09-07-2013, 03:59 PM
High Performance Bricks from Fly Ash
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ABSTRACT
Bricks whose solid ingredient is 100% fly ash have been manufactured. The manufacturing process uses
techniques and equipment similar to those used in clay brick factories. The bricks produced were about
28% lighter than clay bricks. The bricks manufactured from fly ash possessed compressive strength higher
than 40 MPa. This exceeds some of the best of load carrying clay bricks available by more than 25% and
is several times better than acceptable commercially available common clay bricks. Other important
characteristics of the fly ash bricks have been evaluated. These included absorption capacity, initial rate
of absorption, modulus of rupture, bond strength and durability. The values of these characteristics for fly
ash bricks are excellent and have exceeded those pertaining to clay bricks. Moreover, fly ash bricks have
been produced with a naturally occurring reddish colour similar to that of normal clay bricks. The new
bricks and process have been patented and the new bricks have been given the name FlashBricks. This
paper presents the results of testing and the advantages gained by this type of bricks over conventional
clay bricks.
INTRODUCTION
The ever increasing volume of fly ash quantities in the world has not been remotely matched by its
utilisation. Australia is an example where such utilisation has been minimal. The most important and
popular use of fly ash in Australia has been the partial replacement of portland cement. Australia shares
most of the Western countries in similar methods and traditions as far as residential buildings are
concerned. These include bricks as the main constituent. It is therefore natural that the brick industry
presents an opportunity for the efficient utilisation of the vast quantities of fly ash. Conservative attitudes
are among the factors that limited the use of fly ash in concrete to generally a maximum of 25%
replacement of Portland cement. This conservatism can be understood in the context of concrete where the
ash is mixed raw, and the effects 1
of high volume replacement are still subject to research and sometimes controversy. It is however not
quite justifiable that the brick industry should take similar conservative attitude. Environmental concerns
have been raised in some parts of the world where coal is the main power generating resource and where
bricks are also the main building material. Such concerns have resulted in legislation that obliges the
brick industry to incorporate at least 25% by weight of fly ash and /or bottom or pond ash in the brick
making mixture if the industry is within 50 km from a coal power generation plant.1,2 Some successful
ventures have been reported where fly ash was incorporated in the mixture at the rate of 20 to 50%.3
Nevertheless, there is only little evidence that incorporation of fly ash in the brick mixture has exceeded
the 30% by volume even when the legislation was obeyed.4 Reasons behind such reluctance are not clear.
A most probable reason is the fear of change in many small factories and the ingrained conservatism in
the attitude of stake holders of the large producers. Added to this is the fact that with an existing clay
brick factory, the incorporation of fly ash is a potential addition of cost. The possible incompatibility of
the ash with the clay and shale during the various processes of production including the crucial one of
firing may be a legitimate difficulty. At high temperatures beyond 1000o C, the temperature and length of
time of firing become very sensitive to the type of ash and of course to the clay and shale if in the same
mixture.
BRICK PRODUCTION
The bricks, produced according to the patent 7, have been given the name FlashBricks. Essentially, the
only solid ingredient of the bricks is the ash. The main liquid ingredient is water. Other ingredients that so
far are commercially protected are cheap, commonly available and, though essential, are only minor in
quantities. The technology, subject of the patent, includes the method of mixing, forming into moulds,
curing and firing. These are easily adaptable by existing clay brick factories. The technology uses less
energy than that needed in the manufacture of clay bricks. Furthermore, it requires less manpower and less
area is needed for material processing than in the case of clay brick production.
THE INITIAL RATE OF ABSORPTION AND THE ABSORPTION CAPACITY
Two important properties of building bricks are the initial rate of absorption (IRA) and the absorption
capacity. The IRA is of great importance for the laying of the bricks and bonding with the mortar. A high
IRA results in too quick drying of the mortar and thus weakens the mortar and reduces its adherence to the
brick. On the other hand if the IRA is too low, the surface of the brick adjacent to the mortar would not
absorb the excess water and would result in very weak layer of the mortar that would not have penetrated
enough into the surface crevices and pores of the brick.
The property of total absorption capacity is also very important for the performance of the brick. A high
absorption results in vulnerability to volume changes that would result in cracking of the bricks and
structural damage in buildings. It also would lead to cracking in the event of freezing and thawing of the
water inside the pores. Too little absorption however is also not desired.
This is because rain water, rather than getting partially absorbed by the brick, would tend to run off very
quickly towards the joints and may find its way into the building as well as reduce the durability of the
mortar joints.
CONCLUSIONS
1. The results are indicative of the satisfactory performance of FlashBricks as load bearing elements. This
type of bricks uses 100% fly ash without mixing with clay and shale. It, therefore provides a large venue
for the disposal of fly ash in a very efficient, useful and profitable way.
2. The mechanical properties of FlashBricks have exceeded those of the standard load bearing clay
bricks. Notable among these properties are the compressive strength and the tensile strength. Compressive
strength was 24% better than good quality clay bricks. Tensile strength was nearly three times the value
for standard clay bricks.
3. Comparison between the bond strength of FlashBricks to mortar and that of comparable shaped and
commonly used solid clay bricks showed that the FlashBricks have a bond that is 44% higher than the
standard clay brick.
4. There is evidence that the microstructural feature of the surface of FlashBrick is characterised by a
rougher texture than that of clay bricks.
This characteristic is believed to be responsible for the increased bond strength with mortar.