India produces around 170 billion bricks per year, in the process consuming around 442 million tonnes of soil. Considering that the majority of the bricks produced will be burnt clay bricks, this volume of production will require about 230-240 million m³ of agricultural soil. This is equivalent to 25,500 hectares of fertile agricultural land, with the exploitive depth being 1 m. Such exploitation will have a tremendous impact on national food security. Recognising the importance of restricting the excavation of top soil for manufacture of bricks and promoting the use of Pulverized Fuel Ash (PFA) in brick making, the Government of India has taken several measures to enable the same.

There are several programmes of the Government of India (like the Fly Ash Mission) that promote the use of PFA in clay brick making and consequently reduce soil usage. However, lack of knowledge and operational hazards restricts the use of PFA in clay brick making. Generally, most brick making units across India follow manual mixing process for green brick making. This tradition has been followed due to access to cheap labour. Due to its highly powdered form, PFA has a low specific gravity and bulk density. On the other hand, soil (in all its forms) has higher density. It is impractical to manually achieve proper mixing characteristics of these two materials of varying density. Initial trials have shown agglomeration of these two materials in patches, even within a single green brick. This type of improper mixing results in inconsistent properties after firing and in higher breakage.

Pulverized Fuel Ash

Flyash mixing with pugmill

Soft mud mixing and moulding

These are some of the major reasons that have contributed to the development of a common incorrect perception of traditional clay brick manufacturers: addition of fly ash results in degradation of brick properties. Addition of fly-ash, therefore, has poor acceptability among brick producers. Various processes exist to efficiently mix dual density materials and enable the use of PFA in clay brick making. The manual process of mixing PFA with soil is locally termed as the “GHOL” process. In this method, soil is mixed with water and made into a thin slurry. It is then sieved through a very fine mesh to filter out the coarser materials. The fine clay slurry is channelled into shallow rectangular storage pits built in the ground, where various additives are mixed manually to reduce the plasticity of the soil. Since the raw materials are in a slurry form, the mixing process is easy and a relatively homogenous consistency can be achieved. The mixed soil is taken out from the pit after it dries to the requisite extent and kneaded into soft dough for moulding.

In the eastern India, particularly West Bengal, soil for green brick making is usually processed through a pugmill. This mixing process can be a suitable opportunity for medium and small scale manufacturers to uniformly mix soil and PFA. The soil and PFA is batched as per quantity required, watered and mixed in the pugmill. The resultant quality of the mix is uniform as compared to the consistency after ordinary hand mixing; this is evident by the uniform colour of the mixture. When compared to hand mixing, this process increases the strength of the brick by 50% - a clear advantage.

In recent years, the soft-mud moulding technique has been revived based on increased demand from brick manufacturers. Ostensibly, this has been to meet the shortage of skilled moulders. The soft-mud moulding machine uses the twin advantage of mechanised mixing and moulding. Mixing is done in a horizontal barrel mixer, followed by extrusion from an opening in the bottom into pre-placed moulds. This machine can also be used to mix soil and PFA into a uniform and compacted green brick. The increased strength, excellent finish and sharp corners of bricks made this way are clear advantages over traditional hand moulded clay bricks.