Building and Environment, Vol. 23, No. 4, pp. 303-308, 1988. 0360-1323/88 $3.00+0.00 Printed in Great Britain. © 1988 Pergamon Press pie.

Pozzolanic Properties of Rice Husk Ash, Burnt Clay and Red Mud

M. R. YOGANANDA* K. S. JAGADISHI"

In this paper materials like rice husk ash, burnt clay and red mud are examined for their pozzolanic properties. Rice husk ash, obtained from various sources, is analysed by X-ray diffraction. Compressive strength properties of lime-pozzolana mortars with rice husk ash, burnt clay and red mud as pozzolana are studied. Influence of grinding of rice husk ash and intergrinding with lime are also investigated. Combination pozzolana with partial replacement of burnt clay and red mud by rice husk ash are examined for their pozzolanic properties. Long term strength behaviour of lime-pozzolana mortars is investigated to understand the durability of lime-pozzolana cements.

INTRODUCTION

ABOUT 50% of the portland cement used in building construction is consumed for secondary construction applications such as masonry and plastering. The strength potential of portland cement is never fully util- ized in such applications. The strength requirements in such works are of the order of 4.0 MPa, while portland cement is ideally suited for applications with strength requirements in excess of 15.0 MPa. It is also not adequately appreciated that pure portland cement mor- tars are harsh and lack the plasticity that is very much needed in masonry construction.

Lime-pozzolana cements can replace portland cement in such secondary construction activities. The term poz- zolana designates reactive siliceous and/or aluminous materials, which react with lime (calcium hydroxide) in the presence of moisture to form stable cementitious com- pounds. The arguments in favour of lime-pozzolana cements are: (1) mortar strength requirements of the order of 4.0 MPa can be easily attained at favourable costs ; (2) they can be produced from second grade lime- stones or kankar deposits; (3) they do not depend on pulverized coal. Firewood can be used as fuel; (4) they can be produced in small decentralized plants which can generate a greater volume of rural employment while reducing transportation costs.

Earlier work by Mehta [1] and Cook et al. [2] have shown that alternative cements can be made from rice husk ash for use in building construction. However, there is not enough information regarding the influence of ash grinding before intergrinding with lime on the strength of lime-rice husk ash cements and also on the long term behaviour of lime-rice husk ash cements. It is also necess- ary to examine the utilization of other pozzolana like burnt clay and red mud. It is now well understood that

* Karnataka State Council for Science and Technology, Indian Institute of Science, Bangalore 560 012, India.

1" Centre for ASTRA and Dept. of Civil Engineering, Indian Institute of Science, Bangalore 560 012, India.

rice husk, when burnt under controlled conditions, can produce an amorphous ash of high lime reactivity. A number of methods of rice husk burning are available [3-5]. However, most rice husk is used in India as a fuel for operations like parboiling of paddy, brickmaking, cooking in hotels etc. and the process of burning is rarely controlled to produce amorphous ash. There is a need to explore the processing of such ash to increase its reactivity. It would also be useful to develop new burning processes to produce amorphous ash while using the heat from the husk in an industrial operation.

This paper discusses the pozzolanic properties of different types of rice husk ash, burnt clay and red mud under the influence of the factors mentioned above.

TYPES OF POZZOLANA

In this paper, the behaviour of rice husk ash (RHA) produced in a variety of ways, both in laboratory and field conditions is considered. Samples of burnt clay (BC) and red mud (RM) have also been considered for pur- poses of comparison. The various pozzolana tried may be briefly described as follows.

R H A - A 1 Rice husk is burnt in a small enclosure of size

0.3 m x 0 . 3 m x0.38 m constructed using fire bricks. The burning of rice husk is allowed to take place in the open air. The burning proceeded from the top surface slowly downwards till the rice husk at the bottom of the enclosure is burnt. The maximum temperature attained is observed to be around 500°C.

R H A - A 2 Rice husk is used to fill a cylindrical pit of diameter

1.0 m and height 1.0 m dug in the ground. The burning is allowed to take place from the top surface in a similar manner to RHA-A1. The maximum temperature observed is of the order of 500°C. The X-ray diffraction

8AE 23:~-D 303

304 M. R. Yoyananda and K. S. Jagadish

O =Quar tz

C5= Crystall ine silica in the form of ¢ristobati te

RHA-A2

RHA-A3

O

RHA-A4

O Q

• . o . , . , 32 266 219 207 15 °

C5

C5

C5

I I 32" 21.9" 20.9"

Diffraction angle 20 in Diffraction angle 2e in degrees (CuK~r) de~'ees (CuKcC)

Fig. 1. XRD of the various RHA samples.

(XRD) pattern of the ash indicates that the ash is con- taining silica in the amorphous state (Fig. 1).

RHA -A 3 An annular enclosure as shown in Fig. 2 is used to

burn rice husk and the heat utilized for producing hot water. Rice husk fills in the space between the two coaxial cylinders of wire mesh, the grating being being placed at a height of 20 cm above the ground. Rice husk is set on fire at the bottom and the burning is allowed to take place from bottom to top. Hot water is produced through

I I I

: : : : : : : . : : I ' " . " ' , ' : I , ' ' " - . ; ;,:1

1" . . r : ",1

0.75 m ,~

0.27 m - ~

" - ' " - - - ; : " : " , " ' : : ; ':S ' / I Annul . . . . h enclosure

i < ~ , - , i r : :L ' . " ' - ' . J I " : '-" : - - " 't

h" ; " . ' ' . : . i

t . . , , . . , - . , . ' , 1 I . " . , . . . '_" ' I . - . ~..'?.~,.'l

~". :., '~L'- - . ' t ]" ,:"-: , ' , " ; : . "J

0 2 m : . " " - , " .'1 I ? ' " ; ~ : ~ " " I Hot w&ter • ' " : ' : . O . : .." I :-~. ' ~ . , , ;

Cold water ~ " ~ Copper lube

Fig. 2. Annular rice husk burner.

2.95 m ~ -

~ .65 ~ ~- - ~

~ R i c e husk

~ t w a t ~ e r

Control v&tve

H II II II II II II II i ~ ~ .

II il I1 II II II II 11 lt-------~5¢rnl [ wide II II It II II It II II rang }t II II 11 11 I ope

SIDE ELEVATION

Fig. 3. Rectangular rice husk burner.

the coiled copper tube placed in the enclosure. Tem- peratures of the order of 600°C are observed. The per- centage heat extraction in producing hot water is deter- mined taking the calorific value of rice husk as 3000 kcal kg-~ and is found to be equal to 12.0. The XRD of the ash (Fig. 1) shows the silica particles to be of an amorphous nature. The quartz peak in the XRD is due to contamination [6].

RHA-A4 Another experiment to produce hot water in a narrow

rectangular enclosure (Fig. 3) is attempted. This en- closure is built using soil blocks with a number of small openings in the walls for air entrance. Rice husk is filled to the top and a G.I. pipe is placed about 10 cm below the top. Burning of rice husk is started from the top and as the burning zone decends the G.I. pipe carrying water sinks with it. Temperatures recorded are of the order of 600°C. The heat extracted from this operation is found to be 10.0%. The XRD of the ash powder shows the presence of amorphous silica (Fig. 1). The quartz peaks are due to contamination.

RHA-A 5 Rice husk is burnt for cooking operations in a hotel at

Kunigal, Tumkur district, Karnataka, India. The ash obtained from this hotel is analysed and the XRD shows the presence of a mixture of amorphous and crystalline silica. The crystalline nature can be noticed by the pres- ence of a cristobalite peak in the XRD (Fig. 1).

RHA-A6 This sample of ash is obtained from a hotel at Yeda-

vani, Kunigal Tahik, Tumkur district, Karnataka. The XRD of the ash powder again shows the presence of a mixture of amorphous and crystalline silica. A cristo- balite peak can be seen in the XRD (Fig. 1).

RHA-A7 This sample of ash is produced in a parboiling oper-

ation. It is obtained from Tamilnadu Civil Supplies Cor- poration Ltd., Tamilnadu, India. Parboiling is a process in which scalding of paddy is done to produce boiled rice. The ash is found to contain mainly crystalline silica which

Pozzolanic Properties of Rice Husk Ash, Burnt Clay and Red Mud 305

can be seen from the pronounced cristobalite peak in the XRD (Fig. 1).

RHA-A8 Rice husk is also used for brick burning operations.

One such sample of RHA is procured from Davangere, Chitradurga district, Karnataka. The XRD of the ash powder show that the silica is present mainly in the crys- talline state. A sharp cristobalite peak can be seen from the XRD (Fig. 1).

Burnt clay(BC) Clay, when burnt at 700-800°C, produces amorphous

silica and alumina. Broken tile pieces when ground to a fine powder ( < 90 #m) offers itself as a good burnt clay pozzolana. One such sample is obtained from a title fac- tory at Bangaiore, Karnataka. It contains mainly 50.0% silica and 20.0% alumina.

Red mud(RM) Red mud, a waste product of an aluminium factory, is

collected from Belgaum, Karnataka. It contains mainly a lumina-- 19.8 %, silica--7.9 %, ferric oxide--40.5 % and titanium oxide- - 15.5 %.

DETAILS OF TESTING

The lime reactivity of various pozzolana is tested by evaluating the compressive strength of standard 5 cm cubes of lime-pozzolana mortar. The various steps involved in the testing are described below.

Grinding of pozzolana and intergrinding it with lime Grinding of various types of RHA, AI -A8 , is carried

out in a laboratory ball mill for definite durations. Burnt clay is obtained in a fine powder form from tile factory, where broken tile pieces are ground in a factory ball mill. Red mud in a fine form is obtained from aluminium factory. Intergrinding of lime (Laboratory grade) and pozzolana is carried out in the laboratory ball mill for specific durations.

Preparation of lime-pozzolana mortar The interground lime--pozzolana is mixed with stan-

dard sand in a ratio of 1 : 3 by weight and the mixing is done by hand. The quantity of water for mortar pre- paration is controlled to achieve workable consistency.

Moulding and curing the specimens Standard cube moulds of 5 cm size are used to prepare

the test specimens. Moulding of the specimens is started soon after the mortar preparation. Moulding is done in two layers, each layer of mortar being tamped 32 times with a tamping rod [7]. On completion of tamping, the top of the moulds are smooth finished after removing the excess mortar. The moulds are covered with wet burlap for 48 h. The mortar cubes are then removed from the moulds and placed in moist burlap. At the end of 7 days, the specimens for 28 and 180 day tests are kept immersed in clean water in a storage tank.

Compressive strength tests At the end of 7, 28 and 180 days the specimens are

removed and tested on their sides up to failure in a compression testing machine. The maximum load at fail- ure is noted. A minimum of three cubes are tested for each curing period and the average compressive strengths are reported.

COMPRESSIVE STRENGTH RESULTS AND DISCUSSION

Compressive strength of lime-pozzolana mortars The results of the compressive strength tests are

reported in Table 1. It can be seen from the table that lime-pozzolana mortars produced from RHA, BC and RM as pozzolana have yielded 28-day strengths ranging from 7.5 to 21.6 MPa, much higher than the required strength of 4.0 MPa for secondary construction appli- cations.

Lime-RHA mortar using RHA-A1 containing highly reactive silica has produced the highest 28-day strength of 21.6 MPa. The 28-day mortar strengths in case of RHA samples A2, A3 and A4 containing amorphous silica are 17.9, 15.3 and 16.0 MPa respectively. The effect

Table 1. Compressive strength of lime-pozzolana mortars

Type of pozzolana

Mortar proportion by weight

lime : pozzolana : sand

Duration of grinding of pozzolana (min)

or fineness of pozzolana

Duration of intergrinding Average compressive of lime and pozzolana strength (MPa)

(min) 7 days 28 days

A1 A2 A3 A4 A4 A4

A5 A6 A6

A7 A8

BC RM

1:3:12 1:2:9 1:3:12 1:3:12 1:2:9 1:1:6

1:2:9 1:2:9 1:3:12

1:2:9 1:2:9

1:2:9 1:2:9

120 120 120 120 120 120

120 120 120

120 120

< 90/~m < 90/tm

60 60 60 60 60 60

60 60 60

15 60

60 60

17.9 21.6 16.5 17.9 10.3 15.3 10.0 16.0 10.2 13.9 10.4 13.0

4.9 15.5 8.9 15.8

10.9 14.6

4.1 8.3 4.1 9.1

5.3 9.4 6.5 7.5

306 M. R. Yogananda and K. S. Jagadish

Table 2. Influence of grinding of RHA and intergrinding of RHA with lime

Type of pozzolana

Mortar proportion by weight

lime : RHA : sand

Duration of grinding Duration of intergrinding Average compressive of RHA of RHA and lime strength (MPa)

(min) (min) 7 days 28 days

A1 A1 AI

A4 A4

A7 A7 A7 A7

1 3 : 1 2 1 3 : 1 2 1 3 : 1 2

1 2 : 9 1 2 : 9

1 2 : 9 1 2 : 9 1 2 : 9 1 2 : 9

60 60 15.2 17.0 0 180 13.4 15.3

120 60 17.9 21.6

0 5 2.2 3.3 120 5 11.4 16.1

0 15 1.0 2.6 120 15 4.I 8.3 240 15 4.6 10.i 360 15 5.7 13.0

of mortar proportion on the compressive strength in the case of RHA-A4 can be seen from the table. The mortar with lime to ash ratio of 1 : 3 by weight has yielded a 28- day strength increase of 15.1 and 23.1% as compared to proportions of 1 : 2 and 1 : 1 respectively. The mortars with RHA-A5 and A6, containing a mixture of amor- phous and crystalline silica have resulted in a 28-day strength of 15.5 and 15.8 MPa respectively. Lime to ash proportion of I : 2 by weight in the case of RHA-A6 has shown an increase of 8.2% in the 28-day strength as compared to 1 : 3 by weight. It appears that lime to ash proportion of 1:3 by weight is suitable for amorphous ash, while it should be reduced to 1 : 2 in the case of a mixture of amorphous and crystalline ash as far as 28- day strength is concerned. Lime-RHA mortars with A7 and A8, containing mostly crystalline silica, have pro- duced relatively lesser 28-day strengths of 8.3 MPa and 9.1 MPa respectively. Lime-pozzolana mortars with BC and RM as pozzolana have given a 28-day strength of 9.4 MPa and 7.5 MPa respectively.

In general, lime-pozzolana mortars with RHA as poz- zolana produce higher 28-day mortar strengths than BC or RM as pozzolana [8]. However, RHA containing mostly crystalline silica is less reactive and hence lead to relatively lower 28-day mortar strengths as compared to amorphous RHA. The RHA A1 and A2 are more reac- tive than other RHA samples where heat from the burn- ing rice husk has been utilized for various purposes.

Effect of pregr&din9 of RHA The influence of grinding of RHA prior to inter-

grinding with lime on the compressive strength of mor- tars is presented in Table 2. The 28-day compressive strength in the case of RHA-A1, with 120 min grinding and 60 min intergrinding with lime, is 21.6 MPa. This is observed to be 41.2% more than that with only inter- grinding with lime for 180 min and 27% more than that with 60 min grinding and 60 min intergrinding with lime. These results clearly indicate the necessity of pregrinding of RHA in order to achieve higher mortar strengths. Tests carried out on RHA-4, ground for 120 min and interground for 5 min with lime, shows nearly five times increase in the 28-day mortar strength compared to only intergrinding with lime for 5 min. It must also be observed from the table that mortars with RHA-A7, contain- ing mostly crystalline silica, have also yielded higher strengths with continued grinding of RHA. The 28-day strengths for different durations of grinding viz. 0,

120, 240 and 360 min with 15 min intergrinding with lime are respectively 2.6, 8.3, 10.1 and 13;0 MPa. It is apparent from these results that even though the RHA contains crystalline silica the lime-reactivity of the RHA can be greatly enhanced by prolonged durations of grinding.

Combination pozzolana Lime-pozzolana mortars using BC and RM have given

lesser 28-day strengths compared to most of the lime- RHA mortars. It is hence felt that a combination poz- zolana by replacing a portion of either BC or RM by RHA may improve their compressive strengths. Accord- ingly mixing of BC and RM with RHA is attempted. The results are reported in Table 3.

In the case of BC, a partial replacement of 25.0% by RHA-A1 resulted in doubling of the 28-day strength of lime-BC mortar. It may also be noted that a similar strength increase in the 28-day strength is observed in the case of RM with a partial replacement of 25.0% by RHA- A4. However, the 28-day strengths of mortars with only RHA are slightly higher than the 28-day strength of mortars using combination pozzolana.

Long term strengths Long term behaviour of lime-pozzolana cement is very

important to assess its durability. Accordingly long term tests (6 months) are conducted and the results are as shown in Fig. 4. The 6-month tests on lime-RHA mortars

28

24

2 0 .C

P

o = ~2 .==

E 8 g

A5

A6 AI A3 A?

A2

R N

c t I I '7 28 180

Duration of cu r i ng in days

Fig. 4. Long term strength of lime-pozzolana mortars.

Pozzolanic Propertiokof Rice Husk Ash, Burnt.~lay afzd Red Mud

Table 3. Compressive strength of mortars using combination pozzolana

307

Type of pozzolana

Duration of Fineness Duration of Mortar proportion grinding of BC intergrinding of lime

by weight of RHA or RM and pozzolana lime : pozzolana : sand (rain) (gm) (rain)

Average compressive strength (MPa)

7 days 28 days

BC 1:2:9 - - A1 I : 3 : 12 120

25% replacement of BC by RHA-A1 1 : 3 : 12 120

RM 1:2:9 - - A4 1:2:9 120

25% replacement of RM by A4 1 : 2 : 9 120

<90 60 5.3 9.4 - - 60 17.9 21.6

<90 60 13.2 18.8

<90 60 6.5 7.5 - - 60 10.2 13.9

<90 60 9.8 13.7

using A1 and A2 which contain highly reactive silica have shown a decrease in the compressive strength after 28 days of curing by an amount equal to 8.0 and 31.5% respectively. This decrease in the long term strength may be due to the presence of unreacted reactive silica [9]. However, the reasons are yet to be fully understood. It can be seen from the figure that long term strength in all other cases show an increase over the 28-day strength ranging from 6.0 to 140.0%. Lime-RM mortar yielded the lowest 6-month strength gain of 6.0% over its 28- day strength. Lime-RHA mortars with A3 and A4 have shown a 6-month strength increase of 26.8 and 18.8% over the 28-day strength respectively. The 6-month strength increases in the case of RHA-A5 and RHA-A6 over their corresponding 28-day strengths are 61.5 and 43.7% respectively. It must be noted that higher 6-month strength increases of 117.0 and 140.0% over the 28-day strength have been observed in the case of RHA con- taining mostly crystalline silica, viz. A7 and A8 respec- tively. The 6-month mortar strength of 27.0 MPa in the case of BC (75%)+A1 (25%) as against 20.0 MPa with only RHA-AI and 13.0 MPa with only BC supports the idea of using combination pozzolana.

CONCLUSIONS

(1) The results of the compressive strengtl~ tests pre- sented in this paper indicate that lime--pozzolana mortars using RHA, BC or RM as pozzolana satisfy the require- ments for secondary construction applications like masonry and plastering.

(2) The RHA samples containing amorphous silica have" shown variations in their pozzolanic properties. The RHA samples, where the rice husk is burnt without utilizing its heat value, have lead to higher 28-day strength mortars. They attain 80-90% of the 28-day strength at the age of 7 days only. However, the long term strength results do not appear to be encouraging as there is a decrease in strength after 28 days. In case of RHA, where the heat from the husk has been utilized for producing hot water, the lime-RHA mortars have shown an increase in the long term strength. The 7-day strengths are however lower than in the earlier case. The reasons for these variations is yet to be fully understood.

(3) Lime-RHA mortars with RHA containing a mix- ture of amorphous and crystalline silica lead to higher long term strengths.

(4) Results of l ime-RHA mortars with RHA con- taining mostly crystalline silica have shown that pro- longed grinding of crystalline RHA enhances its lime- reactivity. The long term strength gain over 28 days is much higher than in case of other RHA samples.

(5) Pregrinding of RHA before intergrinding with lime is essential to achieve higher strength mortars.

(6) Partial replacement of BC and RM by RHA greatly improves the compressive strengths of lime-BC and lime- RM mortars.

Acknowledgements---The authors would like to acknowledge the very useful discussions they had with Profs R. Kumar, A. K, N. Reddy and M. Subba Rao and Dr J. James. The financial sup- port provided by the Karnataka State Council for Science and Technology is gratefully acknowledged.

R E F E R E N C E S

1. P.K. Mehta, The chemistry and technology of cements made from rice husk ash. Proceedings of a Joint workshop held in Peshawar, Pakistan, pp. 113-122 (1979).

2. D.J. Cook, R. P. Pama and B. K. Paul, Rice husk ash-lime-cement mixes for use in masonry units. Bldg Envir. 12, 281-288 (1977).

3. P.K. Mehta and N. Pitt, A new process of rice husk utilisation. Proc. Fourth International Conference of Rice By-products Utilisation, Valencia, Spain (1974).

4. Arjun Das and Mohan Rai, Prospects and problems in the production of cementitious materials from rice husk. Proceedings of a Joint workshop held in Peshawar, Pakistan, pp. 49-56 (1979).

5. R.D. Shrestha, Industrial utilisation of agro-wastes for making cement like materials. Proceedings of a Joint workshop held in Peshawar, Pakistan, pp. 77-87 (1979).

308 M. R. Yogananda and K. S. Jagadish

6. J. James and M. Subba Rao, Characterisation of silica in rice husk ash. Bull. Am. Ceram. Soc. 65, 1177-I 180 (1986).

7. Indian Standards Institution, Specification for lime-pozzolana mixture. IS 4098 (1967). 8. M.R . Yogananda, K. S. Jagadish and R. Kumar, Studies on surkhi and rice husk ash pozzolana,

Alternative Building Series-9, ASTRA, Indian Institute of Science (1983). 9. J. James, Ph.D. Thesis, Indian Institute of Science, Bangalore, India (I 986).