APD Wp 5 Ayeh Paper1

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OPPORTUNITY FOR THE INCREASED USE OF CLAY POZZOLANA

THE BRRI EXPERIENCE

K. A. Solomon-Ayeh

Building and Road Research Institute (BRRI)

Kumasi, [email protected]

Abstract

Pozzolanic properties of bauxite wastes and clay has been researched into by the Building andRoad Research Institute (BRRI) for over 30 years. It has recently come into its own mainly dueto the over 350% increase in the cost of Ordinary Portland Cement (OPC) in over the last sevenyears.

This positive appreciation of pozzolana cement has been influenced by satisfactorycompressive strength results from concretes produced with up to 30% replacement of OPC byclay pozzolana, favourable durability properties, relatively low cost of a prototype house and theeminent expansion of a prototype plant that produces clay pozzolana on a small scale.

With the countrywide availability of suitable clays and escalating cost of OPC, a favourable

environment has been presented for the use of clay pozzolana cement and a collaborative effortresearchers, designers, builders and investors should enable this opportunity to be seized.

This paper briefly presents the BRRIs efforts so far in taking this agenda forward.

Keywords: OPC, cement, clay, pozzolana, strength

1.0 INTRODUCTION

Pozzolan/Pozzolanas are described as any siliceous and aluminous materials, which are themselves not

cementitious, but in their finely-divided form react with lime in the presence of water at ordinary

temperatures to produce cementitious compounds. Natural materials like volcanic ash are pozzolans in

their natural state but materials such as clay, shales, bauxite waste (artificial pozzolana) have to undergo

heat treatment before they become pozzolanic.

Pozzolans have been used in the past as ingredient of Portland cement to construct massive civil

engineering structures such as the Bhakra Dam in India (Palta and Rao, 1964), the Davis and Friant


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Dams in the U.S.A. (Davis, 1949) and are envisaged to be used as dam core material for the proposed

Bui Dam in Ghana in 2008(1)

.

Initial studies on possible pozzolanic materials in Ghana were undertaken by Hammond (1976, 19787)

using bauxite waste from the Awaso mines. Other materials that possess pozzolanic properties were

identified as the vast clay deposits in the Greater-Accra region of Ghana (Hammond, 1978) and

agricultural wastes such as rice husks, coconut fibres, groundnut husks, sugar-cane bagasse etc.

(Hammond, 1987a, 1987b). The research on bauxite wastes indicated that with a 20 to 30% replacement

of OPC with calcined (700-900oC) bauxite-wastes, mortars and concretes produced with these blended

cements produced strengths comparable to those using OPC only. Studies carried out in India

(Srinivasan, 1964) and the U.S.A. (Davis et al., 2949) and in Ghana (Atiemo, 1994) have confirmed that

satisfactory strengths are obtainable with up to 30% replacement of OPC with pulverized burnt clay and

additionally such pozzolanic cements have shown better performance in saline atmospheres.

With the Building and Road Research Institute (BRRI) in particular, lack of equipment and other

resources in the past meant that only four clay deposits in Ghana were studied for their pozzolanic

potential. The consequence of this is that the potentials of the vast clay deposits in almost all regions in

Ghana for cement production could not be studied and research stalled, leading to the use of the clay

deposits mainly for the production of brick, tiles, earthenware and ornamental pottery. Needless to say,

the difficulty in the availability of cheap fuel to fire bricks have also led to the collapse of several brick

factories.

The rather high cost presently of OPC (GH8.5/GH2.4 for 2007:2000) has revived the interest in the

potential of clay for the production of pozzolanic cements. A prototype production plant has been in

existence at the BRRI since 2001 and the potential for increased use of the clay pozzolana in

construction has been informed by:

(1)Personal communication with Mr. Bekoe, Bui Dam Secretariat, Ministry of Energy, Ghana.


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(i) increased studies of clay deposits in other regions (rainforest and transitional savannah and

savannah zones) besides Greater-Accra;

(ii) performance and costs of prototype houses built using clay pozzolana for blocks and rendering;

(iii) performance and cost of prototype house built using clay pozzolana for blocks and rendering;

(iv) the setting up to prototype production plant at the BRRI, Kumasi.

This paper summarises the studies carried out as indicated in the foregoing and points the way forward in

the work on clay pozzolana.

Threats to the expansion of the clay pozzolana industry are technical, financial, marketing and political

and these are also briefly highlighted.

2.0 GENERAL FEATURES OF HYDRATION OF CEMENT

Ordinary Portland Cement (OPC) or derivatives of it contain calcium silicates and aluminates

which are basic elements of clinker (C3S, C2S, C3A) and to these are added 4-5% of gypsum

(CaS04 . 2H2O), mainly to control the rate of setting of cement.

The chemical process that takes place when water is added to cement (hydration) involves the

release of calcium hydroxide (saturated lime solution) into the solution. Hydrated silicates in

contact with calcium hydroxide undergoes hydrolysis liberating more lime into the solution (Lea,

1970).

The released hydrated lime finds voids in the cement paste and in the presence of atmospheric

carbon dioxide forms calcium carbonates (CaCO3). The deposits of Ca(OH)2 and CaCO3 are

causes of weakness of cement products since they can be easily attacked by sulphates and

chlorides (Uppal and Singh, 1964; Pallota and Mantegazza, 1988).

Fig.1 shows a schematic representation of the production of OPC and the active elements that

are beneficial for the combination with clay pozzolana.


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HeatAddGypsum

+Grind

+WATER

HYDRATION

+ ATMOSPHERIC C(OH)2 + CLAY POZZOLANICMATERIAL

+ CALCIUM IN PASTE (Active Siliceous+ Aluminate materials)

Attack bySulphates

+Chlorides

WEAKNESS IN CONCRETE WATER STRENGHTIGHTNESS

In the chemical reaction of hydration, the major constituents of C3S and the phases C-S-H and

Ca(OH)2 in the chemical chain are the most important regarding the strength properties of

cement paste. When a pozzolanic material is blended with the cement, it will start consuming

the formed Ca(OH)2 (Hammond, 1987a).The intensity of the pozzolanic activity is a measure of

CLAYS + LIMESTONES CLINKER

OPC

C3S + C-S-H + Ca(OH)2

Release of LIME In Solution

C CO + C OH HYDROUS CALCIUM SILICATES

has low solubility

Fig. 1.0: Schematic Chemical Reaction of Cement and Pozzolana and properties

of products


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how much Ca(OH)2 the material is able to combine. In a pozzolana cement mix, the released

calcium hydroxide reacts chemically with the active constituents of clay (amorphous siliceous

and aluminous materials) to form hydrous calcium silicates. These compounds have the

important properties of low solubility and thus contributes to water tightness as well as strength

(Davis, 1949). Clay components of pozzolana have these active siliceous materials. The

fineness of clay is also a vital factor as far as pozzolanic activity is concerned (Palta and Rao,

1964 and Puri and Srivivasan, 1964). Thus, the finer the clay material, the more active its

pozzolanic action.

3.0 DEVELOPMENT MADE IN THE PRODUCTION AND USE OF CLAY POZZOLANA

CEMENT

3.1 Deposits of clay in Ghana

Clays suitable for the production of clay pozzolana abound throughout Ghana, especially in the

southern half, where most rivers flow (Fig.2.0). It implies therefore that small-scale production

plants can be set up countrywide. Cheaper cement can potentially be produced, on account of

shorter haulage distances of raw materials to factories.

Fig. 2.0 Map of Clay deposits in Ghana


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3.2 Prototype Plant for Production of Pozzolana at the BRRI

In 2001, the BRRI built a small, prototype plant to produce clay pozzolana from clay deposits at

Mfensi (north-west of Kumasi). The plant consisted of a ball mill for both the grinding of faw clay

and palm kernel shells and the pulverizing of calcined clay. The plant also included a small

nodulizer that nodulized a ground clay/palm kernel shell mix. Calcination was by a vertical, up-

draft brick kiln which handled 1.2 tonnes/batch.

Since early 2007, the prototype plant has been upgraded with the installation of the following:

- hammer mill for grinding of raw clay

- addition of 1500mm diameter nodulizer

- horizontal mixer for ground clay and palm kernel shells, prior to nodulization

- installation of 9 tonnes/batch brick vertical up-draft kiln, for calcinations

- a pulverizing plant with the capacity of pulverizing 5 tonnes of pozzolana per hour.

At full capacity, the plant can employ 15 persons per 8hr shift.

The role of palm kernel shells (waste products of the oil-palm production industry) is that using

its innately high calorific value to boost the calcination temperature to the required 700-900oC

range. Ash, which is the end-product of the kernel burning, has some low level lime content,

which is a plus to the pozzolanic process. Its main role however is that of providing fuel.

A schematic representation of the production process is shown in Fig. 3 and photographs of

units in the upgraded plant are shown in Figs. 4 to 13.


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NODDLEZER

BLOWER

. .

Fig. 3: Clay Pozzolana Production Process

3.3 Strengths and Physical Properties of Cements and Concretes Produced

From Clay Pozzolana

Tests have been undertaken by many researchers to ascertain the setting time and strengths

achievable with cements and concretes made with part replacement of OPC by clay pozzolana.

Atiemo (1994) used clay pozzolana:OPC blend of between 20 and 30% replacement of OPC by

RAWCLAY

PALM KERNELSHELLS

CLAYPOWDER

SHELLPOWDER

HORIZONTALMIXER

PULVERIZER

CLAYPOZZOLANA

Air dry(3 days)

Ball MillHammer Mill

< 150m size

9 tonnes/batch 5 tonnes/

VERTICALKILN

NODULES

HEAT

FUEL


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weight to produce blended cement:sand mortars (1:3). Control tests used OPC:sand (1:3)

mortars. The blended cement mortars tests were repeated for clay pozzolanas produced at clay

calcinations of 700oC, 800oC, 900oC and 1100oC. Compressive tests were carried out between

7 and 60 days of curing (by water ponding).

The test showed that the control tests had the highest compressive strength results at 28 days

but strengths dipped slightly beyond 28 days. The blended cement mortars showed that :

(i) the clay used was highly pozzolanic reactive at 900oC;

(ii) the optimum replacement range of clay pozzolana with OPC was between 20 to 25%;

(iii) the compressive strength achieved ranged between 24.1 N/mm2 and 26.0 N/mm2 at 28

days and continued to increase even after 60 days, and for these strength levels, the

blended cement can be used for masonry joints, screeding and for concretes of low

(blinding, ground mass concrete) and lower normal (reinforced lintel, reinforced slab

under low load) strength;

(iv) setting times (initial and final) were less than for OPC, but increased with percentage

pondary replacement and temperature of calcinations;

(v) water absorption were higher than for OPC but in all cases less than 5%.

A second set of tests involved concrete mix designs to strengths C25 and C40 using 20mm

maximum-sized aggregates. The binder for the mixes consisted of 10% replacement of OPC

and 20% replacement of OPC with clay pozzolana and the use of OPC only.

The results showed a low early gain of strength of pozzolana cement concrete, but at 28 days,

concretes with 10% OPC replacement gave the highest values. Also, the designed strength was

only attained for C25 with 10% replacement of OPC ( Figs.15 &16). It can thus be concluded

that, for normal strength concretes, an optimum 10% part replacement of OPC by clay

pozzolana will give the best results (Solomon-Ayeh et al., 2009).


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Fig.4.0 Palm kernel shells Fig.5.0 Grinder for palm kernel shells


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Fig.8.0 Mixer Fig. 9.0 - Nodulizer

Fig. 10.0 Nodulized clays Fig. 11.0 Calcination in kiln


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Fig. 12.0 Calcined clay pozzolana Fig. 13.0 - Bagging process

Fig. 14.0 Finished building using

clay pozzolana cement


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Prototype 2-Bedroom House

A two-bedroom house was constructed in 2002 for the Ejisu-Juaben District Assembly. The

house was built in sandcrete blocks (using clay pozzolana cement) and plastered/rendered with

clay pozzolana cement :sand mortar. The clay pozzolana cement used was made up of a 30%

replacement (by volume) of OPC by clay pozzolana (Fig. 14).

The building cost 52 million cedis (52 million/$6,100) in 2002 as compared to 150 million cedis

($17,650) for a similar building with OPC; a ratio of about 1:3. This represents a huge saving in

the provision of shelter and if replicated in the 166 districts of Ghana, would

save a lot for the government, which can be used in other equally pressing sectors. Also, of

equal importance, is that the building continues to satisfactorily perform the functions for which it

was designed.


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3.4 Technical

The vertical kiln used in the prototype plant is not very efficient and considerable losses of heat

occur. More efficient kilns need to be developed. These kilns should be able to use local

agricultural wastes as fuels for firing.

The process of initial grinding and mixing of ground clay and shells tend to be dusty and thus

pose a health hazard, although the work force have protective nose masks. Further

improvement in the production would require a means of reducing the dust emission.

It is recommended that a maximum pozzolana replacement of OPC will give satisfactory

strength and durability properties of resultant mortar/concrete. At present, this replacement is

left to the builder. This option can lead to abuse either out of technical illiteracy or deliberate for

economic gain. Both reasons will give a negative advertisement to pozzolana cement. It is

recommended that further improvement in the production process will be the mixing of

pozzolana and OPC by the manufacturer.

Political

Political will is needed to replicate pozzolana plants nationwide, as the forces for the production

of OPC tend to be powerful and very well established and connected worldwide. In the late

1970s there was the attempt to use the smaller of two OPC production plants in Ghana (at

Takoradi) for the production of pozzolana. This was not possible as the political will was not

sustained. Since the BRRI plant has demonstrated that a large plant (as for OPC) is not

required if they are to be district-based, it is expected that the push from government will be

more forth-coming.


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4.0 CONCLUSION

The present high cost of OPC in Ghana has provided new impetus for the revival of past efforts

at producing pozzolana cement. The almost nationwide abundance of suitable clays, makes

clay pozzolana cement an obvious choice. Research on the strength and physical properties of

clay pozzolana cement and the successful start of a medium-scale prototype clay pozzolana

production plant by the BRRI makes this objective comes nearer to fulfillment.

The replication of similar plants countrywide will provide employment, skills training and result in

shelter of good building materials. However, this can only be realized if the threats of low

financial support for start-up ventures, need for technical improvement of the available

production methods and seemingly laid-back political support are confronted early.

5.0 REFERENCES

DAVIS, R.E. (1949).A review of pozzolanic materials and their uses Symposium on the Use

of Pozzolanic Materials in Mortars and Concretes. ASTM Special Technical Publication, No.90,

pp: 3-5.

DAVIS, R.E., HANA, W.C., AND BROWN, E.H. (1949). Strength volume pozzolana cements

Symposium on the Use of Pozzolanic Materials in Mortars and Concretes. ASTM Special

Technical Publication, No.90, pp: 131-153.

HAMMOND, A.A. (1976). Bauxite wastes in building. Building Research and Practice,

Vol.4No.2, pp: 80-83.

HAMMOND, A.A. (1978). Flexural and compressive strength properties of bauxite waste

pozzolana cement concrete. International Conference of Materials of Construction for

Developing Countries. Bangkok, Thailand, pp: 77-87.


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HAMMOND, A.A. (1978). Clay evaluation in the neighbourhood of Tema for pozzolanic cement

production. BRRI Project Publication.

HAMMOND, A.A. (1976). Survey of possible sources and extent of materials with pozzolanic

properties in Ghana: Some local raw materials for low-cost housing in Ghana Part One.

Commonwealth Science Council Publication.

SRINIVASAN, N.R. (1964). A new approach to the problems of Surkhi as a pozzolana.

Proceedings of Symposium on Pozzolanic clays in India, their Industrial exploitation and use in

engineering works. Central Road Research Institute (CRRI) Special Reports, No.1, India, pp:

175-181.

PALTA, B.R. AND RAO, P.S. (1964). Experience on the use of pozzolanas, their survey,

manufacture and utilization. Proceedings of Symposium on pozzolanas, their survey,

manufacture and utilization, New Delhi, CRRI Publication, pp: 185-201.

HAMMOND, A.A. (1987). Hydration products of bauxite-waste pozzolana cement. International

Journal of Cement Composites and Lightweight Concrete, Vol.9, No.1; p. 21.

Lea, F.M. (1970). The chemistry of cement and concrete. Edward Arnold, London, pp: 414-453.

PALLOTA, S. AND MANTEGAZZA, M. (1988). Durable concrete and modern technology.

Materials Engineering, Vol.1, No.3. Italian Association for Engineering Materials, RILEM, Italian

Group. Pp: 823-838.

UPPAL, H.L. AND SINGH, M. (1964). Some theoretical consideration regarding strength

development of cement-pozzolana or lime-pozzolana paste during hardening. Proceedings of

Symposium on Pozzolanas, Their Survey, Manufacture and Utilization, New Delhi, CRRI

Publication, pp: 85-94.

ATIEMO, E. (1994). Clay as pozzolana for building purposes. Journal of Building and Road

Research, Vol.2, Nos. 1 &2, June/Dec., 1994. The Building and Road Research Institute.

SOLOMON-AYEH, K.A., ATIEMO, E.,AMOAKO-KUTIN, T. (2009). Comparative strengths of

concrete from clay pozzolana and other blended cements. Unpublished paper.


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