Efforts to improve the environment through conversion of waste materials into useful construction products are fortified by the advent of ‘Cellular Light Weight Concrete’. This has created a large scope for sustainable development among building material suppliers and high-end contractors. Although the word ‘concrete’ is a misnomer here, it is the most common nomenclature used to explain aerated lightweight building blocks.

In areas with high demand of burnt clay bricks, CLC bricks compete favourably as an alternate material. These are very useful building products that reduce dead weight of structures and accelerate the pace of construction when used as pre-cast elements for walling and flooring. Unfortunately, the basic cost of the product (as delivered to the construction site) has been too high to be offset by the economic gain accruing due to speed and lighter structure. It is for this reason that commonly available varieties of CLC have not proved to very successful in India.

The basic raw materials used in making CLC bricks are fly ash and sand. Lime or cement (or a judicious mixture of the two) is used as binders. The aeration is produced by a foaming agent. The most common and widely used foaming agent is German developed and patented ‘Neopor’. There are various techniques of making a CLC brick depending upon the investment to be made and the productivity required. However, the most economic option as per Indian conditions does not require high-end sophisticated equipment. Major equipment required for production of high quality CLC bricks are a mixer, a hydraulic press, a tunnel dryer and autoclaves. All of this equipment is indigenous and available in industrial cities. Utilities required for the production are power, steam (10-12 kg/sq.cm pressure) and water.

Aerated flyash blocks

Assembly line for producing aerated flyash blocks

Construction using aerated flyash blocks

The choice of raw materials depends upon the quality of the product required. The best quality of CLC blocks are produced from dry fly ash and sand of a defined gradation. Initially, certain quantities of fly ash and calcined gypsum are mixed in a dry state. Once a uniform colour is achieved, cement is added. The foamer and lime, along with water, is added at the last stage to form a thin slurry. This slurry is then poured into moulds and allowed to set for a period of 24 hours. The set blocks are then de-moulded and autoclaved in steam for a period of at least 8 hours. Steam-cured blocks are then water cured for a period of at least 7 days before being readied for dispatch. Depending upon the mix ratio, bulk density of the block can vary between 0.4 – 1.8 gm/cm³.

There are many commercial companies providing the technology, equipment or even turnkey services for setting up a CLC brick production unit. An average capacity of 150,000 m³ of CLC blocks per year (equivalent to 150 million normal sized bricks), will require a tentative investment of Rs200 million.

The major disadvantage of CLC blocks is the high cost. Despite its lightweight nature benefiting transportation costs, the production cost per unit is significantly higher than normal bricks.

Additionally, the shear stress and compressive strength are not as high as conventional bricks.

CLC blocks use less material than conventional concrete blocks, making them more environmentally friendly. However, CLC bricks might not be the most eco-friendly material for use. The mixing, pressing and autoclaving process requires significant amount of energy, thereby increasing the carbon footprint over traditional burnt-clay bricks.

The most prohibitive aspect under Indian conditions is the excessive investment required, which is beyond the reach for even a medium scale entrepreneur.