Tiles from waste glass

Clay Technology magazine
,
13 Feb 2009
Microstructure of the cathode ray tube glass ceramic tile

Researchers in Italy have developed a ceramic tile that uses difficult to recycle waste glass in place of feldspar minerals, creating a product that can be fired at lower temperatures while providing an environmentally friendly way to dispose of the glass.

The team from the University of Padova and the Bologna Centre of Ceramics have produced two tiles – one containing 60%/wt soda-lime glass, another using 80%/wt cathode ray tubes (CRT).

The soda-lime tile is fired at 880-1,000ºC, while the CRT tile is fired between 750-825ºC. These temperatures are much lower than those used for traditional ceramics, says Dr Enrico Bernardo at
Padova.

Both soda-lime glass and CRTs must be purified prior to recycling to remove metallic or ceramic contaminants.

For creating ceramic tiles, however, no pre-processing is required, and the entire glass content is ground down to a fine powder and added to the clay mixture.

Mineral substituting

Replacing the more expensive feldspar sand materials used in ceramic tiles reduces costs, Bernardo adds, but creates problems with the final product’s stability. Ordinarily, ‘feldspar minerals melt upon firing, and the tiles do not collapse, due to the formation of mullite crystals from the clay fraction,’ he explains.

‘If we use glass, we can fire at much lower temperatures, but mullite does not form. So it is important to get something that could act like mullite in controlling deformation and imparting
good mechanical properties.’

The team tested two options – a calcium hydroxide precursor to create calcium silicate crystals in the soda-lime tiles when fired, and alumina platelets in the CRT mixture. The team found that the use of five per cent/weight calcium hydroxide created a constant shrinkage rate between 880-960ºC.

Compared to feldspar sand tiles, the mechanical properties of the glass tiles are also ‘only slightly lower than those for a conventional porcelain stoneware,’ notes Bernardo. ‘As an example, the bending strength is 10% lower, but the hardness is almost the same.’

Dr Geoff Edgell, Head of Building Technology at advisory organisation CERAM, based in Stoke-On-Trent, UK, notes that waste materials, particularly glass, have been successfully used in ceramics before. ‘It does offer good energy savings, but the economics is an issue,’ he adds. ‘Grinding the glass down to the right size adds processing costs, and you also need to have a large source close to the factory, otherwise you’re facing high transport costs.’

But despite the financial drawbacks, Edgell believes such technologies will become more important as recycled building materials grow in demand.

The Padova group is also developing ceramic products containing waste glass that comes from the vitrification of hazardous waste, such as fly ash from solid waste incinerators.