Smooth grains might seem unsuitable as a filtration medium in that they are less likely to trap particles But they are surprisingly effective. “When you grind up the glass,” Dryden explains, “you break molecular bonds. The effect of this is to leave spare electrons on the surface. The glass grains have a net negative electrical charge.” This enables them to attract particles in the way that a balloon that has been brushed a few times against a sweater will stick to the ceiling.A second effect then occurs. Glass is made green by the addition of salts of chromium, and brown by adding iron Both metals have catalytic activity. They can split oxygen molecules into single, highly reactive oxygen “radicals”.

So in filters made from coloured glass grains, microbes are first drawn to the surface of the grains electrostatically and, once there, are zapped by oxygen radicals, which kill them.”We think that if glass replaced sand as a filtration medium, the potential market could be in the region of 750,000 tonnes a year,” says Dryden. “And we are interested only in coloured glass.”Meanwhile, at the Centre for Cement and Concrete at the University of Sheffield, Dr Ewan Byars, in collaboration with 30 UK companies, is finding new uses for waste glass in the concrete industry. Of particular interest is the possibility that finely ground glass could be useful as an additive in cement “Cement chemistry is extremely complex,” says Dr Byars. “Essentially, calcium, silicates and water react together to form calcium silicate hydrates, the building blocks on which concrete’s strength and engineering properties develop.”There are a number of silica-based waste materials – pozzolans – that can be added to cement.

Pozzolans may increase the cohesiveness and workability of concrete mixes. The Sheffield researchers have shown that powdered glass has pozzolanic activity, and could be a useful additive to cement without compromising its properties, and possibly improving them.Larger particles of glass are being investigated as coarse aggregates for high-value concrete products. One company, for example, specialises in architectural masonry. Here, glass aggregate can be incorporated into the concrete and the surface given a fine polish to display the glass facets.Over in Stoke-on-Trent, Dr Andy Smith is Technical and Research Manager in the Building Technology division of the research organisation Ceram.