Ab-IO 0.36 0.30

31.5 47.0 39.3 61.0 47.7 71.0 48.8 72.4

2.3 2.0

Aa- 0.33

35.3 46.8 57.3 57.4

0.18

Ab-20 Ab-30 0.32 0.34)

46.7 43.1 56.0 51.7 70.2 63.0 70.3 63.2

0.14 0.32

Mix number Aa- Ab-IO 0.36 0.30

24.2 35.7 31.0 45.7

_ _

Aa- 0.33

26.3 36.7

_

Ab-20 Ab-30 0.32 0.34)

33.8 32.5 45.0 39.7

Mix number Aa- Ab-10 0.36 0.30

Aa- 0.33

Ab-20 Ab-30 0.32 0.34)

3.17 2.97 3.80 3.72 3.44

Mix number Aa- Ab-10 0.36 0.30

Aa- 0.33

Ab-20 Ab-30 0.32 0.34)

0.56 0.62 0.59 0.60 0.63 0.64 0.65 0.64 0.64 0.63

different ages are shown in Tubles 7 and 8 and in results were obtained by Chopra” by grinding RHA of Figures 2 and 3, respectively.

Studied by Metha’” had confirmed that the best con- crystalline form, the authors tried to study the effect of crystalline RHA on HSC obtained by intense grinding of

tribution of RHA on the strength of concrete was RHA in a ball mill. The authors were of the opinion that obtained when the RHA was in amorphous form the fineness of RHA may activate the pozzolanic prop- obtained by low burning of rice husks. Since good erty of RHA even in crystalline form and for this reason

524 Effect of rice husk ash on high strength concrete: M. S. lsmail and A. M. Waliuddin

Table 7 Effect of fineness of RHA on three-day strength

w/c

3 day strength (MPa) % of 28 day strength

A Aa- Aa- 0.24 0.31 0.33

54.3 46.2 35.5 75.0 67.9 61.6

Mix number Aa- Ab-10 Ab-20 Ab-30 0.36 0.30 0.32 0.34

31.5 47.0 46.7 43.1 66.0 66.2 66.5 68.4

Table 8 Effect of fineness of RHA on seven-day strength

Mix number A Aa- Aa- Aa- Ab-IO Ab-20 Ab-30

w/c 0.24 0.31 0.33 0.36 0.30 0.32 0.34

7 day strength (MPa) 62.3 56.0 46.8 39.3 61.0 56.0 51.7 % of 28 day strength 86.0 82.0 82.0 82.0 86.0 80.0 82.0

the RHA was ground up to the fineness equivalent to sieve #200 (that of OPC) and sieve #325 (that of high early strength cement).

RHA obtained from a local market was burnt slowly for about 24 h at a temperature of 400-700°C in a drum providing sufficient ventilation at the top and bottom. It was then ground in a ball mill and sieved over #200 and #325 sieves. Tuble 3 shows the physical and chem- ical prOpertieS Of RHA.

(a) (b)

100 0 RHA passing # 325

r n RHA passing I200

wing t 325 usiog # 200

Concrete mix

Based upon the properties of materials determined, mix proportions (after casting and testing some trial batches) were established with and without the replace- ment of cement by RHA. The batch without the replace- ment of cement was denoted by Mix-A and using this mix two other groups were prepared. The groups in which cement was partially replaced by RHA, passing

0 RHA passing # 325 n RHA passing # 200

(4

100

RHA passing # 325

0 IO 20 30 . Rm (‘k)

Figure 2 Effect of fineness on strength gain at 3 days; (b) effect of fineness on strength gain at 7 days; (c) effect of fineness on strength gain at 28 days; (d) effect of fineness on strength gain at 150 days

Effect of rice husk ash on high strength concrete: M. S. lsmail and A. M. Waliuddin

(a) (b)

RHA passing t 325 m RHA pusing # 200

525

RHA (%I RHA (%)

(cl q Without RHA 0 RHA passing # 325

t25,- n RHA passing # 200

RHA (%)

Figure 3 (a) Effect of fineness on 3 day f,,; (b) effect of fineness on 7 day,&,; (c) effect of fineness on 150 day, /E,

#200 and #325 sieves, were denoted by Mix-Aa and Mix-Ab. The above two groups (Aa and Ab) were again divided into three sub-groups with variations of quanti- ties of RHA, i.e. Mix-Aa in which cement was replaced by 10, 20 and 30% of RHA passing #200 sieve were denoted by Aa-10, Aa- and Aa-30. Similarly the Mix-Ab was denoted by Ab-10, Ab-20 and Ab-30. Mix proportions are given in Table 4. The concrete was mixed by tradi- tional method in a tilting type concrete mixer. While observing the characteristics of fresh concrete of different mixes, it was noted that, in the concrete matrix when cement was replaced in different proportions of RHA, the workability decreased with increasing quantity of RHA.

This happened because the quantum of fine material in the concrete mix increased and it became difficult to have the same level of workability (good) even using plasticiz- ers. The authors had two options, of either keeping the workability level constant or keeping the w/c ratio con- stant. The latter opinion was tried first and in order to obtain the same level of workability with a w/c = 0.24 the doses of plasticizer were increased beyond the doses recommended by the manufacturer. This resulted in increased setting time and as such in subsequent stud& this was discarded. As such in subsequent tests the w/c ratio was varied and the workability was kept constant.

Test specimens From the above mixes 100 X 100 X 100 mm cubes and 100 x 200 mm cylinders were cast in standard moulds, consolidated on a vibrating table fixed with two one-hp motors accommodating 14 specimens at a time. The period of vibration was fixed by varying from 60-120 s with 30 s increments. The specimens which were sub- jected to vibration for 90-120 s resulted in segregation. The optimum results were noted when the vibration period was 60 s. Though in a few cases it was observed that the vibration period should be increased it was kept constant to maintain the same compaction period for all the specimens. Specimens were cured in the curing tank and were tested for compressive and splitting tensile strength of concrete after 3, 7, 28 and 150 days. Characteristics of fresh and hardened concrete are given in Table 5 and Table 6.

Results and discussion

The results confirm that it is quite possible to get a strength of 70 MPa in Pakistan with locally avail- able ingredients of concrete. The results also confirm that it is possible to get high strength concrete economically using RHA by burning locally available rice husk.

526 Effect of rice husk ash on high strength concrete: M. S. lsmail and A. M. Waliuddin

It is also observed that, even in crystalline forma- tion of RHA, good results may be obtained by fine grinding (Tubles 7 and 8). The results indicate that optimum replacement of cement by RHA will be around 10% to 20% with finely ground RHA.

The rate of hydration in concrete made with part replacement of cement by RHA is slow as compared to concrete with OPC only (Figure 3). This fact is very dominant during the initial three days of age of concrete. This rate of slow hydration also effects the 150 day strength of concrete made by part replace- ment of cement by RHA in the mix. The lower strength of concrete made with part replacement of cement by RHA is because of higher w/c ratios. Though the w/c ratio for samples Ab-10 and Ab-20 is 23 and 28% higher than w/c ratio in specimen-A, the 28 day strength is 98% and 97% of the concrete of Mix-A where no replacement was made. An improvement in mix design by keeping the w/c ratio constant in all types of sample will make an excellent study for further research. The effect of amorphous RHA as compared to finely ground crystalline RHA, which will be costly, will be another area of further study in the local conditions of Pakistan. Variation of strength between cube strength and cylinder strength decreases as the strength increases. This finding is reported by Neville”. The observa- tion of the authors do not match. On the contrary our observations indicate that the variation is more even compared to normal strength concrete. This aspect requires further in-depth study.

Conclusions

1. Both the broad objectives of the research, (a) possi- bility of achieving concrete strength over 70 MPa with locally available materials and (b) possibility of partial replacement of cement by an agro-waste (RHA) for HSC, were achieved in the study.

2. The constituents of concrete as available locally are sufficient to produce concrete of 70 MPa or more.

3. In this study the maximum strength was obtained with 10% replacement of OPC by RHA but the

authors feel that the optimum strength may lie with part replacement of RHA between 10% and 20%.

4. A durability study of HSC made with part replace- ment of cement by RHA should be conducted along with its economic aspects.

Acknowledgements

The authors would like to express their thanks to Dr A. Maher, Director General, NBRI, Karachi for allowing the collaborative R&D works with NED University of Engg., Karachi. The assistance of Sharf Naz, Seema Zameer, Asma Shaheen, Rana Anjum, Musarrat Naz and Darakhshan Raza, final year (civil) students of NED University of Engg. Karachi is also acknowl- edged. We are also thankful to Engr Minhajuddin Nasri, Mr Saleem Khatri, Mr Anwar and Mr Bashir of NBRI for their assistance.

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