The present invention concerns the application of a novel process for the physical and chemical conditioning of chemical gypsum or phospho gypsum, derived from the production of phosphoric acid, and its use as a retarding agent in the setting of Portland type cements or in the preparation of other construction materials. This conditioning is based on solid-state reactions or reactions in the presence of small amounts of water, between phospho gypsum and agglutinating-neutralizing agents, principally through the application of oxy and hydroxy compounds, without discounting use of the respective carbonates of magnesium, aluminum and principally calcium, in addition to mixtures of these, and the subsequent preparation of self-hardening pellets which facilitate transport and handling. The technological process associated with the invention notably simplifies the number and type of individual operations, and the preparation of pellets from mixtures of phospho gypsum, in addition to reducing energy costs by operating at low temperatures (150.degree. C.) during short periods (1-2 hours), reducing the quantity of neutralizing agents, and also minimizing the volume of water required for the process.