Binding a future for waste bricks

Materials World magazine
3 Jun 2011
waste bricks on shelves

A test manufacturing facility is producing ‘carbon-negative’ masonry bricks from recovered aggregates, reinforced by an innovative binder. This could provide an alternative to clay fired bricks.

Research Director Stuart Shaw at Encos in Leeds, UK, explains, ‘We used vegetable oil and glycerol to bind graded aggregates recovered from various waste streams. The binder is extremely adaptable and is capable of binding crushed waste brick, furnace bottom ash, pulverised fuel ash, incinerated sludge ash, glass cullet and rice husk ash. As we are using recovered aggregates, there is no extraction of raw materials or minerals, and we can subsequently reduce the amount of material going to landfill’.

To form the bricks, the aggregate is loaded into a 300kg standard pan mixer. A load cell is attached to the loading bucket to determine how much aggregate has been added. The vegetable oil is then pumped into the pan mixer and the aggregate and oil are mixed for several minutes and then discharged onto a conveyor belt. The mixed material is then loaded into a hopper and attached to the vibratory press. It is fed into the mould, and a combination of vibration and compaction is used to manufacture the brick to the correct height. The bricks are ejected onto a curing plate and placed on a racking system within the oven before curing at 150°C for 24 hours.

Typically, the composition of a coursing brick is 90% furnace bottom ash and 10% pulverised fuel ash, with the addition of a small amount of binder.

Shaw argues that the bricks are more energy efficient to manufacture as the curing process involves low heat and no predrying. ‘In comparison, clay bricks require a two-stage drying and curing process, with curing carried out at over 1,000°C for 24 hours,’ he adds.

Furthermore, as the curing temperature is low, heat recovered from other heat generating sources, such as incinerators, can be used. In addition, as the binder is made from polyunsaturated oils, there is no need to manufacture it.

Shaw says, ‘An independent study also revealed that the amount of CO2 emitted from the production of clay brick is 239kg CO2e (carbon dioxide equivalent) and the aggregate brick in comparison is 87kg CO2e.’

Using waste at such a high percentage can result in differing physical properties in the brick, yet Shaw adds that this is a positive factor.

He points out, ‘With traditional clay bricks, the compressive strength/durability/water absorption/freeze thaw properties vary depending on the source of clay and how they are made. By using waste aggregates, we do not anticipate the resulting products being any different to traditional bricks, and we have the flexibility of altering these properties by using waste aggregates in different combinations, thus manufacturing products to meet specific requirements’.

According to Shaw, tests have been completed to meet BS EN771-1 requirements at the University of Leeds, CERAM, WSP and Warrington Fire, UK. These include tests on compressive strength, dimensional stability, density, shear strength, creep, bond strength, fire resilience, leaching, freeze/thaw and thermal conductivity.

Waste building material consultant Charles Fentiman of Fentiman Consulting, in Southwater, UK, says, ‘This is a very interesting development and, if the claims are substantiated, it is a technology that will help the drive towards zero carbon homes. Careful lifecycle analysis will be a must. It will also be interesting to compare with masonry made from recycled aggregates and cements. Since this is a new technology, testing will be vital to ensure that there are no unexpected durability or health and safety issues’.

The next stage will look at reducing the amount of vegetable oil required for the binder. The company also plans to investigate the addition of colouration, marking and texture for use as facing bricks.