THE RAW MATERIAL AND PRODUCT QUALITY IN THE BRICK INDUSTRY (The small scale producer)

BY

D. P. Katale and Peter Tawodzera Department of Civil Engineering,

University of Zimbabwe, P. O. Box MP 167 Mount Pleasant Harare.

Abstract An investigation was carried out to investigate the raw materials used and the quality of bricks produced. Of special interest was the performance of small-scale producers. Tests were conducted on materials used and the behaviour of the products produced both at the green and fired stages of brick making. The results showed that all the producers were able to select the right type of raw materials to use. The result from testing of the fired brick however showed that almost all the small-scale producers were not able to meet the performance requirements.

INTRODUCTION The Department of Civil Engineering, University of Zimbabwe together with ITDG Zimbabwe carried out an investigation into the product quality and raw material used in the Brick production industry. Scope of work The scope of work included: (a) Determining the product characteristics, both the intermediate characteristics; the green

bricks and the final products; the fired bricks. The parameters used for this part of the investigation included

(i) the average density of the green bricks (ii) the average moisture content of the green bricks (iii) the average density of the fired bricks (iv) the average water absorption of the fired bricks (v) the average compressive strength of fired bricks. (b) Determining the raw material characteristics. The parameters used here were (i) the particle size distribution of the soil (ii) the plasticity characteristics of the soil. Procedure Soil samples, green bricks and fired bricks were obtained from a number of brick producers, in a number of locations. The average properties were determined using at least three bricks except where the number of bricks available was limited. The tests that were conducted are listed below. Average density The densities of the bricks both the green and fired bricks were determined by weighing the bricks using a balance accurate to 1 g. The volume of the bricks were determined by measuring the length, the breadth and the height of each brick to an accuracy of 0.1 mm using vernier callipers Three readings were made for each of these linear dimension and the average values used in calculating the volume of each brick. The density was then calculated as the weight in kilograms divide by the volume in cubic meters. Moisture content The moisture content for each green brick was measured using the method detailed in close 6.1 of SAZ 185: part 1: 19981. The whole brick was used to measure the moisture content. Water absorption

The water absorption of fired bricks was obtained using the method detailed in SAZS 221 : 19912. The fired brick was dried in an oven at 105 - 110 degrees centigrade. The oven dry weight of the brick was recorded and then the brick submerged in cold water for twenty four hours. The brick was removed from the water, quickly surface dried and weighed immediately. The absorption of the brick was then calculated as the water absorbed by the brick expressed as a percentage of the dry weight of the brick. The compressive strength of fired bricks The compressive strength of the fired bricks was measured as detailed in SAZS 221 : 19912 . Immediately after measuring the water absorption the brick was used to determine its compressive strength. The brick was placed between two plywood boards and crushed using a universal compressive machine capable of providing the required force. The loading surface was the one that would be used for mortar bedding in a normal wall. The compressive strength was then calculated as the failure load divided by the loading area. Particle size analysis The particle size analysis was conducted using two methods. These were the wet sieve analysis and the hydrometer analysis. The results from the two methods combined to give a description of the grain size distribution for the various soils. The detailed procedures for these two tests are given in close 5 of SAZ 185: part 1: 19981.. Plasticity indices The liquid limit was measured using method A, close 7.2 of SAZ 185: part 1: 1998. While the plastic limit was measured according to close 8 of the same standard. The results of the liquid limit and the plastic limit tests together with the grain size distribution were used to calculate the plasticity index and plasticity product. The calculation procedures for these are given in close 9 of SAZ 185: part 1: 19981. RESULTS The results from all the tests are given in the tables and figures below. The grain size distribution results are shown in figure 1 and the plasticity results are given in table 1. The properties of the green bricks are shown in table 2; these are the dimensions, the moisture content and the dry density. The dimensions, water absorption rate, the dry density and the compressive strength of the fired bricks are shown in table 3.

Table 1. Soil Plasticity (Raw materials) Soil sample Liquid Plastic Plasticity Linear Fineness Plasticity

Location limit limit (%) index (%)shrinkag

e Index Product wL (%) wP (%) IP (%) IS (%) FI(%) Large scale producers ALP 40.2 24.8 15.4 5.3 75 1155GWE-RS 31.8 17.2 14.6 5.3 61 891MUT 36.6 17.4 19.2 4 39 749 Medium scale producers KWE 49.3 22.9 26.4 8.6 -- --NORT 46.1 19.6 26.5 8 -- -- Small scale producers CR-LY 51.3 32.8 18.5 7.3 51 944CR-B 60.8 30.8 30 10 -- --EPW 26.6 12.2 14.4 2 43 619EPW-OG 28.3 15.3 13 4.7 33 429NPEPW 38.9 22.7 16.2 6.7 30 486FAM 31.5 16.4 15.1 6 72 1087MHO 36.6 20.4 16.2 6.7 -- --SEK 18.9 14 4.9 1.3 -- --

Figure 1 Gra in size distribution of the soils used.

0

10

20

30

40

50

60

70

80

90

100

0.0001 0.001 0.01 0.1 1 10 100 1000

g ra in s ize (m m )

ALPCR-BCR-LYEPWEPW -OGFAMGW E-RSMUTNPEPW

FINE COARSEMEDIUMFINEMEDIUM

SILTCOARSECOARSEMEDIUMFINE

GRA V ELSA NDCLA Y COBBLES

Table 2. Dimensions, Moisture content and dry density for the green bricks Location Average dimensions

Length

(L) Width (W)Height

(H) Moisture

Content Dry

Density (mm) (mm) (mm) wg (%) (kg/m3) Large scale producers ALP 229.4 109.5 73.8 1.5 1664.8 GWE-RS 229.5 113 75.2 1.4 1859.6 MUT 232.5 114.9 76 1.3 1846.8 Medium scale producers KWE 225.5 113.7 71.2 3.7 1779.8 NOR 223.4 107.6 74.1 4.92 1780.3 Small scale producers CR-LY 220.6 108.7 73.5 5.1 1468.1 CR-B -- -- -- -- -- EPW 230.2 116.9 75.3 2.2 1632.3 EPW-OG -- -- -- -- -- NPEPW FAM 215.7 104.6 67.6 3.3 1837.5 MHO 229.2 115.6 70.3 3.28 1507.5 SEK 211.4 117.1 70.8 0.08 1573.6

Table 2. Dimensions, Water Absorption, Dry density and the average Strength of fired bricks

Location Average dimensions

Length

(L)Width

(W)Height

(H)

Water absorptio

nDry

Density Average Strength

(mm) (mm) (mm) wg (%) (kg/m3) Mpa

Large scale producers

ALP (common) 230.5 108.5 75.3 19.3 1601.3 8.2

ALP (hard burn) 222.6 107.8 73.4 13.7 1741.5 23.7

ALP (combined) 226.5 108.1 74.3 16.5 1671.4 15.9

GWE-RS 230.0 112.8 75.6 11.1 1643.6 14.1

MUT 231.2 113.7 75.1 14.3 1795.9 13.7

Medium scale producers

KWE 229.5 117.4 72.0 15.3 1582.0 13.3

NOR 223.3 107.7 75.1 17.3 1637.8 13.8

Small scale producers

CR-LY 220.0 113.3 75.1 19.4 1402.2 4.3

CR-B -- -- -- -- -- --

EPW 231.4 115.2 76.8 15.3 1590.3 3.3

EPW-OG -- -- -- -- -- --

NPEPW -- -- -- -- -- --

FAM (common) 217.8 106.3 70.3 17.7 1675.9 10.7

FAM (hard burn) 212.7 102.3 69.1 8.2 1807.0 23.3

FAM (combined) 215.2 104.3 69.7 13.0 1741.5 17

MHO 230.2 114.3 71.5 22.6 1424.5 3.2

SEK 212.7 113.1 76.8 12.3 1417.6 2.2

NB. Two producers sorted their products into two classes, common and hard burns. DISCUSSION In order to produce high quality bricks the raw material must have suitable properties. Soils with these properties are found in numerous places making it possible to make bricks in almost any locality. The brick making process include mixing the soil with water and kneading it to produce a plastic workable material which can be moulded into the brick shape. The moulded brick must be moved out of the moulding equipment and placed in the drying area without distorting its shape. The wet brick must then be dried without producing cracks in the brick. Finally the dried brick is fired to produce a strong well shaped crack free and undistorted unit ready for use in a building. The type of soil used affect the results obtained at each stage of the brick making. Suitable soils are composed of sand silts and clays. The clays provide the plasticity, which allows for easy preparation and moulding of the clay into any required shape without breaking. The more clay there is in a soil the easier it is for it to be moulded into any shape and for it to keep that shape. The clay also acts as the glue for the soil. Unfortunately clay expands and shrinks with changes in moisture content. Too much clay in the soil means that the bricks will crack while being dried and this will render them useless. The shrinking properties of soils do not only depend on the quantity of clay but also on the type of clay minerals that the clay is composed of. Montmorillonite has a very high shrinkage rate in comparison to kaolin. It is therefore necessary to know the composition of the soil and how to change it if required. Good brick making soil should contain about 10% to 50 % clay3 the rest being silt and sand. Looking at figures 1 only the soil from CR-B failed this criterion, having a clay content of about 60%. The rest fell within a range of 10% to 35%, which is acceptable for brick making. The samples from NOR, FAM, KWE, MUT and one of the samples from EPW had up to 10% gravel. This might have to be sieved out. The plasticity of the soil is another important property of the soil for making bricks. If a soil has very low plasticity it will be very difficult to mould and remove from the mould. Pure sand is an extreme example of this. The soils with high plastic limit need a lot of water to make them workable. This means that the drying time of green bricks will be long. Another even more serious reason is that soils with high plastic limits tend to be the ones with high activity and the

ones more likely to crack on drying. These factors mean that the soils to use should not be of very high plasticity. The plastic limit range of 12% to 22%, liquid limit range of 30% to 35% and a plasticity index range of 7% to 18% have been suggested as adequate for brick making using traditional methods3. Soils with values outside these ranges can and have been successfully used. Table 1 shows the results of the plasticity test on the soils used by the brick makers in this study. Most of the small-scale producers satisfied these criteria or violated them only marginally; the only exception being the soil from CR-LY. Of the large-scale producers only GWE-RS satisfied the criteria while all the soils from the medium scale producers did not. The soils that did not satisfy these criteria were more plastic than required and would require more water to work with or tend to crack while drying. But these are soils that large scale and medium scale producers can work with since they have the capacity to mix and mould soils using little amounts of water. Why is it important to know the properties of soils being used? This is a question large scale producers never ask themselves because they know that the information they get from testing soil enable them to adjust the materials they use and their methods of working in order to produce their bricks as effectively and as efficiently as possible. Small scale producers on the other hand need to be made aware of the importance of understanding the properties of the soils they use and how they affect their operations. When unsuitable soils, both in terms of the soil composition and its plasticity, are used a lot of bricks will break while they are being made. The ones, which do not break, tend to be weak and might have limited use. This might look just like a waste of the brick makers time and may be money but it goes beyond this. A lot of energy is used to fire the bricks and whenever fired bricks break and become unusable this energy is wasted. Most of the energy sources used to fire bricks have an adverse impact on the environment, both in terms of the exhaust gases they produce and the depletion and destruction of forests when wood is used. It therefore stands to reason that among other things, efficient use of energy reduces the impact of brick making on the environment. Knowing the soils and how to work with the soil helps in efficient utilisation of energy. There are quite a number of parameters that are used to measure the quality of bricks worldwide. Zimbabwe has a set of standards specifying the requirements that must be satisfied by various types of bricks. The better the quality of the bricks the higher the price that has to be paid for them. A major problem with bricks produced by small-scale brick makers is their quality. This poor quality is reflected in the market price of these brick in comparison to those produced by large-scale producers. Currently these bricks often referred to as farm bricks fetch only about a fifth of the price of those produced by the large-scale producers. The first quality indicator that a customer has of the bricks he is buying is their shape, appearance and finish of the bricks. This has a very strong bearing on the price that he will be willing to pay for these products. Facing bricks are differentiated from industrial ones by just their attractive appearance. This indicator was related to the control the brick makers had over their moulding and this can be seen in dimensional variation of the bricks from the various sites. SAZS 221 19912 gives dimensional tolerances for bricks of 229-232 mm for the length, 110-116 mm for the width and 71-75 mm for the height. Looking at the dimensions of both the green bricks and the burnt bricks in table 2.1 and 3.1 it is evident that the large-scale producers have the best quality control. The largest variation for the small-scale producers was in the length of the bricks. The overall impression however is that closer control of the brick dimensions need to be exercised by all the producers. Energy loss is a waste in terms of money lost but also has a negative impact on the environment. An effort must therefore be made at all times to minimise this loss. The energy is lost from the kiln surface area. Reducing the surface area in relation to the volume of the kiln reduces the energy lost per brick made. To achieve this, large compact kilns should be used. The height of the kiln that can be made depends on how easily the bricks at the bottom of the kiln break. The stronger the bricks the higher the kiln can be made. The strength of the brick in this position and condition depends on how dry the green bricks are and how true the shape of the bricks is. The drier a

brick is the stronger it is. Bricks to be fired should therefore be as dry as they can be made. Bricks which are distorted provide local support points so that when they are piled on top of each other in a kiln they are supported at these points instead of the whole surface of the brick. As a result the height to which the kiln can be built is reduced and the efficiency of energy utilisation is likewise reduced. Again the lack of close control of the dimensions indicates that the bricks are likely to be distorted and therefore limit the height to which the kilns can be built. The first stage of firing the bricks does no more than drive off the moisture remaining in the bricks after the drying stage. If a lot of water is left in the bricks when the firing begins it means that a lot of energy will be used just to drive off this excess water. It is therefore important to make sure that the bricks are as dry as possible before they are put into a kiln for firing. This way the sun energy which is not paid for and pollution free is used instead of using the firing fuels that have a monetary as well as an environmental cost. Large-scale producers performed better in this respect as can be seen in table 2.2. Small-scale producers need to improve this facet of their production procedure. They would save both their money by using less energy and reduce the impact they have on the environment in the process. The density of bricks depends on the nature of soil used as well as the compactive energy utilised. It is therefore difficult to compare the practices of the different brick makers without letting them use the same soil type. What can be concluded from the results however is that the density of green bricks is increased with firing. A comparison of table 2 and 3 shows this density change. This is to be expected since the soil contracts first on loosing water and then continue to do so as the temperature is increased to produce ceramic changes as well as vitrification. The second observation that can be made from the results for the same soil types is that the compressive strength and the water absorption of the fired brick is correlated to the density of the fired brick. This can be seen in table 3. The results of the common and hard burnt bricks from ALP and FAM show that the strength increased and the water absorption decreased with an increase in the density of the bricks. Table 3 shows the compressive strength of the finished product. SAZS 2212 specifies that common bricks should have a minimum average strength 7 Mpa, industrial 15 Mpa, and Engineering bricks 25 Mpa. Of the small scale producers only one produced bricks which satisfied these criteria. With sorting this producer managed to make bricks that satisfy two classes, commons and industrial bricks. The remaining small-scale producers achieved compressive strength of between 2 and 5 Mpa. These are still bricks that can be used for non-load bearing partition walls3. Large and medium scale producers on the other hand had bricks that satisfied commons and industrial bricks criteria. Again they had a wide variation in their strength and sorting would be necessary to efficiently sell and use the bricks in an appropriate class. CONCLUSION 1. All the soils used by the various brick makers both large scale and small-scale producers except CR-B had the required clay content for making bricks. 2. The plasticity of the soils used by small-scale producers and one large-scale producer were in an acceptable range for traditional brick making or just marginally outside this range. The remaining ones had soils with higher plasticity; nevertheless they are still soils that can be used. These two factors indicate that selecting suitable soils was not difficult for all the brick producers 3. There was a lot of variation in the brick dimensions produced by the various brick makers. 4. The average density of green bricks varied between 1468 kg/m3 and 1860 kg/m3 while that of fired bricks varied between 1402 kg/m3 and 1807 kg/m3.

5. The average water absorption of the bricks ranged between 8.2% and 22.6%. For the same soil type this quantity had a correlation with the dry density of the fired bricks. 6. The compressive strength of the fired bricks varied between 2.2 Mpa and 23.7 Mpa. The Large and small-scale producer made bricks which were stronger than those of the small scale producers and satisfied the SAZS 221 specification for common and industrial bricks. The small-scale producers failed to satisfy these compressive standard. The exception being FAM which produced bricks suitable for both the commons class and the industrial class. The strength achieved by the other small-scale producers was nevertheless good enough for the bricks to be used in non-load bearing partition walls. This indicates that small scale producers have not yet mastered their technology well enough to consistently produce the required performance. With training however it should be possible for them to do so as evidenced by FAM. 7. Brick making negatively impacts the environment due to the green house gases produced during the firing stage and the cutting down of trees for fuel purposes. Any method used to increase the efficiency of energy utilisation during the firing stage and any reduction in the loss of the finished bricks will reduce this negative impact. REFERENCES 1 SAZ 185: part 1: 1998 Method of Testing Soils for Civil Engineering

Purposes 2 SAZS 221 : 1991 Zimbabwe standard specification for burnt clay building

bricks and blocks, 3 ILO 1984 Small-scale brick making, Technology series, Technological

memorandum No 6