THE USE OF FL Y ASH ANO FURNACE-BOTTOM ASH TO PROOUCE BRICK

Philippe Gleize1 and Humberto R. Roman2

1. ABSTRACT

A research programme has been developed to investigate the addition of either pulverized fly ash or furnace-bottom ash to c1ay for brick production. The ashes are wastes of a coal-burning power station of a southern state of BraziL The c1ay was obtained trom brickyard placed near the station. Three types of specimens were made: pure c1ay, c1ay and furnace-bottom ash and c1ay with pulverized fly ash. The mechanical and physical characteristics observed were the drying and firing shrinkage, the drying rate, the compressive strength, the water absorption, the suction rate and the apparent density. The structure of the material was observed through scanning electron microscopy The works compare the physical and mechanical testing results with the micro structure of the materiais

2. INTROOUCTION

A Brazilian coai buming e1ectric power station produces a large arnount of pulverized fly ash (PF A) and bottom slag furnace ash. The PF A has been largely used by the cement industry, but still part ofit together with the whole production ofthe bottom slag furnace slag ash are disposed in open air causing serious environrnental pollution problems

Keywords c1ay bricks, PF A, Furnace-bottom ash, material

:Visitor Researcher ofThe Civil Engineering Department ofThe Federal University of Santa Catarina.

2Lecturer ofThe Civil Engineering Department ofFederal University of Santa Catarina, Florianópolis, Se. , Brazil- 88040-900

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Close to the power station there are many small brickyards which produce low quality bricks of high cost The compressive strength of the bricks are normally below the Brazilian Standard specifications l , and the dimensions and shape are generally not in accordance with the standards2.

Buming clay with ash for brick making is not new as shown in the literature review3. The use of ash have reduced the shrinkage of the clay and decreased the need of energy to fire the bricks.

lnvestigation carried out by Brooks et al4 have found that the use of PF A could lead to economic savings in the manufacture of bricks, and that the compressive strengths were not affected by the decreasing of density and increasing porosity caused by the PF A addition.

The use of differenl coarse PF A particles with glass, silica sand and clay was sludied by Day and Bergman5 The results showed that it was important to add substantial quantity of grog to reduce lhe absorption, the ash coarse was not significant to better bricks properties and the initial rate of suction of bricks was high.

The authors did not find any research work developed with bottom-furnace-ash to produce brick. Considering the brickyards situation and the pollution problems caused by this waste material, it was decided to investigate the possibility of using this type of by­product to produce bricks. Some rnixes with PF A were also made for comparison. The testing programme and results were shown previously<J. This work describes the material structural analysis carried oul and the main conclusions,

3. EXPERIMENTAL WORK

The chemical analysis ofboth clay and ashes are shown in Table 1.

Table 1 - Chemical composition of clay and ashes

Compound Material (% by weight) Clay Fly-ash Bottom-ash

Si02 70,50 55,00 53 ,80 A120, 12,50 29.30 25 .60 CaO 0.03 1.50 0.67 MgO 038 0.80 0.65

Sulphate 0.06 0.20 0.08 Loss on Ignition 5.40 - -

Cylindrical specimens with 34.4 mm diameter and about 36.5 mm high were molded up to apressure of 20 N/mm2

Six different mixes of 5 samples were molded every time: one of pUfe c1ay specimen for lesling contrai and the others with the anlOunt of ash ranging from 10 lo 50 % in weight

The moisture conlent used was either 8 % or 10 % of lhe dried mass,

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Two firing cycIes of either 10 hours or 22 hours were used to bum the specimens. The first was made with 3 hours of maximum temperature, whilst the latter for 10 hours The maximum bunúng temperatures used were 750 oe, 950 0e and 1050 0e

The physical and mechanical characteristics of materials observed were: the drying and firing shrinkage, the apparent density, the suction rate, the water absorption and the compressive strength.

4. TEST RESUL TS

There was no significant difference between the results of the specimens with either 8 % or 10 % moisture Therefore, it was decided to use the mixing moisture of 10 % to continue the experimental programme.

Some ofthe resuIts obtained can be seen in Table 2. These are averages of5 specimens

Table 2 - Test results for firing cycle of 10 hours and bunúng temperature of950 0e

Amount of Ash in the mix (% in weight) Type ofTest Bottom-fumace-ash Pulverized fly-ash

O lO 20 30 40 50 O lO 20 30 40 Firing Shrinkage .2 .2 .5 .7 .5 .2 O .1 .6 .9 .7

Suction Rate 3.4 4.1 5.2 5.6 6.4 6.4 3.6 3.8 4.8 5.5 5.3 Absorption 15 .0 16.5 17.5 18.4 22.3 24.3 15 .6 16.1 16.7 18.0 22.5

Comp. Strength 18 .2 15 .6 11.0 10.3 G.9 5.7 17.4 17.8 136 111 7.9 Porositv 27.8 29.1 29.5 29.8 33.2 34.5 28.6 28 .6 28.5 29.5 33 .7

Fired densitv 1.86 1.77 1.69 1.63 1.49 1.42 1.84 1.78 UI 1.64 1.50 • ,L • ,L , Note. Shrinkage {V.). suCUon rate (kg/m .nun). Absorpuon (Vo). CompreSSl~e Strength (N/mm ) .

Porosity (%) and Fired Density (g/cm3)

The firing cycIes and the bunúng temperature have shown little influence on the mechanical characteristics of the material . The latter was significant only for the firing shrinkage and material porosity . Specimens at smaller temperature gave consistently smaller values of shrinkage and higher values of porosity The specimens burned at 750 0e have shown small expansion instead of shrinkage

Figure 1 shows results for specimens bumed in aIO' hours cycIe and burning temperature of 950 0e lt was not found difference of the results between specimens either with bottom-fumace ash or pulverized fly-ash.

The compressive strength decreased by increasing addition of both ashes. The reason for that may be explained by the micro structure of the specimens for different amounts of ashes. Figure 2 shows a 100 11m photograph of apure clay specimen, whilst Figures 3 and 4 show respectively cIay with 30% ash and cIay with 50% addition of ash.AlI of them were bumed for lO hours in a temperature of950 0e

The difference of aspect is clear. The layers and compacted structures of the cIay is substituted by a more irregular structure and specially more porouS.

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50 .2

6.1 24.9 5.6 35 .1 1.41

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26

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2:2

20

4

2

o

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1 I

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o 20 40 60

AMOUNT OF ASH IN THE MIX (%) ::::: BC)TTOM-F"CJRNAC E ASH Pt .. JL.VERI ZEI:) FL.·~'-ASJ-1

Figure 1 - Compressive strength of specimens burned in 950°C

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Figure 2 - Pure clay specimen (100 11m)

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I ..

Figure 3 - Brick with addition of 30% of ash (100 Ilm)

Figure 4 - Brick with addition of50 % ofash (100 Ilm)

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Indeed, spherical ash particles of amorphous silica (10 < 0 11m < 100) penetrate in the c1ay matrix concurring to increase the number of fractures, the number of cracks (Figures 3 and 4) and therefore, the porosity and water absorption. The increasing of absorption and porosity as function of the rate of ash in the rnixes can be seen in Figure 5.

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li' \J

LI: O Q,

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.'3 o ....,~; =~-=,!;!:::===:~e:=:::~ ,l,= =;l ;-==-25 J

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Porosity

i Water Absorption

o I~----~------._----_.------,_----_.------._------o 20 60

AMOUNT DF ASH IN THE MIX ( % ) o BOTTOM-FURN .A.CE ASH PULVERIZ ED FLY - AS H

Figure 5 - Influence of ash addition in the porosity and water absorption

The consequence is the fragility of the material and the decrease of the compressive strength with ash addition. The values are not in accordance with results found in the literature. This may occur due to the type of c\ay used, which is not proper to produce high strength bricks.

A1so the absorption increased with the addition of ash . The same occurred to the suction rate.

5. CONCLUSIONS

The type of c1ay used in the experiment is not suitable for production of high strength bricks. In addition, the spherical shape of the ash partic\es seems to contribute to decrease the compressive strength, since there wilI be more holes inside the bricks The use of smaller size particles of ash could lead to better results .

However, the compressive strength of the specimens are well inside the Brazilian Standard. As the bricks produced nearby the power station are normally used for c1adding walls, the low compressive strength is not the most important factor on their production.

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The type of molding may have influenced the results. The use of extrusion could allow the use of higher moisture contents with probable improvement of results

The results have shown that, as has been seen for pulverized fly-ash, fumace bottom-ash is suitable to be used for production of c1ay bricks.

5. REFERENCES

I . ABNT, Associação Brasileira de Normas Técnicas Tijolo maciço cerâmico para alvenaria: NBR 7170, 1983, Rio de Janeiro

2. ABNT, Associação Brasileira de Normas Técnicas. NBR 8042.Bloco cerâmico para Alvenaria - Formas e dimensões Nov. 1992

3. Barber, E. G , Jones, G. T. , Knight, P . G. K, and Miles, M. H., PFA Utilization, Central Electricity Generating Board, 1976

4 . Brooks, 1 J, Cabrera, 1 G, and EI-Badri, M., Properties of ClaylPF A Bricks, Proceedings of 8th International BrickIBlock Masonry Conference, Dublin, Ireland, Sept 1988, pp. 64-74.

5. Day, R. L , Bergman, J W. , Fly Ash as a Substitute for Clay in Brick Manufacture, Proceedings of8th International BrickIBlock Conference, Dublin, Ireland, Sept. 1988, pp. 14-25

6. Roman, H. R , Negreiros, F. T. , Bricks Made with Clay and Power Station Ashes, Proceedings 3rd International Masonry Conference, October 1992. London (in press)

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