Alkali-silica reaction (ASR) is one of the most important weathering processes in cement-based materials. The damages caused by ASR have been qualitatively investigated with a number of different techniques. In this study, we present a procedure to obtain quantitative morphological parameters of the ASR reaction effects using synchrotron X-ray microtomography data. We found three different kinds of voids due to the effect of three different mechanisms: (i) cracks from ASR expansion, (ii) irregular-shaped voids due to the aggregate particles dissolution, and (iii) bubbles due to the cement paste preparation. We were able to separate them using morphological parameters (such as surface/volume ratio and aspect-ratio) calculated for each object, thus obtaining, e.g., volume fractions for each kind of voids. From the orientation data, we also studied if any shape preferred orientation was present in the sample, concerning the fractures network, and we found no appreciable preferred orientation. The new analysis procedure we applied in this study proved to be an effective approach for the quantitative characterization of the effects (cracks and porosity development by aggregate weathering) of the ASR reaction in mortars.