Abstract: The application of a new test method for aggregate alkali activity‚so-l gel expansion method‚ was presented‚which was used to investigate the alkali activities of opal and natural aggregate．The experimental results show that by using the method‚opal shows significant alkali activity‚and at the pessimum proportion of opal‚both SiO2 content and SiO2／Na2O mole ratio in the reaction solution reach their maximum values‚and so is the expansion．

Key words: so-l gel expansion method, opal, alkal-i aggregate reaction, aggregate activity