An alkali-reactive basalt from Queensland, Australia

Abstract

Basalts are basic igneous rocks which are normally non-reactive with alkali in concrete. However, a glassy olivine basalt of Tertiary age from Queensland, Australia has been found to be highly reactive with high alkali cement and unsuitable as a concrete aggregate. Examination of the rock petrographically, by scanning electron microscopy and electron microprobe analysis revealed the presence of 1.6% opaline silica. Also present in the rock was a glass (about 34%) of a Si-rich composition, and this together with the opaline silica is suspected as the components causing reactivity of the basalt. However, the 1.6% opaline silica alone would be sufficient to render the rock reactive to alkali hydroxide.