USE OF CLASS F FLY ASH IN THE MANUFACTURE OF BUILDING BRICKS By Wenyi Hu Research Assistant and

Tarun R. Naik Director, Center for By-Product Utilization Department of Civil Engineering and Mechanics College of Engineering and Applied Science University of Wisconsin-Milwaukee P.O. Box 784 Milwaukee, WI 53201 Telephone: (414) 229-6696 Fax: (414) 229-6958

Abstract Satisfactory bricks for building purposes can be made from Class F fly ash, using lime and/or cement as a binder, and conventional aggregates. Methods of preparing the mix, molding and curing are described in this paper. Bricks were made by making a semi-dry mixture, placed into molds and compacted by a pressing machine. The bricks were cured under the ambient air conditions, no firing or

autoclaving was applied. The results of laboratory test carried out on bricks, as well as performance assessments in field use are presented. _________________________________________________________________ *Presented and Published at the ACI/CANMET - EPRI Sponsored Fourth

International Conference on Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete, Istanbul, Turkey, May 1992.

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INTRODUCTION

Stabilized fly ash brick is a type of no-burnt brick, which is

made by unclassified Class F fly ash together with cement as a

stabilizing agent. It was developed as a new wall construction

material by considering economic benefit and reduced housing

construction costs in China.

In China, burnt clay bricks have become a traditional building

material for a long time. Its output is the highest in the world.

With more and more energy consumption in industry and shortage of

fertile lands to excavate clays, it would cause increased energy

consumption and farmlands would be destroyed if the burnt clay bricks

were produced in large quantities continuously. Therefore

utilization of industrial by-products is extremely important to

produce sufficient brick wall materials for low-cost housing.

Laboratory research was carried out at the Shanghai Research

Institute of Building Sciences, and experimental building was built

in Jandin County, Shanghai. The results of the laboratory research

and an inspection of the experimental building has proven to be

remarkably beneficial in allowing to use stabilized fly ash brick

in rural areas where there might be lack of building materials and

energy shortage for producing traditional building materials.

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MATERIALS

There were two kinds of unclassified Class F fly ash used in

this project. The chemical composition of these fly ashes is given

in Table 1. The particle-size analysis of Bao Gang and Shiyan

unclassified fly ash (containing slag) are given in Table 2. The

physical properties of low-strength cement are shown in Table 3.

Lime, which contains effective CaO above 60%, and F-gypsum (a kind

of by-product, when hydro-fluorided is made, contains above 88% to

96% content of CaSO4 2H20) as chemical admixtures of cement, were used

in a few mixtures.

EXPERIMENTAL DETAILS

The manufacturing procedures were: fly ash and cement were dry

mixed in a mortar mixer; after initial mixing, the dry components

were sprinkled with 13% to 17% water by weight of components; the

mix was then further mixed for two to three minutes; and mix was removed

to a brick mold and consolidated by pressing in a universal testing

machine. Load was slowly applied to obtain a designed stress which

was held for a short time. After unloading, the brick was removed

from the mold and stored indoors and covered by plastic sheets in

order to minimize moisture loss.

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Table 1: Chemical Composition of Unclassified Class F Fly Ash

TYPES LOI SiO2 A12O Fe203 CaO MgO S03 K2O Na2O Specific Gravity g/cm

3

Bao Gang

Shiyan

6.2

6.6

47.2

56.5

34.4

24.6

5.9

4.7

2.4

2.2

0.9

1.4

0.8

0.6

0.2

1.8

0.1

0.2

2.14

2.03

Table 2: Particle-Size Analysis of Unclassified Fly Ash

╔════════════════════════╤═══════════════════════════════════════════════╗ ║

│ Percent Retained ║ ║────────────────────────┼───────┬───────┬────────┬──────┬────────┬──────╢ ║

Size │>150 um│ 150~ │ 125~ │ 100 ~│ 90~ │ <45 ║ ║ │ │125

um │ 100 um │ 90 um│ 45 um │ um ║ ╟────────────────────────┼───────┼───────┼────────┼──────┼────────┼──────╢

║Bao Gang │ 20.5 │ 2.0 │ 2.0 │ 5.2 │ 48.8 │ 21.5 ║ ╟────────────────────────┼───────┼───────┼────────┼──────┼────────┼──────╢

║Shiyan │ 50.8 │ 14.5 │ 6.8 │ 4.4 │ 13.5 │ 10.0 ║

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╚════════════════════════╧═══════╧═══════╧════════╧══════╧════════╧══════╝

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Table 3: Physical Properties of Cement

FINENESS,

(80 μm, sieve residual, %)

10.5

Flexural Strength, MPa 7 - days

28 - days

4.0

6.0

Compressive Strength, MPa 7 - days

28 - days

22.8

39.9

Setting TimeInitial

Final

5 hr. 20 min.

7 hr. 40 min.

RESULTS AND DISCUSSIONS

The mix proportioning and compressive strength of the stabilized fly ash bricks are given in

Table 4. Fig. 1 shows the relation between pressure of machine and compressive strength of the

stabilized bricks. Table 5 shows improved compressive strengths of the stabilized fly ash bricks

with lime or F-gypsum (Type: B-LF or B-GF) by comparison with that of control bricks (Type:

B-F).

The Table 4 indicates that compressive strength of the bricks increases with increasing

cement content. Since the fly ash bricks consist of mainly two components, fly ash and cement,

not only does fly ash become a filler material in bricks, but it also has a hydration reaction with

calcium hydroxide generated by hydration of cement. The more cement content increases, the more

fly ash content reacts with hydration products of cement.

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Table 4: Influence of Compressive Strength due to Cement Content and Source of Fly Ash

╔════════╤════════╤════════════════════╤═══════════╤════════════╤═════════════╗

║ Type │ Cement │ Mix proportions │ Machine │ Compressive│ Dry Unit ║

║ of │ Content│ by Weight (%) │ Pressure │ Strength │ Weight of ║

║ Fly Ash│ (kg) ├────────┬───────────┤ (MPa) │ of Brick │ Bricks ║

║ │ │ Cement │ Fly Ash │ │ (MPa) │ (kg/m3) ║

╟────────┼────────┼────────┼───────────┼───────────┼────────────┼─────────────╢

║ Bao │ 94 │ 0.9 │ 10 │ 25 │ 6.1 │ ║

║ Gang │ 131 │ 1.2 │ 10 │ 25 │ 7.8 │ 1138 ║

║ │ 148 │ 1.5 │ 10 │ 25 │ 8.6 │ ║ ╟────────┼────────┼────────┼───────────┼───────────┼────────────┼─────────────╢

║ Shiyan │ 88 │ 0.9 │ 10 │ 25 │ 4.6 │ ║

║ │ 114 │ 1.2 │ 10 │ 25 │ 5.5 │ 1068 ║

║ │ 139 │ 1.5 │ 10 │ 25 │ 6.1 │ ║ ╚════════╧════════╧════════╧═══════════╧═══════════╧════════════╧═════════════╝

Table 5: Improving Brick Compressive Strength by Cement Modified with Lime and Gypsum ╔═════════╤════════╤══════════════════════════════════════╤══════════════════╤═══════════╗ ║ │ │ Mix Proportion │ Compressive │ ║ ║Bao Gang │ Cement │ by Weight (%) │ Strength (MPa) │ Notes ║ ║Fly Ash │Content ├────────┬───────┬──────────┬──────────┼────────┬─────────┤ ║ ║ │ (kg) │ Cement │ Lime │ F-gypsum │ Fly Ash │14 days │ 28 days │ ║

╟─────────┼────────┼────────┼───────┼──────────┼──────────┼────────┼─────────┼───────────╢ ║ B-F │ 104 │ 1 │ 0 │ 0 │ 10 │ 5.4 │ 7.3 │ - ║ ╟─────────┼────────┼────────┼───────┼──────────┼──────────┼────────┼─────────┼───────────╢ ║ B-LF │ 104 │ 1 │ 0.1 │ 0.005 │ 10 │ 7.5 │ 10.3 │ * ║ ╟─────────┼────────┼────────┼───────┼──────────┼──────────┼────────┼─────────┼───────────╢ ║ B-GF │ 104 │ 1 │ 0 │ 0.1 │ 10 │ 9.0 │ 14.6 │ * ║

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╚═════════╧════════╧════════╧═══════╧══════════╧══════════╧════════╧═════════╧═══════════╝ * Before mixing bricks, the low-strength cement was combined with lime or F-gypsum by mixing in a ball mill.

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The laboratory tests showed that suitable water content added

an important factor which not only influenced the process of pressing

on the green bricks, but also influenced the outward appearance of

these bricks. When the water content was below 12% by weight of all

components, it was difficult to form bricks, with dry component

materials remaining on the surface of bricks. When the water content

was over 18% by weight of components, it was difficult to continuously

increase the pressure on the green bricks and achieve a fixed pressure

value while in machine pressing, because the components paste became

extruded along the edges of the mold. The results of the tests showed

that the suitable range of water content in producing fly ash bricks

was 13% to 17% by weight of all components.

Figure 1 indicates that compressive strength of bricks increased

with increasing compression pressure by the molding machine. By

considering both the costs in manufacturing and compressive strength

of bricks, a suitable and cost-effective pressing value by the molding

machine was suggested as 25 Mpa at which a break in the relationship

occured, Figure 1. Under a fixed cost of raw materials and recipe

for making bricks, properties of these bricks were mainly dependent

upon the density of green bricks which was controlled by the pressing

techniques. The pressing techniques include three aspects: pressing

speed; pressing method; and the magnitude of the pressing load. In

this project, the pressing method of "double-pressed face" was

adopted. Which means in order to achieve proper pressure

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distributions in green bricks, test specimens were subjected to

pressure from both top and bottom face of the bricks.

To expel air content within the material components and to prevent

hair cracks from occurring in green bricks, an uniformly slow load

speed should be used until the desired pressure is achieved. This

applied pressure should then be held for a few seconds. The pressing

process for this project indicated that the suitable pressing speed

was 5 to 8 mm/s.

After the low-strength cement was modified by other cementitious

materials, activity of the cement was improved and the hydration

reaction between cement and fly ash was better and more "active".

Table 5 shows that when the CaO and/or F-gypsum was added, with or

without lime, the compressive strength of (B-LF and B-GF) bricks was

much higher than that of the control bricks (B-F).

Table 6 gives the main physical properties and durability indices

of the bricks and experimental brick walls constructed with these

bricks.

After evaluating physical properties and shapes of the bricks,

a two-story experimental building was built in Jia Ding county,

Shanghai in 1988. The fly ash bricks were made on site using two

"BREPAK" brick machines which were obtained from England. The brick

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size was 29x14x10 cm. The green bricks were covered immediately by

plastic sheets for least three days in order to maintain sufficient

humidity within the green bricks after they were made. The building

up to now has remained in good condition and good durability. It

met the user's expectations.

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Table 6: Properties of Stabilized Fly Ash Bricks and Brick Wall ╔════════════════╤════════════════════════╤═══════════════╗ ║ │ │ ║ ║ ITEMS │ PROPERTIES │ RESULTS ║ ║ │ │ ║ ╟────────────────┼────────────────────────┼───────────────╢

║ │ Compressive │ ║ ║ │ Strength │ 6.3 MPa ║ ║ ├────────────────────────┼───────────────╢ ║ │ Flexural │ ║ ║ │ Strength │ 1.2 MPa ║ ║ ├────────────────────────┼───────────────╢ ║ │ Water │ ║ ║ │ Absorption │ 21.3 % ║ ║ BRICKS ├────────────────────────┼───────────────╢ ║ │ Dry Bulk Density │ 1138 kg/cu.m. ║ ║ │ │ ║ ║ ├────────────────────────┼───────────────╢ ║ │ Freeze-thaw │ ║ ║ │ Resistance │ >15 cycles ║ ║ ├────────────────────────┼───────────────╢

║ │ Thermal │ ║ ║ │ Conductivity │ 0.3233 w/m.k ║ ║ │ Coefficient │ ║ ╟────────────────┼────────────────────────┼───────────────╢ ║ │ Compressive │ ║ ║ │ Strength │ 3.4 MPa ║ ║ │ │ ║ ║ BRICK WALL ├────────────────────────┼───────────────╢ ║ (L x W x H: │ Modulus of │ ║ ║ 120x10x75 cm) │ Elasticity │ 1,060 MPa ║ ║ │ │ ║ ╚════════════════╧════════════════════════╧═══════════════╝

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CONCLUSIONS

(1)Fly ash can be used as a filler material, along with its use as

a cementitious material. This could lead to cost savings and

reduce energy consumptions in the manufacture of fly ash bricks.

(2)Compared with burnt clay bricks, the fly ash bricks presents a

good appearance and dimensional stability during manufacturing.

The technology is also simple.

(3)The semi-dry pressing is one of the main influencing factors to

improve compressive strength and other physical properties of

bricks. It was established that the pressing speed should be

roughly 5 to 8 mm/s, and the cost effective pressing stress is

25 MPa.

(4)The results of inspection of the experimental building shows that

the fly ash bricks not only met the needs of design, but also

has a good appearance and durability.

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REFERENCES

(1)"BREPAK Operating Manual", 0cesds Development Administration,

British Research Establishment, England, 1986.

(2)He Qian, "A Study of Forming Unburnt Bricks", New Building Material

Journal, China, Vol. 11, No. 12, pp. 12-16, 1987.

(3)Hu, Wenyi, "A Study of Unburnt Brick Made by Local Natural

Materials", Shanghai Research Institute of Building Sciences,

December 1987.

(4)Hu, Wenyi, "Inspection of the Experimental Building Made by

Stabilized Fly Ash Bricks", Shanghai Research Institute of

Building Sciences, April 1988.

REP-68