Journal of the Network of African Countries on Local Building Materials and Technologies  Volume 1, Number 4 (HABITAT, 1991, 48 p.) 
^{*} By A.A. Andam, B. Sc (Eng.), Ph.D., C. Eng., M.I.C.E., School of Engineering, University of Science and Technology, Kumasi, Ghana.This paper was presented to the Seminar on Local Materials for Housing, Third International Seminar of the African Network of Scientific and Technological Institutions (ANSTI), Civil Engineering Subnetwork, held at the University of Mauritius, March 1990. ANSTI is a UNESCOsponsored network.
Abstract
The paper identifies underfiring of clay bricks as one problem facing the relatively new brick industry in Ghana. A method, outlined in this paper, can be used to determine the residual firing temperature of a given clay brick. This method relates the inplane compressive strength of the clay brick to the firing temperature. The method shows that if vitrification has not taken place in the given fired clay brick, then, a calibration curve could be constructed from the results of the method and the same used to determine the residual firing temperature of the given clay brick. However, if vitrification has already taken place in the firedclay brick, then, a further method is outlined that can be used to determine the optimum firing temperature of the given clay brick.
Introduction
The most popular material used in the construction of buildings in Ghana is the sandcrete block. At present, all the cement that is used to manufacture sandcrete blocks is imported. As foreign exchange has become scarce, the need to develop an alternative local building material that will not rely on foreign inputs has, therefore, become paramount.
Ghana’s brick industry is about 10 years old and is plagued with developmental problems. One such problem, which is common to emerging brick industries, is the durability of the end product. Bricks used in external areas, such as fence walling, do not usually have their exteriors rendered, thus, exposing the bare bricks to rain and sun. In such cases, if the bricks have not been adequately fired during their manufacture, they will deteriorate first by flaking and, eventually, by total disintegration of the brick mass.
When a brick is fired, it undergoes both chemical and mechanical changes. Notably, the colour usually changes from dull clay texture to the aesthetically pleasing brick colour depending on the chemical composition of the clay. Ideally, it is desirable that during the firing process of the brick, vitrification should take place and be sustained long enough to enable the brick to develop its inherent resistance to rain and frost. If vitrification is achieved at the desired rate, the brick also acquires beneficial inplane compressive strength and could function in a dual role as a loadbearing and an infill material.
Clay deposits vary in their chemical composition. Thus, the temperature at which vitrification takes place is peculiar to every clay sample. In practice, clay samples from the same deposit may be regarded as having a similar optimum firing temperature at which vitrification would occur.
Test procedure
Ten solid bricks, each measuring 215 mm x 102.5 mm x 65 mm, were moulded and pressed under laboratory conditions. Two clay samples from Gomoa Mprumaim (Central Region of Ghana) and from the Accra plains, were used. These bricks were air dried and, then, fired in a kiln to a constant temperature of 400°C. Similarly, four sets of 10 bricks per set from the same clay deposits were moulded, pressed, air dried and fired to 500°C, 600°C, 700°C and 800°C, respectively. After the firing, for each brick, the weight and dimensions were measured. Each brick was then tested to failure for compressive strength.
All the debris from the crushed clay bricks fired at 400°C were carefully collected from the compression machine and ground into fireclay powder. The same was done for all the bricks fired at 500°C, 600°C, 700°C and 800°C.
Sixty briquettes, each measuring 130 mm x 25 mm x 25 mm, were formed out of the clay that had been ground from the 400°C brick debris. Sixty briquettes were also formed out of the clay that had been ground from the 500°C and 600°C brick debris, respectively. At this stage, however, the clay that had been ground from the 700°C and 800°C brick debris was found to have lost nearly all its clay fraction and could not possibly be used to form briquettes. This clay was thus discarded from the first group which was now subjected to further tests (see table 1).
All the 60 briquettes with residual firing temperature of 400°C were grouped as shown in table 1. The briquettes in group 1 were air dried and fired to a constant temperature of 300°C in the kiln (see table 1). Similarly, the briquettes in groups 2, 3, 4, 5, 6 were air dried and fired to constant temperatures of 400°C, 500°C, 600°C, 700°C and 800°C respectively. After the firing, the weights and dimensions of each briquette were measured. Finally, they were tested to failure for compressive strength.
Table 1. Briquette groups for bricks fired at 400°C (similarly for bricks fired at 500°C and 600°C)
Group 
Number of briquettes 
Residual firing temperature (°C) 
Final firing temperature (°C) 
1 
10 
400 
300 
2 
10 
400 
400 
3 
10 
400 
500 
4 
10 
400 
600 
5 
10 
400 
700 
6 
10 
400 
800 
Loss of clay fraction
It is generally known that the clay content in a brick provides the needed binding force to hold all particles together. As the clay is fired and vitrified, the essential plasticity is progressively lost. This phenomenon is not immediately known to the observer who rather finds the wellfired brick strong and dense. However, if this fired brick is crushed and ground, it will be found that the clay particles had, during vitrification, disintegrated into cohesionless particles.
In this particular test, loss of clay fraction resulted in the weakening of the briquette which developed cracks during the various stages of the test. This phenomenon, as should be expected, varied in various degrees with the two clay samples.
The clay from the Accra Plains generally showed more susceptibility to clay fraction loss during vitrification than the clay from Gomoa Mprumaim. With a residual brick firing temperature of 400°C, all the briquettes from the two clay deposits sustained sufficient binding strength throughout fabrication and firing processes. With a residual brick firing temperature of 500°C about 60 per cent of the briquettes from the Accra Plains cracked during the drying stage. The remaining 40 per cent of the briquettes from Gomoa Mprumaim clay deposit, although visibly weaker than the previous set where residual firing temperature was 400°C, did not crack during the drying and firing stages. Finally, briquettes with residual firing temperature of 600°C behaved exactly in the same way as those with a residual firing temperature of 500°C. That is why the briquettes from the Accra Plains cracked whilst those from Gomoa Mprumaim did not crack though they were visibly weak.
Test results
A summary of compressive strength for both clay deposits is given in table 2. Figure 1 shows the variation of compressive strength with firing temperature for the Accra Plains clay samples for briquettes with residual firing temperature of 400°C. A regression analysis for the results in figure 1 gave the expression in equation 1:
C_{i} = 0.00775T + 0.15 [1]
where C_{i} is the compressive strength and T the firing temperature.
Figure 1. Strength curve for
400°C residual firing temperature  Accra Plains.
Figure 2 shows similar results for clay samples from Gomoa Mprumaim. The briquettes had a residual firing temperature of 400°C and a regression analysis gave the expression in equation 2:
C_{400} = 0.0106T + 0.6 [2]
Figure 2. Strength curve for
400°C residual firing temperature  Gomoa Mprumaim.
Table 2. Compressive strength of laboratory manufactured solid bricks
Clay deposit 
Average compressive strength (N/sq mm)^{a}  

400°C 
500°C 
600°C  

Mean 
Standard deviation 
Mean 
Standard deviation 
Mean 
Standard deviation 
Accra Plains 
2.9 
0.3 
10.2 
1.5 
18.0 
3.3 
Gomoa Mprumaim 
4.9 
0.5 
18.7 
2.0 
20.3 
3.8 
^{a} Mean often solid bricks sized 215 mm x 102.5 mm x 65 mm.
Figure 3 shows similar results for briquettes with residual temperature of 500°C from Gomoa Mprumaim clay deposits. It is clear from the graph that vitrification is taking place with consequent loss of clay fraction and low compressive strength. A regression analysis gave the expression in equation 3:
C_{500} = 0.05T + 0.043 [3]
Figure 3. Strength curve for
500°C residual firing temperature  Gomoa Mprumaim
Figure 4 shows the results for briquettes with residual firing temperature of 600°C. The results are similar to figure 3 and indicate that vitrification is taking place. A regression analysis gave the expression in equation 4:
C_{600} = 0.035T + 0.024 [4]
Figure 4. Strength curve for
600°C residual firing temperature  Gomoa Mprumaim
Optimum firing temperature
In table 2, the mean compressive strength of solid bricks from the Accra Plains clay deposit fired at 400°C was 2.9 N/mm^{2}. If this compressive strength is substituted in the regression formula in equation [1], the firing temperature obtained is 355°C which is only 11.3 per cent below the magnitude of the residual temperature of 400°C. Similarly, from table 2, the mean compressive strength of solid bricks from Gomoa Mprumaim clay deposit fired at 400°C was 4.9 N/mm^{2}. If this compressive strength is substituted in the regression formula in equation [2], the firing temperature obtained is 405°C which is 1.3 per cent above the magnitude of the residual temperature of 400°C. However, if this exercise is repeated for equations [3] and [4], the correlation is very poor. It is observed that in equations [3] and [4], vitrification has already taken place whilst in equations [1] and [2] there is no vitrification in the clay brick. Thus, figures 1 and 2 could be regarded as calibration curves for the various clay deposits and the T in equations [1] and [2] should thus be regarded as residual temperatures of the original clay brick.
Figure 5 has been extracted from figures 2,3 and 4. Figures 2, 3 and 4 show the progressive path of the Gomoa Mprumaim clay samples from previtrification to postvitrification. The parabolic curves in figure 5 are lower bound and suggest an optimum firing temperature value of about 530°C for this particular clay sample.
Figure 5. Optimum firing
temperature
Conclusions
In this paper, a test procedure has been outlined which can be used to determine the optimum firing temperature of a clay brick given a specific clay deposit.
The experimental results indicate that if vitrification has not taken place in a clay brick, then, calibration curves similar to those in figures 1 and 2 could be used to determine the residual firing temperature of a given clay brick. However, if vitrification has already taken place in the clay brick, then, the correlation is very poor and these curves should not be used as calibration curves.
The optimum firing temperature of a given clay sample can be obtained by the method outlined in this paper.
The method outlined here is invaluable to any brick industry that has the developmental problem of brick underfiring.
Acknowledgements
Acknowledgement is given to the technical staff of the Department of Civil Engineering, University of Science and Technology, Kumasi, for their assistance in field clay sampling and to Mr. S.I. Akinbola, who helped with some of the laboratory experiments.