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Secret binder immobilises heavy metals

TESTS ON heavily contaminated land stabilised by a revolutionary accelerated carbonation technique have shown that more than 99% of the heavy metals have been safely immobilised, it was claimed this week.

Developed by the University of Greenwich and Blue Circle Special Cements, the technique involves mixing contaminated soil with up to 20% by weight of a still secret cementitious binder in an airtight mixer.

Gaseous carbon dioxide is then added and the mixture agitated for between four and 15 minutes. The technique effectively immobilises contaminants by locking them into the cement matrix. This means that the soil can be reused on site.

Site trials began earlier this month at a disused fireworks factory near Dartford, Kent.

'The site was used for explosive and fireworks manufacture for nearly 90 years and is heavily contaminated with lead, nickel, copper, chromium and zinc, ' said University of Greenwich senior research fellow Dr Colin Hills. 'As the soil is a heavy clay it needed extra mixing time.

Leaching tests have now shown that virtually all the metals have been locked up as stable compounds.'

A modified 8m 3truckmixer was used for the trials, producing 2-3m 3batches. Hills said the reaction between the CO 2and the binder happened very quickly. 'In some soils it's all over in five minutes, with the granular final product containing up to 50% by weight of CO 'Once compacted back into position no further reaction takes place, as the accelerated carbonation leaves behind no active cementitious compounds.

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