Significant research efforts have been dedicated to understanding the evolution of damage produced in concrete by the alkali silica reaction (ASR), with the final goal of predicting its development in real-world structures. Nevertheless, the mechanisms for aggregate swelling, followed by its cracking, are still only partially understood. In this study, we present a non-destructive, time-lapse characterization of ASRcaused cracking in concrete at the meso-scale using X-ray micro-tomography (XMT). To accelerate ASR in the laboratory, a specific protocol was executed comprising the immersion of the specimens in an alkaline solution. The tomographic imaging of the ASR-induced cracking was performed at the beginning of the ASR acceleration and continued at specific time points later on (12, 20 and 36 weeks). To make the ASR products visible in the tomograms, caesium was used as a tracer to increase the Xray contrast. Finally, 3D image analysis workflows were applied to investigate ASR cracking and identify ASR products. Our results confirm the usefulness of caesium for X-ray absorption-based "labelling" of ASR products, without any spurious perturbation of the ASR process. Such labelling allowed observing an extensive and long range filling up of the pore space by the ASR products. Not only cracks but also other types of pores were filled by the products, starting from those at the aggregate-cement paste interfaces. The observed long range transport of products both outside and inside the aggregates suggests the products having significant viscoelasticity, which still requires characterization, if progress in understanding and model ASR damage wants to be achieved.