Abstract
This work aims at studying the hydration process and the properties of wollastonite-based brushite cements. These binders may indeed offer new prospects for the treatment or conditioning of low- or intermediate- level radioactive wastes characterized by a strong acidity and/or a contamination by strontium. First, a study of the hydration process is carried out with a commercial cement. Then, the influence of the mixing solution composition on the setting and hardening process is determined. Finally, a first assessment of the potential of this material for the decontamination and immobilization of strontium-containing aqueous effluents is performed.Wollastonite-based brushite cement pastes are prepared by mixing ground wollastonite and a phosphoric acid solution containing metallic cations (Al3+ and Zn2+) and borax. Wollastonite reacts through a dissolution/precipitation process, which leads to the formation of brushite, amorphous silica and amorphous zinc and calcium aluminophosphate whose structure has been investigated by 31P and 27Al MAS-NMR. The brushite precipitation is preceded by the transient formation of monophosphate calcium monohydrate. The thermodynamic simulation of the wollastonite reaction with a phosphoric acid solution, using a geochemical speciation code, fairly well reproduces the precipitation sequence of crystalline phases observed experimentally.It appears that the optimum phosphoric acid concentration in the mixing solution is comprised between 9 and 10 mol.L-1. Boron retards the cement setting whereas zinc accelerates it. The addition of aluminium, which leads to the massive precipitation of amorphous calcium aluminophosphate, is proved to be necessary to obtain a material with high mechanical strength. Thanks to response surface methodology and multi-criteria optimization, a composition domain of the mixing solution is pointed out, leading to a material with good properties (in terms of setting time, self-heating and mechanical strength) for waste conditioning.A crystallographic study also shows that brushite is able to incorporate at least 30 % (mol/mol) of strontium in substitution for calcium in its structure. Strontium retention tests by a ground cement paste in diluted suspension lead to a type S sorption isotherm, meaning that several retention mechanisms are involved. Finally, a leaching experiment performed on a cement monolith containing strontium shows that this species is well confined within the cement matrix, with a retention coefficient at least two orders of magnitude higher than that of a Portland cement paste.