Considerate builders think of the future

BUILDING houses and offices out of toxic waste sounds like a pretty eccentric idea. Yet it may become commonplace if Ana Andrés of the University of Cantabria in Spain has her way. For Dr Andrés and her colleagues suggest, in Industrial & Engineering Chemistry Research, that the humble brick need not be made of pure clay. Instead, up to 30% of its weight could be slag—the toxic gunk left over when steel is made.

Waelz slag, to give its technical name, is composed mainly of silica but is also undesirably rich in poisonous metals like lead and zinc. Getting rid of it safely is thus a problem. Getting rid of it usefully might sound like a miracle. But that is what Dr Andrés proposes. A series of experiments she has conducted over the past three years suggests this is not only possible but will make bricks cheaper and more environmentally friendly.

Her research started after she read of previous work which had shown that many ceramics suffer no loss of integrity when the clay used to make them is mixed with other materials, and that the molecular structure of some ceramics acts to trap atoms of toxic heavy metals. She wondered whether these things might be true of brick clay and Waelz slag, and she began experimenting. The answer, she found, was that they are. Bricks show no loss of useful mechanical properties even when 20-30% of their content is slag. Nor do they leak.

To check that, Dr Andrés and her team ground their bricks into powder and soaked them in water, shook them in special machines for days at a time, and even tried to dissolve them in nitric acid. The pollutants stayed resolutely put. Moreover, adding slag to the clay reduced by a third the amount of carbon dioxide each brick released during its manufacture, because wood pulp is added to clay before it is fired, and less clay means less pulp is needed. The cost, too, fell, because slag is free, whereas clay costs money.

There is, of course, the problem of customers. Whether people will be willing to live and work in structures that double as waste dumps is moot. But for those who want to make an eco-point, what better way could there be than, literally, to build their green credentials?