Characteristics of Wood ASH/OPC Concrete

 

M. Abdullahi

 

Civil Engineering Department, Federal University of Technology, P.M.B. 65, Minna, Niger State, Nigeria, abdulapai@yahoo.com

 

 

Abstract

The study presents the behaviour of wood ash / OPC concrete. Chemical analysis of wood ash, bulk density, sieve analysis and specific gravity of wood ash and aggregates, consistency, setting time and slump test of the fresh paste were conducted to determine the suitability of the materials for concrete making. Mix ratio of 1:2:4 and percentage replacement level of 0, 10, 20, 30 and 40 percents of cement by wood ash were used. 150mm150mm cubes were cast, cured and crushed at 28 and 60 days to determine their compressive strength. Test result indicates that the wood ash is slightly pozzolanic, water demand increases as the ash content increases and the setting time of the paste increases as the ash content increases. Compressive strength of wood ash / OPC concrete it increases with age at curing with optimum replacement of cement by wood ash of 20%.

Keywords

Wood ash, OPC concrete, Chemical analysis

 

 

Introduction

 

Concrete is a construction material composed of Portland cement and water combined with sand, gravel, crushed stone, or other inert material such as expanded slag or vermiculite [1]. The major constituent of concrete is aggregate, which may be natural (gravel or crushed rock with sand) or artificial (blast furnace slag, broken brick and steel shot). Another constituent is the binder, which serves to hold together the particles of aggregate to form concrete. Commonly used binder is the product of hydration of cement, which is the chemical reaction between cement and water [2]. Admixtures may also be added to concrete mixes to change some of its properties.

Wood ash in this study is an admixture: a pozzolana. A pozzolana is a material rich in silica and alumina which in itself has little or no cementitious value but will, in finely divided form and in the presence of moisture, chemically react with calcium hydroxide at ordinary temperatures to form compounds possessing cementitious properties [3]. Wood ash is obtained from the combustion of wood. It can be related to fly ash since fly ash is obtained from coal, which is a fossilized wood [4]. Rice husk ash is also of plant origin. This implies that wood ash could be used as a pozzolana in concrete. Tarun, Rudolph and Rafat [5] reported the following elements in wood ash: carbon (5% to 30%), calcium (5% to 30%), carbon (7% to 33%), potassium (3% to 4%), magnesium (1% to 2%), phosphorus (0.3% to 1.4%) and sodium (0.2% to 0.5%). The following compound composition limits were also reported: SiO2 (4% to 60%), Al2O3 (5% to 20%), Fe2 O3 (10% to 90%), CaO (2% to 37%), MgO (0.7% to 5%), TiO2 (0% to 1.5%), K2O (0.4% to 14%), SO3 (0.1% to 15%), LOI (0.1% to 33%), moisture content (0.1% to 22%), and available alkalis (0.4% to 20%). The study revealed that all the major compounds present in wood ash are present in fly ash.

 

 

Materials and Methods

 

The wood ash used in this work was powdery, amorphous solid, sourced locally, from a bakery in Minna. The wood ash was passed through BS sieve 0.075mm size. The chemical composition of the wood ash was conducted at the Agric. Laboratory of Federal University of Technology, Minna, Nigeria. Bulk density, sieve analysis and specific gravity tests were conducted on the ash, fine and coarse aggregates in accordance to [6] and [7].

The mix used for all concrete cubes cast in this work is 1:2:4, with the cement partially replaced by wood ash in varied percentages of its volume (0%, 10%, 20%, 30% and 40%). The workability of each of the mixes was determined using slump test according to [8]. Cubes were cast using wooden moulds (150mm150mm) and compaction was done manually. The cubes were cured and crushed to determine their compressive strength at 28 and 60 days according to [9].

Results and Discussions

 

Chemical Analysis of Wood Ash

The results of chemical analysis of wood ash are shown in Table 1. The total percentage composition of Iron oxide (Fe2O3 = 2.34%), aluminium oxide (Al2O3 = 28.0%) and silicon dioxide (SiO2 = 31.80%) was found to be 62.14%. This is less than 70% minimum required for pozzolana [3] (ASTM C 618-94, 1994).

 

Table 1. Chemical Composition of Wood Ash

Constituent

SiO2

Al2O3

Fe2O3

CaO

MgO

K2O

NaO

L.O.I

Composition [%]

31.8

28

2.34

10.53

9.32

10.38

6.5

27

 

This reduces the pozzolanicity of the wood ash. The percentage composition of silicon dioxide (31.8%) is within the range specified by [5]. However, the percentage composition of Iron oxide and aluminium oxide were not in agreement with the work [5]. The loss on ignition obtained was 27%. The value is more than 12% maximum as required for pozzolana [3]. This means that the wood ash contain appreciable amount of un-burnt carbon which reduces its pozzolanic activity. The un-burnt carbon is not pozzolanic and its presence serves as filler to the mixture. The alkali content (Na2O) was found to be 6.5%. This value is higher than the maximum alkali content of 1.5% required for pozzolana. The alkali content is important were the wood ash is to be used with reactive aggregate [10]. Wood ash will not be suitable for construction work where reactive aggregate is to be used.

 

Specific Gravity Test

Table 2 shows the result for the specific gravity test of aggregates and wood ash. The specific gravity of wood ash was found to be 2.13. This value is far less than 3.15 for Portland cement.

The value is close to the value reported by [11] and [12] which was 2.13 for rice husk ash and 2.12 for acha husk ash.

It is also within the range for the specific gravity of pulverized fuel ash [10]. The specific gravity of the sand and gravel were found to be 2.62 and 2.66.

The value obtained falls within the limit for natural aggregates with value of specific gravity between 2.6 and 2.7 as reported in [10].

 

 

Table 2. Specific Gravity of Materials

Data

Wood Ash

Sand

Gravel

Test1

Test 2

Test1

Test2

Test 1

Test2

Mass of gas jar, plate,

soil (RHA) and water (m3) [g]

1595.0

1599.0

1685

1678

1732.90

1730.9

Mass of gas jar, plate

and soil (RHA) (m2) [g]

686.6

689.4

820.0

815.5

876.5

875.5

Mass of gas jar, plate

and water (m4) [g]

1504.6

1506.4

1495.9

1493.5

1507.1

1506.9

Mass of gas jar and plate (m1) [g]

515.6

515.6

515.6

515.6

515.6

515.6

(m2 m1) [g]

171.0

173.8

304.4

299.9

360.9

359.9

(m4 m1) [g]

989.0

990.8

980.3

977.9

991.5

991.3

(m3 m2) [g]

908.4

909.6

865.0

862.5

856.4

855.4

(m4 m1) - (m3 m2) [g]

80.6

81.2

115.3

115.4

135.1

135.9

Specific gravity of the particles Gs

= (m2 m1)/((m4 m1) - (m3 m2))

2.12

2.14

2.64

2.60

2.67

2.65

Mean of specific gravity

2.13

2.64

2.66

 
Bulk Density Test

Table 3 shows the result for the compacted bulk density of wood ash and aggregates. The bulk density of wood ash was found to be 760 kg/m3.

 

Table 3. Compacted Bulk Density of Materials

Data

Wood Ash

Sand

Gravel

Test 1

Test 2

Test 1

Test 2

Test 1

Test 2

Weight of empty cylinder (w1) [kg]

4.35

4.35

4.35

4.35

4.35

4.35

Weight of empty cylinder + weight

of compacted materials (w2) [kg]

5.10

5.12

5.96

5.98

5.85

5.85

Weight of compacted materials (w3) [kg]

0.75

0.77

1.61

1.63

1.5

1.5

Volume of Cylinder (v) [Litre]

1

1

1

1

1

1

Compacted Bulk density (w3/v) [Kg/m3]

750

770

1610

1630

1500

1500

Mean Bulk density [Kg/m3]

760

1620

1500

 

The [11] report obtained the compacted bulk density for rice husk ash to be 530 kg/m3 and [12] reported a value of 740 kg/m3 for acha husk ash. These values are reasonably close to one another indicating that they are all lightweight materials. The silica in pozzolana can only combine with calcium hydroxide when it is in a finely divided state. Pozzolana in this state has uniform particles which are difficult to compact very closely resulting to a low compacted bulk density. The compacted bulk densities for sand and gravel were found to be 1620 kg/m3 and 1500 kg/m3.

 

 

Sieve Analysis

Tables 4 and 5 show the result for the sieve analysis of sand and gravel.

 

Table 4. Sieve Analysis of Fine Aggregate

Sieve

Size

[mm]

Mass of

Sieve

(w1) [g]

Mass of

Sieve + Sample

(w2) [g]

Mass of

Sample Retained

(w3) [g]

Percentage

Retained

[%]

Cumulative

Percentage

Retained [%]

Percentage

Passing

[%]

5.00

478.60

483.10

4.50

0.90

0.90

99.10

3.35

468.87

477.42

8.55

1.72

2.62

97.38

2.36

435.99

459.34

23.35

4.67

7.29

92.71

1.18

390.85

423.25

32.40

6.48

13.77

86.23

0.60

337.30

438.30

101.00

20.20

33.97

66.03

0.30

317.10

558.95

241.85

48.37

82.34

17.66

0.15

296.01

302.06

6.05

1.21

83.55

16.45

0.075

301.08

331.18

30.10

6.02

89.57

10.43

Pan

295.53

347.68

52.15

10.43

100.00

0.00

 

Table 5. Sieve Analysis of Coarse Aggregate

Sieve

Size

[mm]

Mass of Sample

Retained

[g]

Percentage

Retained

[%]

Cumulative

Percentage

Retained [%]

Percentage

Passing

[%]

28

0.00

0.00

0.00

100.00

14

323.79

16.19

16.19

83.81

10

1466.21

73.31

89.50

10.50

6.3

166.12

8.31

97.81

2.19

5.0

21.16

1.06

98.87

1.13

Pan

22.70

1.13

100.00

0.00

 

Test result reveal that the sand is fine grading sand and the gravel is single-sized aggregate of 14mm nominal size confirming the aggregates suitable for construction work [13].

 

Consistency and Setting Times Tests

Table 6 shows the result for the consistency test. Result reveals that the water demand increases with the wood ash content. The wood ash introduced into the cement increases the carbon content and this increases the water required to achieve a reasonable workability.

 

Table 6. Consistency Test

Replacement of Opc by Rha [%]

0

10

20

30

40

Opc [g]

400

360

320

280

240

Rha [g]

0

40

80

120

160

Water [ml]

125

132

138

146

153

Water/Binder Ratio [%]

31

33

35

37

38

The result for the setting times test is shown in Table 7.

 

Table 7. Setting Times Test

Replacement of Opc by Rha [%]

0

10

20

30

40

Initial Setting Time [minutes]

100

218

321

395

436

Final Setting Time [minutes]

160

334

523

698

789

 

The initial and final setting times increases with increase in wood ash content. The exothermic reaction between cement and water result into liberation of heat and evaporation of water and eventually hardening of the paste. The rate of reaction and quantity of heat liberated reduces with the introduction of wood ash leading to late stiffening of the paste. As the hydration process was prolonged, greater amount of water was required in the process.

 

Slump Test

The result for the slump test is shown in Table 8. Test result shows that mixes with greater wood ash content requires greater water content to achieve a reasonable workability.

Table 8. Slump Values

Replacement of Opc by Rha [%]

0

10

20

30

40

Water/Binder Actual Ratio

0.6

0.66

0.67

0.68

0.69

Slump [mm]

30

35

40

40

35

 

The increased water demand was due to the relatively high carbon content in wood ash. The slumps observed were from low (15 to 30 mm) to medium (35 to 75 mm) [8].

 

Compressive Strength Test

Table 9 show the result for the compressive strength of wood ash / OPC concrete at 28 and 60 days. The result shows that the cubes containing 0% wood ash had the highest compressive strength. The mix containing 20% wood ash had higher strength than that containing 10% wood ash at 28 and 60 days.

This was due to the fact that the silica provided by 10% wood ash was inadequate to react with the calcium hydroxide produced by the hydration of cement. Increase in wood ash content beyond 20% resulted in a reduction in strength at 28 and 60 days. In this case the silica present in the mix was in excess of the amount required to combine with the calcium hydroxide from the hydrating cement. The excess silica had no pozzolanic value but only served as filler.

 

Table 9. 28 and 60 day Compressive Strength

Cube

ash

Weight

[Kg]

Crushing Load

[KN]

Compressive Strength

[N/mm2]

Average

[N/mm2]

[%]

 

28 day

60 day

28 day

60 day

28 day

60 day

28 day

60 day

0

A

7.84

7.86

535

498

23.78

22.13

23.96

24.15

B

8.58

8.08

540

590

24.00

26.22

C

8.25

7.92

542

542

24.09

24.09

10

A

7.95

8.08

270

312

12.00

13.87

13.09

14.06

B

7.94

7.8

324

329

14.40

14.62

C

7.8

7.9

290

308

12.89

13.69

20

A

8.07

8.1

312

428

13.87

19.02

14.13

18.60

B

8.08

8.06

322

410

14.31

18.22

C

7.95

7.98

320

418

14.22

18.57

30

A

8.19

7.92

197

198

8.75

8.80

9.02

7.91

B

7.9

8.04

208

186

9.24

8.27

C

8.21

8.08

204

150

9.07

6.67

40

A

8.3

8.3

180

197

8.00

8.76

8.59

7.82

B

8.5

7.98

204

149

9.07

6.62

C

8.1

8.04

196

182

8.71

8.09

 

At 60 days hydration period, the compressive strength of concrete containing 20% wood ash increased considerably indicating that greater strength can be obtained at greater days. The optimum replacement of cement by wood ash is therefore 20%.

 

Conclusions

 

From the findings of this work the following conclusions were made:

        The chemical composition of wood ash fell below the standard for pozzolana;

        The specific gravity and compacted bulk density of wood ash were found to be 2.13 and 760 kg/m3;

        The water requirement increases as wood ash content increases;

        The setting times of wood ash / OPC paste increases as the ash content increases; the 10% and 20% wood ash paste satisfy the recommended standard for ordinary Portland cement paste. 30% and 40% wood ash paste gave higher values of setting times which do not satisfy the standard;

        The compressive strength of the concrete with 20% wood ash content increased appreciably at 60 days. The optimum replacement level was therefore 20%.

 

References

 

[1] Brady G. S., Materials Hand Book, McGraw-Hill, London, p. 126, 1986.

[2] Jackson N., Dhir R. K., Civil Engineering Materials, Macmillan Education Ltd., Hong Kong, 1991.

[3] American Standard for Testing and Materials, Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete, ASTM C 618-94, 1994.

[4] United States Department of Transportation - Federal Highway Administration, Fly Ash Facts for Highway Engineers, http://www.fhwa.dot.gov/pavement, 2005.

[5] Tarun R. N., Rudolph N. K., and Rafat S., Use of Wood Ash in Cement-based Materials, A CBU Report, CBU-2003-19 (REP-513), http://uwm.edu/Dept/CBU/report/, 2003.

[6] British Standards Institution, Sampling and Testing of Mineral Aggregates, Sands and Fillers, BS 812, London, 1975.

[7] British Standards Institution, Methods of Testing for Soil for Civil Engineering Purpose, BS 1377, London, 1990.

[8] British Standards Institution, Methods of Determination of Slump, BS 1881, Part 102, London, 1983.

[9] British Standards Institution, Method for Determination of Compressive Strength of Concrete Cubes, BS 1881, Part 116, London, 1983.

[10] Neville A. M., Cementitious Materials of Different Types, Pearson Education Asia Pte. Ltd., 1995.

[11] Abdullahi M., The use of Rice Husk Ash in Low-cost Sandcrete Block Production, Unpublished M. Eng. Thesis, Department of Civil Engineering, Federal University of Technology, Minna, Nigeria, 2003.

[12] Dashan, I. I. and Kamang, E. E. I., Some Characteristics of AHA/OPC Concretes: A Preliminary Assessment, Nigerian Journal of Construction Technology and Management, Vol. 2, No. 1, p. 22-28, 1999.

[13] British Standards Institution, Specification for Aggregates from Natural Sources for Concrete, BS 882, British Standards Institution, London, 1992.