Abstract The external sulfate attack (ESA) of concrete is a disease related to expansive sulfate hydrate formation in a hardened cement matrix. The aim of this research is to study how the choice of a concrete sample size can impact on the kinetics of ESA, by exposing different types of specimen to constant immersion in a solution dosed with 5% Na2SO4⋅10H2O. Monitoring involves mass, dynamic modulus and expansion measurements. It is concluded that 4 × 8 cm concrete cylinders (cored from 11 × 22 cm concrete cylinders) are more quickly damaged by ESA than usual sample types (11 × 22 cm concrete cylinders and 4 × 4 × 16 cm mortar prisms). For all sample types, damage is always limited to the periphery of the sample in the short run. The thickness of the damaged zone is in the region of the size of the largest aggregates. For 4 × 8 cm concrete cylinders, this periphery corresponds to the entire sample because the maximum aggregate size is of the order of the size of the specimen. In this situation, the percolating crack network resulting from swelling is assumed to dramatically damage the cement matrix and to give sulfate solution access to the whole sample. Hence, by using this original type of cored samples, the concrete resistance to sulfate attack can be studied under reliable conditions (concrete formulations and not mortar ones, good sensitivity to ultrasonic tests) and advantage can be taken of the increased kinetics of degradation.