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The employment of non-dolomitic calcareous aggregates in the composition of concrete may serve to cause degradation if the mother rock contains sufficiently disordered silicates in the form of finely dispersed microinclusions. This paper reports the observation of fissures due to the formation of gel as a result of ASR in aggregates with less than 4% of microinclusions and even when this value was less than 1%. It was also found that: i) the formation of cracks is closely dependent on the degree of disorder of the silicate phase; ii) even in extremely compact aggregates, the residual porosity is enough to ensure penetration of the alkaline cement solution so that it reaches and attacks the silicate microinclusions. It also appeared that the gel thus formed exerted a breaking action on the aggregate, whose extent depended on the shape of the pores and the presence of narrowings between intercommunicating pores (even of large volume).