Production And Classification Of Locust Bean Pod Ash (LBPA) As A Pozzolan

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Adama Andrew Yisa (Ministry of Works and Infrastructural Development, Minna, Niger state, Nigeria.)
Dr. Y.A. Jimoh (Department of Civil Engineering Faculty of Engineering and Technology University of Ilorin, Ilorin, Nigeria.)

Locust Bean Pod Ash (LBPA) was produced by burning Locust Bean Pods in an Incinerator under controlled temperature. The solid residue after the combustion process was sieved using a set IS standard sieves after which the sieved sample was analyzed chemically to determine its constituents. The results of particle size distribution and chemical analysis of Locust Bean Pod Ash revealed that the percentage mass retained on IS sieve 44 µm is 29.6 % while the sum total of the combination of the chemical compounds (Sio2 + Al2O3 + Fe2O3) was 63.57%. These indicate that Locust Bean Pod Ash is pozzolanic in nature.

Locust Bean Pod Ash (LBPA), Pozzolan, Chemical analysis, Particle size

As a result of increased industrial and agricultural processes across the globe, there has been significant increase in industrial and agricultural wastes which most often have negative impact on the environment. Much research efforts in recent times are geared towards possible ways of recycling these wastes for re-use to keep the environment clean and safe. The transportation, Construction, and environmental industries have the greatest potential for re-use because they use large quantities of earthen materials annually Basha et al (2002).


Locust bean pod which is a Waste Agricultural Biomass (WAB) and obtained from the fruit of the African locust bean tree (Parkia Biglobosa) is the material resource required for the production of Locust Bean Pod Ash (LBPA). The harvested fruits are ripped open while the yellowish pulp and seeds are removed from the pods; the empty pods are the needed raw material. The pods make up 39% by weight of the fruits while the mealy yellowish pulp and seeds make up 61% see plates 01.

locust bean fruit

For years running, a liquid extract had been obtained by boiling the pod husks of the locust tree fruits in water and is usually reddish brown in colour. The local technology of extracting this liquid admixture from the locust bean pod husks has been a common practice among the Nupe speaking tribe found in present Nigeria. The extract was mainly used as a bonding agent between locally manufactured clay tiles and the soil beneath in the construction of durable floor finishes for their rooms and frontage of the rooms. The construction process involved the filling of the floor area with lateritic soils and compacted, the locust bean pod extract is then poured over the compacted surface at ordinary temperatures after which the clay tiles are laid over the wet floor and is further compacted to a firm finish using specially fabricated wooden rammers. Some floors constructed using the above method have been in existence for the past fifty (50) years and above, see plate 02

plate2-locust bean pod extract as binder

Also evidences still abound across Nupe land of the use of locust bean pod husks on walls built using lateritic soils; in this case the pod husks are usually attached to the external parts of the walls so that when rain falls the extracts from the pod husks normally will drain into the walls and the walls are usually strengthened through this process; see plate 03:

plate3-Locust pod husk

The cementatious properties exhibited by the locust bean pod extracts in the traditional construction processes described above gave rise to the consideration in this research work that Locust Bean Pod Ash (LBPA) that is produced from the same material (Locust bean pod ) could possess pozzolanic properties which could make it suitable for use as a stabilizing agent in weak soils for road construction as it is expected to improve on the engineering properties of these soils and thus, be enhanced and carry greater loads. Hence the primary objective of this study is to establish Locust Bean Pod Ash as a pozzolanic material that could be used as stabilizing agent in weak sub grade soils for road construction.

African Locust Bean Tree and its Economic Importance

The locust bean tree is being cultivated over a wide area within the African sub region it occurs in large numbers in a belt between 5? N and 15?N, from the Atlantic coast in Senegal to Sudan and northern Uganda. The belt is widest in West Africa (maximum 800km) and narrows to the east. About 201,000 ton of the locust bean fruit is being produced in northern Nigeria annually, Sina and Traore (2002).see plate 04:

Plate 04 African Locust Bean -Parkia Biglobosa Tree

African locust bean tree is a multipurpose tree which is equally valued as the Shea butter tree, its fermented seeds (‘Soumbala’, ‘dawadawa’, ‘Netetu’, ‘Kula’, ‘Iru’) serve primarily as a condiment for seasoning sauces and soups in various Nigerian meals among the various tribes of Nigeria. The leaves are sometimes eaten as vegetable, usually after boiling and mixed with other foods such as cereal flour. Young flower buds are added to mixed salads. Also the Roasted seeds are used as a coffee substitute known as ‘Sudan coffee’ or ‘café negre ‘. The mealy pulp from the fruits is eaten or is mixed with water to make a sweet and refreshing drink that is rich in carbohydrates. The boiled pods are used to dye pottery black. The bark is rich in tannins and may be used for tanning hides to moderate quality especially with regard to colour, which is often reddish, uneven and darkens when exposed to light, Sina and Traore (2002).

Also in West Africa the bark, roots, leaves, flowers, fruits and seeds are commonly used in the traditional human medicine to treat a variety of infections or diseases while in veterinary medicine, a root decoction is used to treat concidiosis in poultry .Green pods are crushed and added to rivers to kill fish; although the nutritional value of the fish is not adversely affected so long as they are cooked or dried, Sina and Traore (2002).

African locust bean tree has a reputation for soil improvement; its leaves are applied as manure. It is also important in apiculture, being a good source of nectar and suitable for placement of hives. It may serve as decorative trees in front of buildings and along avenues/motor ways.


Production of Ash from Waste Agricultural Biomass
Waste Agricultural Biomass (WAB) which includes rice husks, saw dust, palm kernel shells, locust bean pod e.t.c constitutes the precious resource of recycled material and or energy to reduce pressure on natural resources and ensures economic development and improved living standards, Report II Waste Biomass Quantification and Characterization, National cleaner production center, Sri Lanka (2010).

For the determination of the most convenient and economical temperature required for conversion of Waste Agricultural Biomass to ash, the losses on ignition (LOI) tests carried out revealed that the optimum burning time and temperature were 2 hours and 500°C, respectively. The loss in weight represents the quantity of unburned carbon present in the material which is often the good indication of how it will affect the air content (Ghani et al, 2008).Also in the combustion process of Rice Husk Ash , the organic constituents are not decomposed at temperatures below 450oc, Garvin and Sheshan (1977).

Silica is usually the major chemical compound contained is most ash residue after the combustion process of Waste Agricultural Biomass and this has health issues arising because all forms of crystalline silica represent a very serious health hazard, Occupational health administration (2002).The forms that develop at high temperatures ie crystobalite and tridymite are particularly harmful. Exposure to crystalline silica via inhalation can lead to a number of diseases, the most common being silicosis, Occupational Safety and Health Administration, US Department of Labour (2002). Although Amorphous ash is the form produced at lower temperatures less than 1000oc does not contain more harmful forms of silica, it can pose respiratory hazard particularly if finely ground Etsu (2003). Crystalline silica is classified as carcinogenic to humans, and the International Agency for Research on Cancer (IARC) concluded that there was sufficient evidence in humans for the carcinogenity of crystalline silica, International Agency for Research on Cancer (1997).

Pozzolanic materials by nature usually react with soil particles to form calcium silicate cement. This reaction is water insoluble. The cementing agents are exactly the same as for the case ordinary Portland cement. The difference is that the calcium silicate gel is formed from the hydration of anhydrous calcium silicate (cement), whereas with the pozzolanic materials, the gel is formed only by the removal of silica from the clay minerals of the soil. The silicate gel proceeds immediately to coat and bind clay lumps in the soil together and to block off the soil voids in the soil structure. In time this gel gradually crystallizes into well defined calcium silicate hydrates and the micro crystals also interlocking .The reaction ceases on drying; as very dry soils will not react with pozzolanic materials or cement, Agus and Gendut, (2002).

The American Society for Testing and Materials (ASTM) classification of Pozzolans
The test to confirm an ash material substance as a Pozzolan can be conducted by determining the quantities of the various chemical compounds present in the ash and then subjecting the result to standard classification of Pozzolans based on their chemical and physical requirements as can be seen in table I.

Table I Chemical and Physical Requirements of Pozzolans






Silicon dioxide, Aluminium dioxide and Iron oxide (SiO2+Al2O3 +Fe2O3), Minimum % 70 70 50
Sulfur Trioxide (SO3), Maximum % 4.0 5.0 5.0
Moisture content , maximum % 3.0 3.0 3.0
Loss on Ignition , maximum % 10.0 6.0 6.0
Available alkalis as Na2O, maximum% 1.5 1.5 1.5
Fineness, maximum % retained on 325-Mesh (44µm) sieve.
34 34 34

*ASTM Specification C618-92a Chemical and Physical Specifications (1994)

The Locust Bean Pods used in this research were sourced from Doko town in Niger state of Nigeria. The material is usually available as a waste product of agricultural processing of the locust bean fruits during the harvest season.

Locust Bean Pod Ash (LBPA) was produced by incineration and using industrial thermometer to ensure that required temperature regimes are attained. The ash produced was sieved using IS set of sieves.

Locust Bean Pod Ash (LBPA)

The Locust bean pod ash is a solid/powder residue prepared by burning large mass of the locust bean pod in the incinerator at temperatures of up to 500o C. The residue is then left to cool and sieved using IS Sieve 44µm to obtain a fine powdery form which is needed for chemical analysis. Temperature control plays an important role in the combustion process as such there should be some degree of confinement of the biomass during combustion.

The ratio of the LBPA residue to the pod produced from the combustion process was 1:25 by weight. In other words if 1kg of the pod is burned the weight of the ash to be produced will be 0.04 kg

Particle Size Distribution of LBPA
The particle size distribution of LBPA is shown in table II, it can be seen that the percentage mass retained on IS sieve 44 µm is 29.6 %.

Table II Particle Size Distribution of LBPA



Wt. Of


Wt. Of





mass retained

%retained %passing
0.088 0 0.0 0.0 100.0
0.075 457.3 463.3 6 6 3.0 97.0
0.063 480.3 488.3 8 14 7.0 93.0
0.053 494.6 504.6 10 24 12.0 88.0
0.044 430.7 465.9 35.2 59.2 29.6 70.4
pan 520.9 661.7 140.8 200 100 0.0

Chemical Analysis Results
The Locust Bean Pod Husks Ash was prepared and analyzed for its chemical composition at the facilities of the Federal University of Technology Minna soil science laboratory. The test results showing quantities of the respective constituents of the ash sample are presented on table III.

Table III: Chemical composition of Locust bean pod ash

Na2O(%) K2O(%) MgO














A 1.21 5.62 2.01 5.82 11.51 13.05 15.71 39.01 6.00

*A= Locust bean pod ash (Solid sample)

From the result of the particle size distribution of LBPA as shown in table II, it can be seen that the percentage mass retained on IS sieve 44 µm is 29.6 % while the sum total of the combination of the chemical compounds (Sio2+ Al2O3 + Fe2O3) was 63.57%, which when compared with standard specifications (ASTM Specification 12 C618-92a) on table I in conjunction with the chemical composition of LBPA as shown in table III; LBPA will fall under the class ‘C’ mineral admixture class and thus can be considered a pozzolan.


The results of the particle size distribution and chemical analysis of Locust Bean Pod Ash (LBPA) confirms that it has pozzolanic properties and can be classified under class ‘’C’’ group of Pozzolans on The American Society for Testing and Materials (ASTM) classification system. As such, it can be recommended for use as a chemical stabilizing agent in weak soils for road construction.

Agus S.M. and Gendut H. (2002): Influence of Rice Husk Ash and Lime on engineering properties of clayey sub grade.; viewed in April, 2010.
ASTM Specification C618-92a (1994): Chemical and Physical Specification. The American Society for Testing and Materials (ASTM).
Emhammed A. Basha & Roslan Hashim (2002): Effect of the Cement Rice Husk Ash on the Plasticity and Compaction of Soil. Department of Civil Engineering. University of Malaya, KualaLumpur, Malaysia.; viewed in April, 2010.
Etsu, U (2003): Rice Husk Ash Market Study:00/0061/0TT/PUB UR Garvin Patrick P. and Sheshan John P. (1977): Method of producing low carbon white husk ash.
International Agency for Research On Cancer, (1997): IARC Monographs on the Evaluation of Carcinogenic Risk to Humans 68. Wikipedia, free encyclopaedia. National cleaner production center, Sri Lanka (2010): Report II Waste Biomass Quantification and Characterization.
Occupational health and Safety Administration (2002): Regulation for Mineral Dusts, Standard 1910, 1000 Table Z-3 Occupational Safety and health Administration, US Department of Labour (2002):
Silica (crystalline). Sina, S & Traore S.A. (2002): Parkia biglobosa (jaccq) R.Br.ex G.Don. (Internet) record, Prota Network Office Europe, Wageningen university, P.O.Box 341, 6700 AH Wageningen, 14
W.A Wan Ab Karim Ghani, M.S Firdaus Abdullah, C.J Loung, C.J Ho, K.A Matori (2008): Vol. 5, No. 2, 2008, pp.111-117 International Journal of Engineering and Technology.

We at are thankful to Engineer Andrew Yisa Adama for submitting this very creative research paper to us. We are hopeful that this paper will open new venues for civil engineers and more study will be done in this field.

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  • Eknath N.H. March 30, 2012 at 4:59 am

    Chemical formula of Parkia pod powder

    • Eknath N.H. September 19, 2012 at 10:40 am

      stil today for us not getting Chemical formula of Parkia pod powder

  • Emmanuella April 10, 2012 at 10:03 pm

    Please what year was this paper published, can’t seem to find…

    • admin April 11, 2012 at 9:18 am

      Published on: Nov 22, 2011

  • iddi Mariam May 6, 2012 at 12:52 pm

    what are the process locust bean grows

  • please what is the chemical and structural formular of Locustbean? July 17, 2012 at 4:36 am

    Does locust bean has chemical formula?

  • A A LIMAN March 31, 2013 at 11:26 am


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