Mg-K phosphate cements (MPC) are used for nuclear waste encapsulation, bone repair, and in civil engineering. The chemical nature of the first amorphous reaction product is matter of research. Another critical aspect is the fast hardening of MPC. The reaction rate is slowed down by adding boric acid. But the retarding mechanism is unknown. Information on reaction products and retarding mechanism is of relevance because directly related to the development of the microstructure, and, consequently, to the properties (e.g. mechanical, adhesive, deterioration) of MPC for applications. The aim of the proposed experiment is to use incoherent inelastic neutron scattering to shed light on the nature of the reaction products and the retarding mechanism, in order to support/complete our previous results (including those from a recent Xpress beamtime) and help in the design of new innovative MPCs.