Why aggregates of low/moderate strength & modulus of elasticity can be valuable in preserving the durability of concrete?
Concrete, during its service life, is subjected to volume changes occuring due to hygral or thermal reasons. If the aggregate is compressible, these volume changes cause lower stress in cement paste as compared to the case when aggregate is not compressible.
Thus compressibility of aggregate would reduce distress in concrete while a strong and rigid aggregate might lead to cracking of the surrounding cement paste.
In this way, aggregate of moderate or low strength and modulus of elasticity can be valuable in preserving the durability of concrete.