OPPORTUNITY FOR THE INCREASED USE OF CLAY POZZOLANA –
THE BRRI EXPERIENCE
K. A. Solomon-Ayeh
Building and Road Research Institute (BRRI)
Kumasi, Ghana
solomonayeh@yahooo.com
Abstract
Pozzolanic properties of bauxite wastes and clay has been researched into by the Building and
Road Research Institute (BRRI) for over 30 years. It has recently come into its 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 satisfactory
compressive strength results from concretes produced with up to 30% replacement of OPC by
clay pozzolana, favourable durability properties, relatively low cost of a prototype house and the
eminent 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 effort
researchers, designers, builders and investors should enable this opportunity to be seized.
This paper briefly presents the BRRI’s efforts so far in taking this agenda forward.
Keywords:
1.0
OPC, cement, clay, pozzolana, strength
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
Dams in the U.S.A. (Davis, 1949) and are envisaged to be used as dam core material for the proposed
(1)
Bui Dam in Ghana in 2008 .
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
o
of OPC with calcined (700-900 C) 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 (GH¢8.5/GH¢2.4 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.
(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.
CLAYS + LIMESTONES
CLINKER
Heat
Add Gypsum
+
Grind
OPC
+
WATER
HYDRATION
C3S + C-S-H + Ca(OH)2
Release of LIME In Solution
+ ATMOSPHERIC C(OH)2
+ CALCIUM IN PASTE
CaCO3 + C(OH)2
+ CLAY POZZOLANIC
MATERIAL
(Active Siliceous
+ Aluminate materials)
HYDROUS CALCIUM SILICATES
Attack by
Sulphates
+
Chlorides
WEAKNESS IN CONCRETE
(has low solubility)
WATER
TIGHTNESS
STRENGH
Fig. 1.0: Schematic Chemical Reaction of Cement and Pozzolana and properties
of products
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
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
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, updraft 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.
RAW
CLAY
PALM KERNEL
SHELLS
Hammer Mill
Ball Mill
CLAY
POWDER
SHELL
POWDER
< 150µm size
HORIZONTAL
MIXER
NODDLEZER
Air dry
(3 days)
VERTICAL KILN
NODULES
BLOWER
•
.
.
9 tonnes/batch
5 tonnes/
hr.
PULVERIZER
HEAT
CLAY
POZZOLANA
FUEL
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 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).
Fig.4.0 – Palm kernel shells
Fig.5.0 – Grinder for palm kernel shells
Fig.8.0 – Mixer
Fig. 10.0 – Nodulized clays
Fig. 9.0 - Nodulizer
Fig. 11.0 – Calcination in kiln
Fig. 12.0 – Calcined clay pozzolana
Fig. 14.0 – Finished building using
clay pozzolana cement
Fig. 13.0 - Bagging process
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.
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
1970’s 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.
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
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