A classical theorem of Hochster provides purely topological characterization of prime spectra of commutative rings. In this paper, we first prove an analogous statement for idempotent semirings, showing that for a spectral space X, we can construct an idempotent semiring A in such a way that the saturated prime spectrum of A is homeomorphic to X. We further provide examples of spectral spaces arising from sets of congruence relations of semirings. In particular, we prove that the space of valuations and the space of prime congruences on an idempotent semiring A are spectral, and there is a natural bijection of sets between two. We then develop several aspects of commutative algebra of semirings. We mainly focus on the notion of closure operations for semirings, and provide several examples. In particular, we introduce an integral closure operation and a Frobenius closure operation for idempotent semirings.