Experimental Studies on Fly Ash-Sand-Lime Bricks with Gypsum Addition

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Brick specimens were produced under the conditions given in Table 2. At least five bricks were made for each specimen type. Before making a brick, each ingredient of the raw materials was dried in a muffle furnace at 110℃ for 24 hour. Required amount of each ingredient was weighed, 14% moisture was added and the components were mixed thoroughly. To ensure uniform size of the bricks a known weight of mixture was used each time to fill the mould cavity. Dimension of the mould cavity opening was 6 X 3.5 cm. A hydraulic press was used to apply pressure for a period of 15 Sec (Figure 1). The bricks (Figure 2) were then ejected and finally cured. Curing was done in air, using water spray, by putting the bricks under wet cloth and by keeping the bricks immersed in water. Bricks formed under various pressures were also cured for different periods in optimum curing condition.

Figure 1. Hydraulic press

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Figure 2. Fly ash-sand-lime-gypsum bricks

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Table 2. Test parameters

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2.1. Test Methods
2.1.1. Compressive Strength

Compressive strength was determined by applying load on the specimen using a Universal Testing Machine. Load was applied on an area measuring 6mm X 3.5 mm [The size of one face of the entire brick].

2.1.2. Microstructure

The samples were observed under an optical microscope (OM) and micrographs were recorded with a digital camera (OPTIKA Microscope B-600 MET) to investigate porosity. No preparation of the samples was done.

2.1.3. Shrinkage Property

Dimensions of the bricks were measured immediately after making the bricks and also after curing to determine the shrinkage of the bricks.

2.1.4. Unit Volume Weight

After 7 days of curing period bricks were dried at 110℃ for 24 hr and then allowed to cool to room temperature. Dry weight D (gm) was then measured. Following that the bricks were immersed for 24 hr in water at room temperature and suspended weight S (gm) was measured. The bricks were then removed, the surface water was wiped off with a damp cloth and the saturated weight W (gm) was measured within 5 min after removing the bricks from the water bath. Unit volume weight B, gm/cm³ = D/ V, where volume V, cm³ = (W - S) was calculated.

2.1.5. Initial Rate of Absorption (IRA)

After measuring dry weight D (gm) as mentioned earlier, the bed surface of the brick (the face measuring 6mm X 3.5 mm ) was caused to absorb water for 1 min. Water was wiped out completely from the surface of the brick within 10 s of removal from contact with the water and weight D’ (gm) was determined within 2 minutes. Initial rate of absorption IRA, % = D’ – D was then calculated [ASTM Designation C 67 – 00].

2.1.6. Absorption Capacity

ASTM Designation C 67 – 00 was followed to measure the absorption capacity A, % = [(W – D)/D].

2.1.7. Apparent Porosity, Open Pore and Impervious Pore

ASTM Designation C 67 – 00 was followed to determine apparent porosity P, % = [(W - D)/ V], Open pore volume, cm³= W – D and impervious pore volume, cm³ = D – S.

2.1.8. Efflorescence Test

This test was carried out according to ASTM C67-08. For this test, one brick was vertically placed in water with one end immersed and another brick stored in ambient condition. After 7 days both bricks were dried in oven at 110℃ for 24 hours. Then both bricks were observed from 10 ft distance under not less than 50 fc with normal vision. If any difference is observed because of presence of any salt deposit then the rating is reported as ‘effloresced’. If no difference is noted, the rating is reported as ‘not effloresced’.

2.1.9. Radioactivity test

Radio activity of the mixture of optimum composition was examined from Health Physics Division of Bangladesh Atomic Energy Commission.

2.1.10. Variation of Curing Process and Time

Effects of four different curing processes i.e. in still air, with water spray twice a day, keeping the specimen under wet cloth and keeping the specimen immersed under water for seven days were examined. Finally the effect of long duration curing was studied. For optimum composition and different compaction pressure one set of bricks were cured in air for five weeks. Another set was cured in air for one week and then for four weeks under water. Then their compressive strengths were compared. Difference in compressive strength was investigated with Fourier transform infrared spectroscopy (FTIR).

3. Results and Discussions

3.1. The Effect of Sand on Compressive Strength

The specimens T1 – T4 (Table 2) were prepared to determine the effect of amount of sand on compressive strength. The compressive strength could be increased from 113.46 kg/cm² to 165.57 kg/cm² by increasing the amount of sand from 10 to 40% (Figure 3). The maximum strength was found for 40% sand – 50% fly ash – 10% lime bricks with 2% gypsum. As bulk utilization of fly ash was the major goal of this investigation, the optimum amount of sand was taken to be 30% that gave a compressive strength of 155.20 kg/cm². The improvement in mechanical strength with increasing sand content has been attributed to a greater extent by the increased amount of free SiO₂ which reacts more easily with lime than fly ash ^[8].

3.2. The Determination of the Optimum Lime and Fly Ash Content

Compressive strength test results conducted on specimens T5 – T9 showed that maximum strength could be obtained with 15% lime and 55% fly ash (Figure 4). Compressive strength found for this composition was 181.75 kg/cm².

Figure 3. Effect of sand addition on compressive strength

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Figure 4. Effect of lime addition on compressive strength

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3.3. Optimum Amount of Gypsum Addition

The variation in compressive strengths of specimens T10 - T13 containing varying amounts of gypsum is shown in Figure 5. It can be seen that the compressive strength increased from 181.75 kg/cm² for 2 percent gypsum to 287.78 kg/cm²for an optimum gypsum content of 14%.

3.4. Effect of Brick forming Pressure on the Compressive Strength

Compressive strength tests on specimens T14 - T16 showed that compressive strength increased with increasing brick forming pressure (Figure 6). Specimen T16 showed maximum compressive strength of 417.96 kg/ cm²for a brick forming pressure of 3000 psi.

Figure 5. Effect of gypsum addition on compressive strength

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Figure 6. Effect of brick forming pressure on compressive strength

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3.5. Microstructure

Specimens T14, T15 and T16 are the specimens with optimum composition and different brick forming pressures. Microstructure and other properties besides compressive strength of these specimens were investigated to optimize brick forming pressure. Figure 7 shows that both individual pore size and total porosity decreased with increasing brick forming pressure. This is to be expected.

Figure 7. Microstructure of bricks formed with pressure (a) 1000 psi (b) 2000 psi (c) 3000 psi [All magnification 500X]

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3.6. Shrinkage property

No noticeable shrinkage occurred as was measured for specimens T14, T15 and T16.

3.7. Unit Volume Weight

With increasing brick forming pressure unit volume weight of the bricks increased (Figure 8). But above 2000 psi the change in density was insignificant. Maximum density of 1.81 gm /cc was found for bricks formed under 3000 psi pressure which is much lower than the density of fired clay based bricks density.

3.8. Initial Rate of Absorption (IRA)

Initial rate of absorption of bricks made under 1000 psi pressure exceeded 30 gm and so according to ASTM C 67 – 00 these bricks should be wetted before laying (Figure 9). With increasing brick forming pressure IRA decreased below 30 gm. Lowest IRA of 14.84 gm was found for a pressure of 3000 psi. So these bricks are not needed to be wetted before laying.

3.9. Absorption Capacity

As shown in Figure 9 absorption capacity was 14.63% for brick forming pressure 1000 psi. With increasing pressure this value dropped down to 11.58% for a pressure of 3000 psi. Above 2000 psi pressure change in absorption capacity was not significant.

Figure 8. Effect of brick forming pressure on unit volume weight

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Figure 9. Effect of brick forming pressure on absorption capacity and IRA

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3.10. Apparent Porosity, Open Pore and Impervious Pore

Effect of brick forming pressure on apparent porosity, open pore and close pore volume is shown in Figure 10. Apparent porosity decreased with increasing pressure. Above 2000 psi the change was insignificant. Open pore volume also followed the same trend. Whatever, impervious pore volume was not affected by brick forming pressure. This result indicted that more densification at same pressure is possible if close pore can be decreased by means of particle size control.

Figure 10. Effect of brick forming pressure on apparent porosity, open pore and impervious pore volume

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3.11. Efflorescence

Bricks of optimum composition were found to be ‘not effloresced’. As seen in Figure 11 no salt was found on the tested brick surface.

Figure 11. Efflorescence test of bricks of optimum composition

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3.12. Radioactivity

Radioactivity of Cesium-137 was below the detection limit of the machine i.e. below 1.54 Bq/kg. So these bricks are not threatening as radioactivity source.

3.13. Effect of Curing Process

Specimen T17 was cured in still air. Specimens T18, T19 and T20 were cured with water spray twice a day, keeping the specimen under wet cloth and keeping the specimen immersed under water respectively. Maximum compressive strength of 442.96 kg/cm² was found for specimen T18 (Figure 12).

Figure 12. Effect of curing process on compressive strength

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3.14. Effect of Curing Period

One set of bricks was cured for seven days in air; one set for five weeks in air and another set for first four weeks in water followed by one week in air. Maximum strength of 877.36 kg/cm²was found for bricks formed under 3000 psi pressure and cured in water for four weeks followed by one week in air. FTIR analysis has shown (Figure 14) that when bricks were cured in water for four weeks followed by one week in air instead of five weeks in air, stronger inter-molecular OH-O bond became dominant over the weaker intra-molecular OH bond.

Figure 13. Effect of curing process on compressive strength

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Figure 14. FTIR analysis of bricks cured under different curing condition

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