In this paper are explored the changes of pore structures of mortar with pozzolanic admixture exposed to aggressive environments, namely 5% wt. sodium sulphate solution. Influence of non-flowing sulphate solution on the test specimens were determined in dependence on the different type of cement and used ground clay brick as pozzolanic admixture. Attention is also paid to the properties of the individual components of the mortar. From the results obtained follows that the additive of ground clay bricks has positive impact on the sulphate resistance.