Examination of microstructural features of Norwegian cataclastic rocks and their use for predicting alkali-reactivity in concrete

The microstructural features of various Norwegian rock samples have been studied and quantified by different techniques. The majority of the rocks studied were different types of cataclastic rocks. Expansion results from a modified version of the NBRI Mortar-Bar Test for alkali reactivity are used for correlation with the quantitative parameters. The aim is to identify microstructural features in the various rock types which promote the alkaliaggregate reaction, and to subsequently use these findings to improve and provide guidelines for engineering practice in order to predict and make more accurate determinations of potentially reactive aggregates. The deformation processes of rocks involve the general process of straining of quartz, grain size reduction and subgrain development. This is characteristic of mylonitisation. This study demonstrates that the grain size reduction of quartz enhances reactivity by increasing the surface area of quartz grain boundaries available for reaction, and thus giving an overall increase in surface energy. Subgrain development will, besides the high surface area, contribute even more to an enhanced reactivity, due to the high dislocation density associated with the quartz subgrain boundaries. The total grain boundary area of quartz, and the mean grain size of quartz appear to be the most favourable quantitative parameters related to the expansion of different rock types. The measurement of the total grain boundary area of quartz, will enhance the value and improve the effectiveness of the petrographical examination as an engineering tool to screen potentially reactive aggregates.