This research is focused on identifying the porosity in concrete manufactured with air-cooled blast furnace slag as coarse aggregate and analyzing its potential use as sound absorption material. Six concrete mixes were manufactured by using three types of aggregate with different particle size distributions and consequently, diverse porosity in the matrix. Acoustic characteristics and mechanical properties of concrete were determined. In a complementary study, an X-ray computed tomography analysis has been carried out in order to establish a connection between the pore structure and its corresponding acoustic and mechanical behavior. Mixes composed of 80% wt of slag with particle sizes between 5 and 10 mm having higher porosity and a better acoustic absorption than those using natural coarse aggregate of the same particle size, although their mechanical properties are lower. A combination of three layers each 4 cm thick containing the larger slag particle sizes in the noise incident face of the concrete, followed by the medium slag particle size and finally, the finest material, presented excellent sound absorption properties, which allowed all the slag to be recycled.