Here, first we introduce and investigate (BD)-sets by using the notion of b-open sets to obtain some weak separation axioms. Second, we introduce the notion of gb-closed sets and then investigate some relations of between b-closed and gb-closed sets. We also give a characterization of b-T1/2 spaces via gb-closed sets. We introduce two new weak homeomorphisms which are important keys between General Topology and Algebra. Using the notion of mX-structures, we give a characterization theorem of mX-T2 spaces. Finally, we give some examples related to the digital line.