Materials and Structures/MatOriaux et Constructions, 1987, 20,361-366

Use of rice husk ash in sandcrete blocks for

masonry units

M . A . R A H M A N

Lecturer in Civil Engineering, University of Ife, lle-Ife, Nigeria

Different mix proportions of sand, cement and rice husk ash (RHA) were studied for use in sandcrete blocks. Optimum water/(cement + RHA) ratios were determined at different mix proportions. Compressive strengths of various mix proportions at 7, 28 and 60 days were also determined. The optimum water/(cement + RHA) ratio increased with rice husk ash contents. Test results showed that up to 40% RHA could be added as a partial replacement for cement without any significant change in compressive strength at 60 days. Compressive strengths of various mix proportions were compared with British Statutory minimum compressive strengths of bricks for various walls and it was found that sandcrete blocks of 1:5 mortar mixes with 40% RHA (by weight of cement) could be used in both load and non-load bearing walls.

1. INTRODUCTION

The construction industry in developing countries has been suffering a depression due to many factors, one of which is the shortage of building materials; another is their increasing cost: there is a continual increase in the cost of conventional materials, particularly cement. As a result, most of the developing countries in West Africa and Southeast Asia are using low cost sandcrete blocks in different types of construction work.

Sandcrete is a mixture of sand, cement and water, and although sandcrete is serving an important role in construction work, yet not much research work has been done in this direction. In 1961, Tyler [1] published a work on sandcrete blocks at the former West African Building Research Institute, reporting the results of tests of their compressive strengths. Thomas [2] carried out tests on solid blocks with varying curing conditions and mix proportions. He recommended a mix propor- tion of 1:6 (cement:sand) for blocks for two-storey houses. Yilla [3] studied the effects of some factors on the shrinkage of sandcrete blocks. He reported that blocks with a cement content higher than 12% are more resistant to moisture absorption. Uzomaka [4] found that the crushing strength increases with decreasing specific surface of sand and that curing of blocks by water sprinkling enhances their strength.

The development of locally manufactured materials is advantageous from the point of view of both increas- ing manufacturing activity and reducing the need for imported materials. In developing countries, building materials should be relatively low cost due to low per- sonal incomes and small government expenditures on housing. Neyertheless, any means or ways by which the cost of materials can be reduced would be a welcome help to the construction industry. The idea of utilizing

waste materials in construction is well established. Apart from getting rid of these materials, their use in construction protects the environment from contamin- ation. Rice husks are a waste material obtained from the threshing of the rice and constitute about 20% of 300 million metric tons of rice produced annually [5].

The primary work on RHA was started at the Asian Institute of Technology by Columna [6]. Cook et al. [7] reported that rice husk ash obtained from the con- trolled burning of rice husk has pozzolanic properties. Mehta [8] reported that RHA in a highly reactive form could be used as a suitable raw material for making hydraulic cement. Mehta [9] also reported that RHA can reduce temperature in high strength mass concrete. Cook et al [10] used RHA with lime and cement for the production of low cost masonry units. A1-Khalaf and Yousif [11] investigated the use of rice husk ash in concrete. They used 1:2 and 1:3 mortar mixes. The local factories in Nigeria are using sandcrete block machines which give high degree of compaction. They practice sand/cement ratios by volume of between 6 and 13 to produce sandcrete blocks [4]. Even these selected mixtures are directly demouldable. Normal concrete gains highest strength when cured under water. However, sandcrete blocks having high sand/ cement ratio do not gain their highest strength when cured under water. This has been reported by Lasisi [12]. He also reported that the best curing method is to have the blocks protected from the sun and rain but to sprinkle them with water every day. This is the usual local practice in Nigeria.

The aim of this research work was to examine the effects of rice husk ash on the strength of sandcrete blocks and also to find the optimum percentage of rice husk ash that can be added without any deterioration in their strength.

362 M.A. Rahman

2. COMPRESSIVE S T R E N G T H TESTING

2.1 Materials

The materials used in this work were ordinary Portland cement, sand and well burnt rice husk ash. Only sand passing through a" US 20 sieve (0.84 mm size) and retained on a US 100 sieve (0.149 mm size) was used. The colour of the sand was light brown. Well burnt rice husk ash passing through a US 200 sieve (0.074 mm size) was used. Rice husks were burnt at the most convenient and economical temperature of 500~ [l l] for two hours. The colour of RHA was whitish grey.

2.2 Procedure

In order to determine the chemical composition of rice husk ash, 1 g of dried and powdered sample was taken and properly digested. The digested sample was diluted as is required for the determination of various ele- ments. The chemical composition of RHA was obtained with the help of an Atomic Absorption Spectrophotometer.

The procedure used in the mixing and preparing of the specimens of various sand-cement-RHA mixes was as follows:

1. Sand was spread out on a non-absorbent tray. The required proportion of cement was added on top of the sand and thoroughly mixed by hand until uniform col- our was obtained. Next, the premeasured amount of rice husk ash was added to the sand--cement mixture. Again, the RHA was mixed thoroughly. Then, the premeasured amount of water was added and mixing was continued until a uniform mixture was obtained.

2. Sand-cement-RHA mix was moulded in a 50 mm cube. The mould was filled with the mix in two layers. Each layer was compacted with a tamping rod at the rate of eight blows for sufficient compaction. Care was taken to ensure adequate and uniform compaction over the surface of each layer.

3. After casting, the specimens were placed immedi- ately in a room maintained at a temperature of about 20~ and with a relative humidity above 90% for 24 hours.

4. Although the selected mixtures were directly demouldable, the specimens were removed from the mould after 24 hours in order to be sure not to spoil any specimen during demoulding. The demoulded speci- mens were stored under plastic cover, then both speci- mens and plastic cover were covered with wet jute bags in order to decrease the evaporation and dehydration of the specimens.

5. Water was sprayed on the specimens twice a day until the day of testing. The specimens were tested after '7, 28 and 60 days.

Optimum water/(cement + RHA) ratios for various mix proportions of sand, cement and RHA were deter- mined. In order to determine the optimum water/ (cement + RHA) ratio for each mix, 15 specimens (3

specimens for each water content) were prepared and their compressive strengths determined after 7 days curing. A total of 150 specimens was tested for the determination of optimum water/(cement + RHA) ratios.

After identifying the optimum water/(cement + RHA) ratios for different sand-cement-RHA mixes, 50 mm cubical specimens were prepared with moulds for the determination of compressive strengths at 7, 28 and 60 days. Two series of specimens were prepared (shown in Table 2), using the same procedure in each case. The first series consisted of 1:4 mortar mixes (cement + RHA):sand. The second consisted of 1:5 mortar mixes (cement + RHA):sand. Each series com- prised 5 batches of 9 specimens each, i.e. 45 specimens per series. All specimens in a given batch were mixed to predetermined water and rice husk ash contents. RHA content was added for 10, 20, 30 and 40% replacement of cement (by weight). 9 specimens were prepared for each RHA content. 3 specimens were usually con- sidered for each set of data, i.e. a total number of 90 specimens in two series was prepared, cured and crushed for compressive strength test.

3. RESULTS AND DISCUSSION

The chemical analysis of rice husk ash by the Atomic Absorption Spectrophotometer showed that its main constituent was silica (89.5%). The ash also contained minor oxides of other elements such as potassium, sodium, calcium, magnesium etc. Loss on ignition of RHA was 3.59%.

On the basis of compressive strength at 7 days, the optimum water/(cement + RHA) ratios of various (cement + RHA): sand mixes were determined and are shown in Table 1. In each mix proportion, 5 different water/(cement + RHA) ratios were used and corres- ponding compressive strengths at 7 days were measured. The trend of changes of compressive strength with increase in water/(cement + RHA) ratio is presented in Figs. 1 and 2. When the water/(cement + RHA) ratio decreases below a certain value, the compressive strength also decreases. The water/ (cement + RHA) ratio corresponding to the maximum compressive strength was taken as the optimum water/ (cement + RHA) ratio. The determined water/(cement + RHA) ratios are 0.66, 0.70, 0.75, 0.81, 0.88, 0.75, 0.81, 0.88, 0.94 and 1.00.

The nature of changes of optimum water/(cement + RHA) ratio with rice husk ash content is presented in Fig. 3. The optimum water/(cement + RHA) ratio increases linearly with rice husk ash contents. It is the opinion of the present author that RHA has more affinity for water. It can be seen clearly from Fig. 3 that the optimum water/(cement + RHA) ratio also increases with the increase of sand proportion. This is expected because extra water is needed to wet the increased total surface area of sand in the mix.

The compressive strengths of the specimens made from various mix proportions at 7, 28 and 60 days are

Materials and Structures 363

Table 1 Determination of optimum water/(cement + RHA) ratio for different mix proportions

Optimum (Cement + RHA): Water/(cement 7-day strength water/(cement sand (by weight) + RHA) ratio (MPa) + RHA) ratio

0.55 8.45 0.60 8.70

(1.0 + 0.0):4 0.65 9.10 0.66 0.70 8.36 0.75 7.44

O.6O 8.08 0.65 8.26

(0.9 + 0.1):4 0.70 8.52 0.70 0.75 7.94 0.80 7.20

0.65 6.33 0.70 6.46

(0.8 + 0.2):4 0.75 6.60 0.75 0.80 6.02 0.85 5.48

0.7O 5.68 0.75 5.80

(0.7 + 0.3):4 0.80 5.98 0.81 0.85 5.54 0.92 4.86

0.76 5.22 0.82 5.38

(0.6 + 0.4):4 0.88 5.60 0.88 0.94 5.22 1.00 4.70

0.65 7.18 0.70 7.37

(1.0 + 0.0):5 0.75 7.68 0.75 0.80 7.15 0.85 6.82

0.70 6.35 0.76 6.43

(0.9 + 0.1):5 0.82 6.55 0.81 0.88 6.28 0.94 5.92

0.76 5.61 0.82 5.75

(0.8 + 0.2):5 0.88 5.93 0.88 0.94 5.60 1.00 5.21

0.82 4.71 0.88 4.83

(0.7 + 0.3):5 0.94 4.99 0.94 1.00 4.72 1.06 4.38

0.88 4.39 O.94 4.5O

(0.6 + 0.4):5 1.00 4.65 1.00 1.06 4.38 1.12 4.02

364 M . A . Rahman

8 "6 el.. : E

= 7 e- O/

II/ >

~ 5 C3.

0.5

x r, (1-0+0.0): ~, 0 - -0 (0.9+0-11:4

(0-8~-2 }: 4 O--O (0.7+031:4 I H (0.6+0.41: ~,

L t I I I . t

0-6 0.7 0-8 0.9 1.0 1.1 Woter/(Cement + RHA) ratio

Fig. 1 7-day compressive strengths for different water/(cement + RHA) ratios and cement-RHA-sand mix proportions (ratio of sand and cement + RHA = 4).

- - 7 13,_ :g r

~ 6 e'-

I&l

s

O . E o

0.6 I

0.7 0-8 0.9 1.0 1.1 1.2

' ' ' ([ement§ Send x ~ (1.0+0-0}: 5 O-O (0.9+0.1h 5

(0-8§ 5 t3--o 10-7+0.3): 5 H ( 0 - 6 + 0 - 4 1 : 5

Water/(Cement * RHA) ratio

Fig. 2 7-day compressive strengths for different water/(cement + RHA) ratio and cement-RHA-sand mix proportions (ratio of sand and cement + RHA = 5).

o 1-I o

_ 1.0 ' - r c~ 0.9 §

~ 0 . 8

0.7

0%

, , , , ,

o~o ( Cement+RHAllSond retie = 1 It, 1 H ( Q~ment§ ratio = 1/5

I

I I . I

10 20 30 ~0 Rice husk ash (%l

Fig. 3 Effect of RHA on optimum water/(cement + RHA) ratios of cement-RHA-sand mixes.

shown in Table 2. It is very clear from the results that the higher the percentage of R H A content, the lower the compressive strength at early ages; but there is no significant reduction in the compressive strength at 60 days ages. It is indicated that up to 40% (by weight of cement) rice husk ash can be added to the mortar mixes, i.e. up to 40% cement replacement is possible in the mortar mixes, without any deterioration in the compressive strengths of the specimens.

Table 2 Compressive strength at optimum water/(cement + RHA) ratio for different mix proportions

(Cement Optimum 7-day 28-day 60-day + RHA): water/(cement strength strength strength sand + RHA) ratio (MPa) (MPa) (MPa)

(1.0 + 0.0):4 0.66 9.07 13.15 16.70 (0.9 + 0.1):4 0.70 8.52 12.70 16.50 (0.8 + 0.2):4 0.75 6.58 10.20 16.35 (0.7 + 0.3):4 0.81 6.00 9.72 16.76 (0.6 + 0.4):4 0.88 5.62 9.44 16.37

(1.0 + 0.0):5 0.75 7.68 11.75 14.40 (0.9 + 0.1):5 0.81 6.58 10.40 14.70 (0.8 + 0.2):5 0.88 5.93 9.84 14.63 (0.7 + 0.3):5 0.94 4.99 8.48 14.60 (0.6 + 0.4):5 1.00 4.65 8.18 14.35

28-day/7-day, 60-day/7-day and 60-day/28-day strength ratios for different mix proportions are shown in Table 3. The trend of changes of these strength ratios with respect to rice husk ash are presented in Figs 4 and 5. These figures show that all these strength ratios increase almost linearly with R H A content. This is expected because of the pozzolanic properties of the RHA.

Table 3 Strength ratio for different mix proportions

(Cement + RHA): 28-day 60-day 60-day sand 7-day 7-day 7-day

(1.0 + 0.0):4 1.45 1.84 1.27 (0.9 + 0.1):4 1.49 1.94 1.30 (0.8 + 0.2):4 1.55 2.49 1.60 (0.7 + 0.3):4 1.62 2.79 1.72 (0.6 + 0.4):4 1.68 2.91 1.73

(1.0 + 0.0):5 1.53 1.88 1.23 (0.9 + 0.1):5 1.58 2.23 1.41 (0.8 + 0.2):5 1.66 2.47 1.47 (0.7 + 0.3):5 1.70 2.93 1.72 (0.6 + 0.4):5 1.76 3.09 1.75

3.0

27

o 2.4 0 ~- 2-I

~ 1 - 5

1.2 0

-x---x 28 day 'strength/7- da'y strength ' _ ~ o - o 60day strength/7- day strength

60 day strenglh/28-day slrencjj.b..--'~ - j ..3_ L - -

10 20 30 40 Rice husk ash(%)

Fig. 4 Effect of RHA on strength ratio at different ages (ratio of sand and cement + RHA = 4).

Materials and Structures 365

3.0~--~ 28doy st(engfn/7-doy strength ' - Io -o 60 doy ~mngth/7-doy strength

2 7 ~ 60 doy stmngtb/28-d:b, stmfi~h/,,, - /

2:t~ .(Cement+ R ~

~~ e - -

C41.5

1.2 0 10 20 30 t~O

Rice husk osh (~

Fig. 5 Effect of RHA on strength ratio at different ages (ratio of sand and cement + RHA = 5).

British Statutory minimum compressive strength of bricks for various walls is shown in Table 4 [13]; the present test results are compared with the British Statu- tory requirements. The minimum compressive strength requirements of load bearing walls according to the 1976 Building Regulations (for one- and two-storey houses) and 1972 GLC Construction By-Laws are 2.75 and 7.00 MPa, respectively. The test results show that the compressive strengths of all specimens at 60 days are much higher (14.35 to 16.76 MPa) than 2.75 and 7.00 MPa. Although the dimensions of the specimens have not been accounted for, it is expected that the compressive strength of the sandcrete block will fulfil the strength requirements. From the point of view of economy and compressive strength, a mix proportion of (0.60 + 0.40):5 can be recommended for load bear- ing walls.

Further investigation is necessary to evaluate the service properties such as durability, water absorption and chemical resistance. It is hoped that the results of such investigation will complement the present investi- gation in implementing the commercial use of rice husk ash in masonry units.

Table 4 British statutory minimum compressive strength of bricks for various walls

Regulations

Minimum compressive strength of bricks (MPa)

(a) Building regulations (1976) Load bearing walls for:

(i) One- and two-storey houses and houses divided into flats 2.75

(ii) Any other building (solid) 10.00 (iii) Any other building (hollow) 5.00

(b) GLC Construction By-laws (1972) (i) Load bearing walls 7.00

(ii) Non-load bearing walls (external) 2.75 (iii) Non-load bearing walls (internal) 1.50

4. C O N C L U S I O N S

On the basis of the test results of this investigation, the following conclusions can be drawn:

1. The optimum water/(cement + RHA) ratio increases linearly with increase in rice husk ash content.

2. The optimum water/(cement + RHA) ratio also increases with increase in sand/(cement + RHA) ratio.

3. The higher the percentage of RHA, the lower the value of compressive strength in both 1:4 and 1:5 mor- tar mixes at early ages.

4. The compressive strength of cement -RHA-sand mix mortar reaches nearly the same value correspond- ing to plain mortar at 60 days.

5. The optimum percentage of RH A that can be added to the plain mortar without any reduction in the 60-day strength is 40 (by weight of cement).

6. All the 28-day/7-day, 60-day/7-day and 60-day/28- day strength ratios are higher for a higher percentage of RHA. It indicates that the R H A used was a pozzolanic material. 7. From the point of view of strength requirements,

sandcrete blocks made of 1:5 (cement:sand) mortar mix with 40% R H A (by weight of cement) can be used in both load and non-load bearing walls.

ACKNOWLEDGEMENT

The author wishes to acknowledge Mr Kuti for his assistance in the testing.

REFERENCES

1. Tyler, R. G., 'Sandcrete blocks', West African Building Research Institute, Note No. 4 (1961).

2. Thomas, K., 'Influence of the curing conditions and mix proportions on the compressive strengths of sandcrete blocks', Mat. Struct. 00 (1964) 149-155.

3. Yilla, I. S., 'The effect of mix proportion and curing condition on shrinkage of sandcrete blocks', Mat. Struct. 00 (1967) 87-90.

4. Uzomaka, O. J., 'Some factors which affect the crushing strength of sandcrete blocks', Mat. Struct. 10 (1977) 45-48.

5. Cook, D.J . , Pama, R.P. and Darner, S.A., 'The behaviour of concrete and cement paste containing rice husk ash', Proceedings of a Conference on Hydraulic Cement Pastes, Their structure and proper- ties (University of Sheffield, 1976), pp. 268--283.

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

7. Cook, D. J., Pama, R. P. and Darner, S. A., 'Rice husk ash as a pozzolanic material', Proceedings of a Confer- ence on New Horizons in Construction Material (Lehigh University, 1976).

8. Mehta, P. K., 'Properties of blended cements made from rice husk ash', J. Am. Conc. Inst. 74 (9) (1977) 440--442.

9. Mehta, P. K. and Pirtz, D., 'Use of rice hull ash to reduce temperature in high strength mass concrete', J. Am. Concr. Inst. 75 (2) (1978) 60--63.

366 M . A . R a h m a n

I0. Cook, D. J., Pama, R. P. and Paul, B. K., 'Rice husk ash-lime-ce.ment mixes for use in masonry units', Build. Envir. 12 (1977) 281-288.

11. Al-Khalaf, M. N. and Yousif, H. F., 'Use of rice husk ash in concrete' , Int. J. Cem. Compos. and Light- weight Concr. 6 (4) (1984) 241-248.

12. Lasisi, F., "Performance characteristics of sandcrete blocks produced with a local hand-operated moulding machine', Stud. Envir. Des. in West Africa 5 (1985) 13-25.

13. Everett, A. , "Materials" (B. T. Batsford, 1978) 125 pp.