EFFECTS OF SAWDUST ASH ON THESTRENGTH OF LATERIZED CONCRETE

Funso Falade *

ABSTRACTThe construction industry in Nigeria is presently operating at a low ebb duemainly to the high cost of construction materials. The use of local materialshas been advocated as a step towards solving this problem. This paperconsidered the use of sawdust ash (SDA) as a partial replacement of cement inlaterized concrete. Compressive strength tests of samples made from mixproportions 1:1.5: 3,1 :2:4 and 1:3:6 at various curing ages (7,14,21 and 28days) for varying SDA content (0,5,10,15,20 and 25%) but at constantwater/cement SDA ratio for each mix proportion were considered. It wasfound that the addition of SDA decreases the strength. The rate of gain ofstrength, however, was more rapid at curing ages of 21 and 28 days,especially in the mixtures with high percentages of SDA. Strengthdevelopment was also found to be higher for mixes with loweraggregate/cement ratios thanfor those with higher ratios.

1.0 INTRODUCTIONOne of the problems facing the construction industry in Nigeria is the scarcityand consequently the high cost of construction materials. This hampers theprovision of adequate housing (quantitatively and qualitatively) for theteeming population of this country.

Cement and aggregates (fine and coarse) are the basic components of theconcrete that is used in building construction. The existing cement factories inNigeria are strained by demand; the quantity of cement produced isinsufficient and adverse economic conditions ensure that the prices are high.Also, the costs of conventional aggregates are high and these materials aregetting more expensive everyday. Thus, there is need for the identification ofalternative construction materials.

An approach to tackling this problem is to attempt to indigenise the methodsand materials of construction. Researchers [1,2,8] have found that laterites

'" Dept. of Civil Engineering. Obafemi Awolowo University. Ile-Ife, Nigeria.

can be used as fine aggregates. Laterite is a weathered red soil foundextensively in the tropical regions of the world and is readily available inNigeria. Stabilisation of this material is important so that its properties willmeet various construction requirements. The use of cement offers the bestand most common method of achieving this stabilisation. However, cementescalates the total cost of this material leaving the industry with the initialproblem of high cost. Hence, any cheap and local stabilizer that can replacesome or all of the cement being used for this purpose will be welcome to theindustry. The recognition of this problem led to the investigation into thesuitability of materials like rice husk ash and pulverised fuel ash, derived fromwastes, for use as stabilising agents. The cost-effectiveness of these agentsis, however, dependent on the quantity available. In Nigeria, large quantitiesof these materials are not easily available so that there is still need to look forcheaper alternatives.

Sawdust ash is one of the materials that has not been properly investigated asa stabilising agent. It is obtained by burning sawdust in air and Nigeria hasthis waste material in abundance. The amount of sawdust presently beingused as fuel and for other purposes is insignificant when compared with thequantity available, and it constitutes a nuisance to sawmills by occupyingmuch of their available land space.

If, after partial replacement of cement with sawdust ash (SDA), the propertiesof the mixture are found adequate for concrete making, then SDA will notonly become of value as a local raw material in construction, and, thereby,help to offset the huge amount of money being spent on cement, but alsosolve the disposal problem posed by sawdust.

The major motivation of this project is to find out the extent to which the ashfrom sawdust can be used to replace cement in concrete for constructionpurposes.

2.0 PREVIOUS WORKIn recent years, a lot of research work has been going on in the field of lateritetechnology. All the efforts are geared towards determining the usefulness oflaterite and other soils in construction and ways of improving their propertiesto make them suitable for construction. One of the early works on laterite wasby Adepegba [1] who compared the strength properties of normal concretewith those of laterized concrete. He found that a concrete in which lateritefines are used instead of sand, can be used as a structural material in place ofnormal concrete. Lasisi and Osunade [2] investigated the effect of grain sizeon the strength of cubes made from lateritic soils. They established that for

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West Indian Journal ofEnginuring. Volume 15. No.1. January 1990

lateritic soils to be of economical use in the industry, the range of particle sizesused in moulding blocks must tend towards the silt fraction.

Ola [3] found that less than 50 percent of the cement requirement for thetemperate zone soils is required for efficient stabilization of lateritic soils forroad sub-grade work. Lasisi [4] found that about 10 percent of cement will beneeded to stabilise lateritic soils to produce blocks with strengths of the sameorder of magnitude as sandcrete blocks, for use as masonry units in buildingconstruction. Medisa [5] also reported that lateritic soils in the Okitipupa areaof Ondo State need only 10-12 percent of cement for stabilising to becomereliable for building purposes. Okunnu [6] established that certain stabilisedlateritic soils can compare favourably with sandcrete. He also concluded thatthe compressive strength of sandcrete blocks increased with curing age.According to Lasisi and Ogunjido [7], the compressive strength decreases asthe laterite/cement ratio increases and the compressive strength is alsoinversely proportional to the grain size.

This intensive research into soil stabilisation has also shown that not onlycement can be used but also other agents like lime, bitumen and rice husk ash.Rahman [10] has reported that the unconfined compressive strengthparameters are found to be highest at around 17 percent rice husk ash and thisis the optimum amount of ash required to stabilise the tested lateritic soil forsub-base materials in highway construction. The use of fibre-reinforcement(local rope with a mean tensile strength of 50N/mm2) as a way of improvingthe properties of units of construction has been reported by Akinmusuru andAdebayo [8]. They found that maximum compressive strength was obtainedusing about 2.5 percent by weight of fibres. This compressive strength wasan improvement of up to 50 percent over that of unreinforced blocks. Thestudy by Lasisi and Ogunjide [7] also indicated an optimum water/cement ratio(for their laterized concretes) with the following relationship:

Y c:: -0.90 + 3.85X

where Y = laterite/cement ratio andX = water/cement ratio

A recent study by Lasisi and Osunade [9] confirmed the existence of theoptimum water content. The work also showed the relationship between thecompressive strength and the lateritic soils to river sand proportions. It wasreported that the maximum compressive strength was obtained when theproportion of river sand in the lateritic soil was 60 percent. Each of thestudies referred to here was geared towards attaining results that wouldenable the proper utilisation of laterite.

73

Falode

3.0 MATERIALS AND EXPERIMENTAL PROCEDUREThe lateritic soil used for this project was collected from a borrow pit alongthe Ife-Ibadan road. The large lumps were crushed and sieved through ASTMsieve No.8 (aperture 2.36mm). The lateritic samples were reddish in colour.The general properties of the laterite were determined by various laboratorytests. These tests were carried out in accordance with British Standardspecifications (BS 1377:1975). Wet sieving and sedimentation were carriedout to determine the grain-size distribution of the laterite. The coarseaggregates were from crushed granite of igneous origin. The particle-sizeranged from 9-14mm. Figure 1 shows the particle size distribution curve ofthe fme and coarse aggregates considered.

The cement used was ordinary portland cement from the West AfricanPortland Cement Company, Ewekoro in Ogun State of Nigeria.

The sawdust was collected from Obafemi Awolowo University Sawmill andthe ash was obtained by burning the sawdust at a temperature of about 800°e.To aid the combustion, the sawdust was air-dry to a moisture content of 6% at1050C. A drum, about 0.8m in diameter and LOrnhigh with one end openwas used (see Figure 2). A central hole about lOOmm in diameter was madeat closed end. The sides and the closed end of the drum were perforated toallow free flow of air during burning thus ensuring complete combustion.After burning the percentage of the ash is about 2% which colour rangingfrom brown to sooty black depending of the degree of combustion. thecarbon-free ash used was obtained by sieving the ash through sieve No. 100(0.15mm sieve size) to remove the unbumt sawdust.

Three mix proportions (by weight) of cement: laterite: coarse aggregated wereused in the work, namely 1:1.5:3, 1:2:4 and 1:3:6 and their respectivewater/cement ratios determined (see Figure 3). The sawdust ash was varied inproportion of 0,5, 10, 15,20 and 25% to the cement in the mix. The cubespecimens used for studying the strength characteristics were prepared inaccordance with BS 1881 Part 3 using 100 x 100 x 100mm cube moulds.The mixing was done by the use of shovel and hand to give a very plastic andeasily worked paste. Water was added at intervals after the cement and theaggregates had been mixed thoroughly and mixing was continued to achievean even paste (of "landcrete"). The specimens were made by filling eachmould in two layers and compacting manually each layer with 35 strokes of asteel road of 25mm diameter. The specimens were given 24 hours ± 1/2 hourto set before demoulding. The specimens were cured in a large tank filledwith clean water until the age of test. The strength characteristics of each cube

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West Indian Journal of Engineering, Volume 15, No.l i January 1990

were tested using a loading rate of 120kN/min on a 600kN Avery DenisonUniversal Testing machine. Three specimens for each curing age of 7, 14,21and 28 days, were brought out of the curing tank and allowed to rest for 2hours and then crushed. The average values of the maximum loads at whicheach group of three specimens failed were found and then the compressivestrengths were determined:C . th Average maximum load

ompressive streng = Average area of the specimen

4.0 ANALYSIS AND DISCUSSION OF RESULTSThe results of the various laboratory tests carried out to determine the generalproperties of the laterite used are presented in Table 1. Table 2 shows theoxide composition of SDA and cement as obtained from the literature. Lackof chemicals hindered the determination of the actual oxides of the SDA usedin this study but it is believed that the percentages of various oxides werewithin the specified percentages contained in the referenced literature. Theoxides present react to form the four main, important compounds of thecementitious material (see Table 3). Tricalcium silicate contributes to thedevelopment of the early strength particularly during the first 14 days whiledicalcium silicate hydrates slowly and is mainly responsible for thedevelopment of strength after 7 days; it remains active for a considerableperiod. Tricalcium aluminate produces little increase in strength after 24 hourswhile tetracalcium aluminoferrite is of less importance to strengthdevelopment.

Generally, the results obtained showed that the compressive strengthdecreases with increase in the SDA content. Hence, the highest compressivestrength values were obtained for specimens having 0% SDA, while thevalues obtained for other percentages of SDA content were lower. Forexample, from Figure 4, for a mix with proportions of 1:2:4, at 7 day curingage, the compressive strength obtained was 9.5kN/mm2 for 0% SDA whilefor the same mix proportion it decreased to 9.4N/mm2, 9.0N/mm2,8.4N/mm2, 8.0N/mm2 and 7.3N/mm2 at contents of SDA of 5, 10, 15,20and 25% respectively.

Figure 5 shows that the strength values are highest for all the different ashcontent percentages at the 28 day curing age and least at 7 days curing. Acomparison of strength developments at 7, 14,21 and 28 day curing ages ofthe mix containing 0% SDA (9.5m 12.8m 14.0 and 15,4N/mm2) with thatcontaining 25.% SDA (7.3,7.7,8.3 and 9.4N/mm2) showed that after 7 dayscuring there was 26.32% strength reduction caused by the introduction ofSDA into the mix. At 14 days, the reduction in strength was 39.84%, at 21

75

Falade

days it was 40.71 % and at 28 days it was 38.96%. At 21 days, the decreasein strength of 40.71 % due to the presence of SDA is greater than at 28 days(38.96%); in other words, after 21 days curing the percentage strengthdevelopment for the 1:2:4 mix containing 25% SDA was higher than for themix with 0% SDA. This behaviour could be due to the proven pozzolanicactivity of the ash. At 0% SDA the cement content is highest, and the rate ofstrength development is initially high, but then becomes lower at 21 and 28days curing ages due to the initial rapid hydration of the cement paste.However, at 25% SDA where there is a substantial amount of ash, the rate ofstrength development is low at the ages of 7 and 14 days but becomes higherat 21 and 28 days due to the slow reaction of the pozzolans.

A pozzolan is a siliceous or siliceous and aluminous material which itselfpossesses little or no cementitious value, but will, when in finely divided forrnand in the presence of moisture, chemically react with lime at ordinarytemperatures to form compounds possessing cementitious properties. In thecase under consideration the product of cement hydration, that is, lime in themix, is not immediately available. Reaction starts when the lime produced inthe mix is present in sufficient quantities to react with the ash.

It is noted that the ratio of the 7 day strength to the 28 day strength increaseswith increase in the percentage of sawdust ash (see Table 4). This trendconfirms the fact that the later strength development increases with increase inpercentage of ash in the mix.

The same trend of strength development observed in the mix with proportionsof 1:2:4 was also observed in the mix with proportionsl:3:6. The specimenswith 0% SDA content have the highest strength values at all the curing agescompared with those containing SDA. At a 7 day curing age, the mixture with0% SDA exhibited a strenght of 6.2N/mm2 compared with strengths of 5.1,4.4, 4.0, 3.8 and 3.6N/mm2 for the mixtures with 5, 10, 15, 20 and 25%SDA contents. The slope of the strength development curve of the 1:3:6 mixcontaining 25% SDA is higher at 21 and 28 days than it is at 7 and 14 dayswhich indicates that strength development is more rapid at later ages formixtures with higher percentages of SDA content (e.g. 25%)

Figure 7 shows that the strength development for 0% SDA content is high at 7and 14 days and becomes lower at 21 and 28 days. Table 5 shows that theratio of the 7 day to 28 day strengths is higher for the 1:3:6 mix at 0% SDAthan for the mixes containing 5, 10, 15, 20 and 25% SDA. This is quite theopposite of what was observed in the 1:2:4 mix. The amount of cement in themix could be the reason for this. The mix with the proportions 1:3:6 containsless cement. The reaction of the pozzolan depends on the amount of lime;

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West lndian Joumal of Engineering, Volume 15, No.1, January 1990

because SDA contains very low levels of lime the quantity of cement availableto produce this lime as a by-product of hydration, is critical.

Figure 8 shows that there was a sharp decrease in strength with increasingpercentages of SDA for the mix with proportions of 1:1.5:3. Figure 9 alsoshows that all the specimens attained the least strength at 7 days curing ageand their highest strengths at the 28 day curing age. Table 6 shows the ratioof 7 day strength to 28 day strengths in percentage terms. It shows that at 0%SDA, the percentage is lower than at 5% SDA and therefrom decreasesprogressively to 59.2 at 25% SDA. The amount of cement in the mix isresponsible for this. There is high cement content which means that thestrength development will be higher at early curing ages (7 and 14 days) thanat the curing ages of 21 and 28 days.

5.0 CONCLUSIONFrom the experimental results the following conclusions can be drawn:

i) The addition of sawdust ash decreases the compressivestrength of laterised concrete.

ii) The compressive strength results of laterized/SDAspecimens cured in water show that the rate of strengthdevelopment for mixtures with 0% SDA is higher at thecuring ages of 7 and 14 days than for the mixtures withSDA, especially those with contents equivalent to 20 and25% SDA replacement of the cement.

iii) The effect of SDA is more pronounced in the mixture withhigh aggregate/cement ratio than in those with loweraggregate/cement ratios.

iv) Despite these disadvantages, SDA may be used to replacesome of the cement, to act as a retarding agent to delay thesetting of laterised concrete, thus compensating for theeffect of hot weather both by helping to offset the loss ofworkability and to minimise the increase in the amount ofwater to produce the required workability.

This paper is intended to generate an awareness that a cementing material suchas sawdust ash is available in the country. Further work is continuing in thearea of developing adequate mix design for cement/SDA concrete to give therequired strength and workability.

77

Table 1. General Properties or the Lateritic soil used withSawdust Asb

Tests Results

Natural moisture content, %Liquid limit, %Plastic limit, %Plasticity Index, %Specific gravity% passing No. 200 BS Sieve

3.1543.8032.0811.722.74

51.40

Table 2. Chemical composition or Sawdust Ash and cement [11]

CementComposition Sawdust Ash

64-6719-244-72-6

53

Ume,CaOSilica, SiD2Alumina, A 1203Iron Oxide, Fe203Magnesia, MgOSulphur Trioxide, S03

2.5-8.540-5315-386-32

0.17- 0.70.8-2.8

Abbreviation

Table 3. Main chemical compounds or Portland Cement [12]

Name or Compound Chemical Composition

Tricalcium SilicateDicalcium SilicateTricalcium AluminateTetracalcium Aluminoferite

3CaO.SiD22CaO.SiD23CaO.A12034CaO.A h03.Fe20:3

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West Indian Jou",aJ of Engineering. Volume IS. No.l.J1JIUIIJTY 1990

Table 4. Ratio or 7 day strength to 28 day strength(mix proportion 1:2:4)

% or SDA in the mix0% 5% 10% 15% 20% 25%

Strength 7 day 9.5 9.4 9.0 ' 8.4 8.0 7.3(N/mm2) 28 day 15.4 14.1 13.5 11.3 10.6 9.4

7 day strength as a% of 28 day strength 61.69 66.67 66.67 74.34 75.47 77.66

Table 5. Ratio or 7 day strength to 28 day strength(mix proportion 1:3:6)

% or SDA in the mix0% 5% 10% 15% 20% 25%

Strength 7 day 6.2 5.1 4.4 4.0 3.8 3.6(N/mm2) 28 day 8.1 7.3 6.1 5.3 5.8 5.0

7 day strength as a% of 28 day strength 76.54 69.86 72.13 75.47 73.08 72.00

Table 6. Ratio or 7 day strength to 28 day strength(mix proportion 1:1.5:3)

% or SDA in the mix0% 5% 10% 15% 20% 25%

Strength 7 day 13.5 12.1 11.01 9.0 8.3 7.4(N/mm2) 28 day 21.3 18.3 16.8 13.8 13.0 12.5

7 day strength as a% of 28 day strength 63.38 66.12 65.54 65.22 63.85 59.2

79

REFERENCES1. D. Adepegba, A comparative study of normal concrete with concrete which

contains laterite fines instead of sand, Bui!din~ Science, 10, pp 135-141 (1975).2. F. Lasisi and J.A. Osunade, Effect of grain size on the strength of cubes madefrom

lateritic soils, Bld~. Envjr .. 19, p55-58 (1984).3. S.A. Ola, Geotechnical properties and behaviour of some stabilised Nigerian

lateritic soils, Journal of En~neerin~ Geolo~v, Vo\. 2, No.2, ppI45-160 (1978).4. F. Lasisi, Masonry units for low income housing from cement stabilised lateritic

soils, Proc. of the ConL on Low Income Housin~ Technolo~y, Thailand, Vo\. 2,pp1037-1046 (1977).

5. E.A. Mesida, Soil stabilisation for housing in Okitipupa area, Ondo Stat'e,Research paper, Department of Geology, University of Ife, Nigeria (1978).

6. K.O. Okunnu, Studies on sandcretetlandcrete blocks in building technology,Student project, Department of Agricultural Engineering, University of Ife, Nigeria(1980).

7. F. Lasisi and A.M. Ogunjide, Effect of grain size on the strength characteristics ofcement-stabilised lateritic soil', Bui!din~ and Envjronment (to be published).

8. J.O. Akinmusuru and 1.0. Adebayo, Fibre-reinforced earth block', Journal ofConst Division, Proc. ASCE, 107, pp487-496 (l~81).

9. F. Lasisi and J.A. Osunade, Factors affecting the strength and creep properties oflaterised concret', Bld~ and Envir .• Vol. 20, No. 12, pp133-138 (1985).

10. M.A. Rahman, Effects of Rice Husk Ash on Geotechnical Properties of lateriticsoil', West Indian Journal of En~ineerin~, Vol. 11, No.2, (1986).

11. S.V. Shestoperove, 'Road and Building Materials', Mir Publishers, Moscow,USSR, 1983, pp189 & 362.

12. N. Jackson, 'Civil Engineering Materials', 3rd Edition, ELBS-MacmilJian, 1984,Table 12.1, pp114.

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West Indian Journal ofEnginuring. Vo"""" IS. No.1. January 1990

~'-"eo.5'" 50'"'"0.0eo5c00~Q..

0

0: Crushed Aggregate

e: Laterite Soil

0.01 0.1 1.0 10.0 100

silt fraction + sand fraction + gravel fraction

Figure 1 : Results of sieve analysis of the laterite soil sampleand crushed aggregate

Drum

Sawdust

100mm ¢J hole

Perforations

Support

Figure 2: Drum used for burning the sawdust on its tripod

81

Falade

12·0

10·0mix w/c ratio1: 1.5: 3 0.6231: 2:4 0.753

---- 1: 3: 6 1.020>- &0•.••....0'':t:! 6'() y= 0.9 + 3.85X...

r = 0.9995EJ! 4"0~·c~ 20j

o 10 20 30Optimum water/cement ratio (x)

Figure 3: Relationship between the optimum water/cementratio and laterite/cement ratio

20% SDA Content

----C'ol .: 0%

~XI 5%0: 10%

6 156: 15%

oS 0: 20%01) Q: 25%e~""«) t)>.;;;""eS-a 5

°O~--~7~--~~~--~~~----~26~Curing age (days)

Compressive strength for different percentages ofSDA at different curing ages (mix 1: 2: 4)

Figure 4:

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W~st Indian Journal o{Engineering, volume 15, No.1, Janwvy 1990

0: 28 days6: 21 "X: 14 ".: 7 "

O~ __~ ~ __~ ~ L-~o 5 10 15 25

SDA content (%)Figure 5: Compressive strength for different curing ages at different

SDA percentages (mix 1: 2: 4)

10

.: 0%x·5%0.10% =8:'~~

~

°0~----~7------~1~4------~2'~----~~=-~Curing age (days)

Compressive strength for different percentages of SDAat different curing ages (mix 1: 3: 6)

Figure 6 :

83

10 028 days621 "X 14 "·7"

o 5 2S10 15 20SDA content (%)

84

Figure 7: Compressive strength for different curing ages at differentSDA percentages (mix 1: 3: 6)

25 :.....,. SDA CONTENT.: 0"1.X: 5'1.

20 0: 10'1.N 6: 15 'I.E.€ 0: 20'1.z \7: 25 "I.

15c:0,cOJ.,

lfl 10

OJ>.~

5a..gu

00 7 14 21 28

Curing age (days), Figure.6 : Compressive strength tor di f terent percenta9t'S

ot SDA at dit ter ern curing oges (mix l1h·J)

25

N~EEz

£(J\c,

~<11 .10_OJ>oilVI•••~. 5 _E0u

00 5

Figure.9

0: 28days6' 21 .'x: 11. ••

.: 7

10 15 20 25SDA CONTENT ('I.)

." Compressive' strength for different cur inoU<JIZS ut <.lilh/lfnt . SUA p~(c('ntoy('s·(rni;·';lh"J)