Alkali-silica reaction (ASR) occurs between certain aggregate types and the alkalis in the pore solution of concrete and forms a silica gel. In the presence of free moisture, the gel will expand and cause cracking and differential movements in structures, as well as other deleterious effects such as reductions in freeze-thaw durability and in strengths properties of concrete. The microstructural characteristics of various Spanish alkali-reactive and potentially reactive rock types, used as concrete aggregate, and other Spanish rock types, with a possible potential reactivity, have been studied and quantified with petrographic techniques and methods of digital analysis of microscopic and macroscopic images. The quartz reactivity index (QRI) values have been obtained by multiplying the percentage of quartz in the aggregate by the specific surface area, which is characteristic of this mineral phase. These results have been related to alkali reactivity as measured by an accelerated mortar bar test. The study has demonstrated that the reduction of grain size in quartz produced by the metamorphic processes greatly enhances alkali reactivity by increasing the surface area of quartz grains available for the reaction.