On the crystalline site relative to Z/p^n, I will explain the construction of two triangulated subcategories of the derived category of complexes of filtered modules on the structural sheaf, linked by a local biduality theorem. For these complexes, one can prove finiteness theorems for inverse and direct images which are analogous to the "non characteristic finiteness theorems" in the theory of complex D-modules, and one can generalize the classical finiteness and duality theorems in crystalline cohomology.