The process of preparing a "Declaration of Performance" is presented for a masonry mortar manufactured with by-products from the steelmaking industry recycled as aggregates, for subsequent CE Marking, with the aim of promoting its commercialization within the European Union. Specific tests are applicable to construction mortars, both for masonry and rendering and plastering, for the verification of their performance, in accordance with harmonized standards EN-998-1 and EN 998-2. The study likewise sets out the Systems of Assessment and Verification of Constancy of Performance (SAVCP): System 4 for Rendering and Plastering Mortars and Systems 4 and 2+ for Masonry Mortars. Special importance in the new procedures for CE Marking is placed on the manufacturers, as they assume responsibility both for determining the properties of the materials and for establishing a control process in the factory. This document describes the technical development of the tests, in accordance with the applicable harmonized standards, for verifying the performance of the mortars designed with steelmaking slags as aggregates. The study concludes with the presentation of a Technical Data Sheet for the preparation of the corresponding CE marking for these new designed mortars.