Bricks and aggregate made from fly ash

Clay Technology magazine
1 Oct 2006

Researchers at the University of New South Wales' (UNSW) Australian Defence Force Academy have developed bricks and building aggregate that can be manufactured entirely from waste fly ash found in coal-burning power plants.

Commonly known as pulverised fuel ash in the UK, this fine powder is already used to partially replace Portland cement in concrete. The latest research project was created due to ‘concern that Australia is producing about 14 million tonnes of fly ash every year. Not more than 10% is getting utilised,' says Dr Obada Kayali of UNSW.

He and his team believe their new products, Flash Brick and Flashag, will find a market in increasingly industrialised countries such as China and India, creating ‘a thriving new green industry'. China produces an estimated 200 million tones of the by-product per annum - most of it sent to landfill. In India, Kayali explains that a law was passed in October 2005 which stipulated that a minimum of 25% of fly ash must be employed in the manufacture of clay bricks for application in construction projects within a 50km radius of coal-burning power plants.

‘Fly ash comes out of the power station and can be fed straight into the brick manufacturing process,' says Kayali. ‘In China, it is difficult to find a clay or aggregate quarry close to a city. Many brick plants are idle due to the lack of clay, yet most power stations have some form of a brick plant close by.'

As well as limiting the quantity of fly ash going to landfill, the technology will reduce the need for extracting natural minerals and aggregates. Another environmental benefit is that upon sintering, the compounds solidify in complex crystal formations, making it difficult for the toxic chemicals within the ash (their quantities are low and usually much below the tolerance limits established by national standards) to leach out. Flash Bricks also take less time to manufacture in kilns, generating fewer greenhouse gas emissions.

According to tests carried out by the team at UNSW, the new bricks are 28% lighter and 24% stronger than comparable clay bricks and, using Flashag, concrete can be manufactured to be 22% lighter and 20% stronger. Kayali adds, ‘Fly ash to start with is lighter than cement and clay. Moreover, when the aggregates or bricks are manufactured, the process results in the formation of extremely small air bubbles uniformly distributed all over the body of the product. This further lowers the density.

The advantages of light building materials include reduced transportation and handling costs, as well as structural benefits due to the potential for shallower foundations and smaller members. Neil Simpson of NewSouth Innovations (NSi), the university's commercialization arm, says, ‘Flashag can be used effectively in high rises where smaller structural columns are needed to maximise floor space and in concrete bridges requiring longer spans.' For such applications, steel reinforcement is usually required to withstand the large tensile and compressive stresses caused by the heavy loads on the columns and foundations. Lighter structures have a lower reinforcement requirement and therefore less CO2 is emitted.

The fly ash technology has two patents and has been licensed for use in the UK and USA. NSi is seeking interest from companies who want to develop the technology for China, Japan, Southeast Asia, Europe and India.