In contrast to foreign practice, in Poland, for many years, there has been a belief, that in prestressed constructions, more expensive concretes on basalt aggregates should be applied. This habit is due to the low quality of national river and fossil aggregates. They allow to obtain low concrete strength and modulus of elasticity. In the paper, basing on two examples of post-tensioned slabs, it has been shown, that it is possible to use concretes with a low value of modulus of elasticity for large-span slabs. These results may be significant in popularizing the use of post-tensioned slabs in buildings in poorer countries, making them cheaper in execution.