USE OF POZZOLANA IN BUILDING

7/29/2019 USE OF POZZOLANA IN BUILDING

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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 theBuilding and Road Research Institute (BRRI) for over 30 years. It has recently come intoits own mainly due to the over 350% increase in the cost of Ordinary Portland Cement(OPC) in over the last seven years.

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

With the countrywide availability of suitable clays and escalating cost of OPC, a favourableenvironment has been presented for the use of clay pozzolana cement and a collaborativeeffort researchers, designers, builders and investors should enable this opportunity to beseized.

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
mailto:[email protected]:[email protected]:[email protected]


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Friant 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).


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

that are beneficial for the combination with clay pozzolana.

HeatAddGypsum

+Grind

+WATER

HYDRATION

+ ATMOSPHERIC C(OH)2 + CLAY POZZOLANICMATERIAL

+ CALCIUM IN PASTE (Active Siliceous+ Aluminate materials)

Attack bySulphates

+Chlorides

WEAKNESS IN CONCRETE WATER STRENGH

TIGHTNESS

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

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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start consuming the formed Ca(OH)2 (Hammond, 1987a).The intensity of the pozzolanic

activity is a measure of 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 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

RAWCLAY

PALM KERNELSHELLS

CLAYPOWDER

SHELLPOWDER

HORIZONTALMIXER

PULVERIZER

CLAYPOZZOLANA

Air dry(3 days)

Ball MillHammer Mill

< 150 m size

9 tonnes/batch 5 tonnes/hr.

VERTICA

LKILN

NODULES

HEAT

FUEL


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clay pozzolanas produced at clay calcinations of 700 oC, 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 (Fig.10):

(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%.

3.4 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


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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.

4.0 THREATS TO THE NASCENT CLAY POZZOLANA INDUSTRY

Threats that may stall or derail this promising industry are financial, technical and political.

4.1 Financial

Cost of machinery is fairly high. If the process is to be mechanized (at least, at the final

milling/bagging stage), the machinery will have to be manufactured locally. Presently, with

the exception of the miller/bagging machine, all other equipment is produced locally, and

that is an encouraging development. The cost per small-size factory (as at the BRRI) is

$300,000 presently and may discourage most district assemblies from replicating the

developments at the BRRI. Some form of government intervention is required.

Haulage costs for raw clay for the prototype plant is high. This has to do with the fact that

the BRRI is at least, 30km from the source of appropriate clay. Production units, when in

operation, are recommended to be sited at or near clay deposits and this should reduce

the cost of winning clay considerably.

4.2 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.


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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.

4.3 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.

5.0 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


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available production methods and seemingly laid-back political support are confronted

early.

6.0 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.

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.


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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.

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