Rice husk and fly ash concrete blocks

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  • The Intemational Journal of Cement Composites and Lightweight Concrete, Volume 9, Number3 August 1987

    Rice husk and fly ash concrete blocks

    Julibn Salas, Marina Alvarez and Janer Veras*

    Synopsis This paper forms part of a project on materials, technology and prototypes of very low cost housing, Carried out at the I nstituto Eduardo Torroja (Spain). Results are presented of 40 x 20 x 20 cm blocks, with a concrete made up of an agricultural waste product, rice husk, and an industrial by-product, fly ash, and formed by means of a manual placement machine. The blocks have a density of about 11 O0 kg/m 3, a K value of 0-20W/mC and failure loads of 7000 to 8000kp.

    Keywords Masonry, lightweight concrete, insulation, rice husk, fly ash, agricultural wastes, concrete elements, compressive strength, thermal conductivity, appropriate technology, strength of materials, mix design

    INTRODUCTION Based on previous research [1-4] on the properties of concrete with a rice-husk base, the present work which is of an experimental nature, expounds the use of this range of materials (lightweight, insulating, and structural) in the form of building components that are easy to produce even in developing countries. For the use of these materials, the following prerequisites were estab- lished:

    (1) adequate manageability, without the need to resort to mechanical means;

    (2) technology to put this into effect that may be easily assimilated;

    (3) minimal investment in equipment; (4) flexibility of use (neutrality of the components); and (5) use of abundant waste materials (rice husk, and fly

    ash) and those that may be easily acquired (cement).

    Two basic elements, capable of satisfying the above requirements, were selected: a hollow block measuring 40 x 20 x 20cm and a 90 x 60 x 6cm panel. The present work describes the development and study of the properties of the hollow blocks.


    Production equipment The production equipment for these blocks involves minimal investment; it is solid and easy to handle and therefore represents an important step in the industriali- sation and rationalisation of the building process in developing countries. It involves a layer machine of the

    * Instituto Eduardo Torroja (C.S.I.C.), Apartado 19002, Madrid, Spain.

    Received 25 June 1980 Accepted 28 January 1987

    (~) Longman Group UK Ltd 1987


    manual placement variety (Figure 1)in which two speci- fic modifications have been introduced.

    (1) To provide wall thicknesses that are greater than normal so that they adapt better to the range of low resistance concretes.

    (2) Longitudinal tongue and groove, to improve behav- iour with respect to horizontal action.

    The block designed consists of a modular frame measuring 40 x 20 x 20cm with an overall horizontal section area of 791.3cm 2 and a net or resistant section of 448.5cm 2 giving an apparent volume of 8.521dm 3 (Figure 2).

    Each placement carried out by the machine pro- duces six blocks. The machine runs on electricity and allows for alterations in the mould for the shape of such elements.

    Mix proportioning Work has been conducted with three families of basic mixes referred to as L, C and M. Later on, a further mix B was definea; detads of these mixes are gwen later for comparative purposes.

    The basic or standard mix L is the same as that used by the authors in making up insulating concretes [1]. In order to obtain concretes with greater resistance and a more homogeneous texture, a 'filler' was added; fly ash in a natural state, originating from the Soto de Ribera plant [2] was used in a quantity equal to 25% of the weight of the cement (mix C) and 54% by weight (mix M) (Table 1 ).

    Five water/(cement + ash) ratios were used for each mix. Since vibration time appeared to be a fundamental parameter, all the mixes were vibrated for one minute; and four mixes were selected to be vibrated again for two and three minutes. Thus altogether twenty-four mixes were tested, with six blocks being made from


  • Rice husk and fly ash concrete blocks Salas, Alvarez and Veras

    Figure 1 Typical blocks and production equipment

    each mix, which were tested in pairs at 7, 28 and 60 days respectively.

    Analysis of results Figures 3, 4 and 5 show the average compression test results of the blocks; the tests were carried out with layers of felt between the block and the plates so as to regulate the load. The blocks were cured at a tem- perature of 15 to 17C. The figures show the breaking loads for the blocks as well as the compressive strength based on the net section of the block; the type of mix, water/(cement + ash) ratio, vibration time and average

    weight of two blocks before testing are also marked on each curve.

    The effect of adding fly ash was evident both in the outward appearance of the blocks, and in the results obtained regarding resistance. Dosages M (with 54% ash) showed marked improvements over dosages C and these" (25% ash), comparatively speaking, surpassed dosages L (without ash).

    The results show average densities of 573-744 kg/ m 3 for mixes L, 748--813 kg/m 3 for mixes C, and 990- 1065 kg/m 3 for mixes M. It should also be pointed out that adopting an average value of O. 12 W/m C [2] for this material, the coefficient of total thermic transmission, K,

    Figure 2

    i! -I-.e


    3 J. 12

    Dimensions of hollow block (all dimensions in cm)


  • Rice husk and fly ash concrete blocks Salas, Alvarez and Veras

    Table 1 Mix proportions by weight for six blocks

    Mixes with substituion of Mix with substitution of cement fly ash by fly ash

    Dosages L C M B

    P-ARI-450 cement (kg) 26.46 26.46 26.46 20.33 Husk treated with lime (saturated surface dry) (kg) 10.17 10.17 10.17 10.17 Fly ash from Soto de

    - - 6.61 14.27 20,33 Ribera (kg) % cement 100 80 66.6 50 % fly ash 0 20 33.3 50

    Water/(cement + ash) ratios 0.45 0.50 0.55 0.60 0.65

    should correspond to an approximate value of 0.20W/ m 2 oC.

    Mixes type L The average unit densities of all the mixes were low, falling within the region of 800 + 50kg/m 3, with the exception of L (0.65) (3'), (573kg/m3). As can be seen from Figure 3, the most suitable water-cement ratio is 0.60. The water-cement ratio of 0.65 was clearly excessive, especially for three minutes' vibration time, since it caused the mortar to segregate considerably, reducing the weight of the block and lowering its strength.

    Mixes type C The average densities of all the C dosages involved hardly any increase over the L mixes, remaining within

    the region of 800 + 50 kg/m 3. The most suitable water/ (cement + ash) ratios were within the range 0.50 to 0.65 for one minute's vibration time, giving resistances at 28 days of 4000 to 4500 kp. The increase in vibration time for the dosage with a water/(cement + ash) ratio of 0.65 was obviously counterproductive, and dropped noticeably between one and two minutes and, most appreciably (around 50%) when vibration time is increased from one to three minutes.

    Mixes type M The average densities of all the mixes were in the region of 950 + 100kg/m 3, with appreciable compressive strengths being achieved in comparison to the previous mixes. The most suitable water/(cement + ash) ratio turned out to be 0.60.

    An attempt was made to determine the influence of vibration time on the two extreme mixes M(0.45) and


    0 CD -






    L [0,55)( I')(6555]

    300( ](1'}(S431

    I II F /


    ,ooo ., %. - - L(O,eS)(3')(4aee:



    ll,O n


    B,8 II


    4,4 ._>

    Q. 2,2 ~: 0 (..}

    Figure 3 Compressive strengths of blocks of different L-type mixes and vibration times


  • Rice husk and fly ash concrete blocks Salas, A lvarez and Veras






    50001. - - -





    C(O, 50)(1')(6535) .______J. C(0,55}(1')(6608)

    C (0,65)(1')(7428)

    C(0,60)(1')(S701)/ C (0,65)(2')(6726)

    c (0,45) (I')(6373 )

    C (0,65)( 3' ) (6930)


    - -13 ,2


    - - 8,8

    - - 6,6


    - - 2,2


    C) tl


    Figure 4 Compressive strengths of blocks of different C-type mixes and vibration times


    Q. E 0 (J

    "o 00 0 {D'} t-



    kp J 90001 - - 19,8 80001 - - _ _ __ M(o ,6o~( -~(8~-~-~1;7 ,6

    7000 - - ~ ' ~ ....

    6000 - - " ~:4"~(3~e6 ~"~T M(0,651(3~(9074)

    5000 . . . . )(1')(7993)


    3000 M!Or65)(]'l166151

    M 10,4j~)(2') (7203

    M(0,651(2'1162031 2000

    1000 - - -

    ' ' 'o 7 28 6

    0 M(O,60)(1')(SlS5)


    - - . - 15,4








    Figure 5 Compressive strengths of blocks of different M-type mixes and vibration times




    E O (_)


  • Rice husk and fly ash concrete blocks Salas, Alvarez and Veras

    ,oTol 60001- -








    ---0 0,55

    --0 0,50



    ~'" 2~ 6b Days





    i 6,7







    E 0

    F igure 6 Compress ive s t rengths o f b locks o f d i f fe rent M- type mixes . V ib ra t ion t ime: one minute Water / (cement + f ly ash) rat io : 0.45; 0.50; 0.55; 0.60; 0.65

    "O D O ED E



    't 8000 -- 7000 i i ~ 6000 I

    5000 ~_





    B (9,460 Kg)a/(c+cv) = 0,45

    M(01185 Kg)q/(c',-cv) - = 0,60

    L(7,292 Kg)a/c =0,60 ~'0

    C {6,701Kg) a/(c cv) ~3 =0,60

    ! 28 6'0


    --'-- 15,6

    - - 13 ,4

    - - 11 ,2

    I 8 ,9

    - - 6 ,7

    i 4 ,5

    - - 2 ,2








    E 0 (D

    Figure 7 Compressive strength of blocks of L, C, M and B dosages, Vibration time: one minute



  • Rice husk and fly ash concrete blocks Salas, Alvarez and Veras

    M(0.65). In both cases, the results are noticeably contra- dictory: a reduction in resistance between one and two minutes of vibration time, and an increase when vibration was increased to three minutes. This phenomenon cannot be accounted for. For the mix M and water/ (cement + fly ash) ratio = 0.60, the increase in vibration time from one to two minutes is seen to be beneficial from the point of view of compressive strengths at all ages.

    Mixes type B It would be appropriate to draw attention, particularly in the light of the results given in Figure 6, to the fundamen- tal influence of an adequate water/(cement + fly ash) ratio: a sharp drop in resistance when passing from 0.60 to 0.65 follows the progressive increase in resistance for increases in such ratio from 0.45 to 0.60.

    In general, it may be concluded that no appreciable advantage of any kind is to be found in moving from mix type L to type C; however, there is a considerable, practical improvement when moving from the latter to M-type mixes. This can be seen clearly in Figure 7, which gives the results for water/(cement + fly ash) ratio = 0.60 and one minute's vibration time.

    In the light of these results it was decided to increase the quantity of ash again by an appreciable amount in such a way that a new dosage B was produced with the following .composition:

    P-ARI-450 cement Soto de Ribera ash Treated husk (S.S.D.) Water Water/(cement + fly ash) -- 0.45

    20.33 kg (40%) 20.33 kg (40%) 10.17 kg (20%) 18.30 litres

    CONCLUSIONS Comparatively speaking, the results of this new mix are

    clearly satisfactory since they include the following points:

    1. An increase in compressive strengths with respect to mix M.

    2. Savings in cement amounting to roughly 30% with respect to mix M, since 4.78kg of cement are used per block instead of 3.38 kg.

    3. A really minimal increase in weight, with respect to mix M, of the order of 3%, since the average weight per block increased from 9.185kg to 9.460kg.

    In light of the above data, mix B has been adopted (4/4/2 by weight of P-ARI-450 cement, fly ash, treated husk) and water/(cement + fly ash) ratio = 0.45, as the most suitable for blocks made up with an industrial by-product (fly ash), an agricultural waste product (rice husk) and a portland cement.

    REFERENCES I. Salas, J. and Veras J. 'Insulating panels with rice

    husk', International Journal for Housing Science and its Applications, Miami, U.S.A., Vol. 10, No. 1, 1986, pp. 27-32.

    2. Salas, J., Alvarez, M. and Veras, J. 'Materiales de construcci6n con propiedades aislantes a base de c~scara de arroz', Informes de la Construcci6n, Madrid, Spain, Vol. 37, No. 372, July 1986, pp. 1-12.

    3. Salas, J., Alvarez; M. and Veras, J. 'Light/insulating concretes with rice husk', The International Journal of Cement Composites and Lightweight Concrete, Vol. 8, No. 3, August 1986, pp. 171-80.

    4. Salas, J., Alvarez, M. and Veras, J. 'Algunas com- probaciones previas para un programa experimen- tal sobre hormigones de baj0 coste', Boletin Oficial del Colegio de Aparejadores, Madrid, Spain, No. 83, October 1985, pp. 52-7.



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