Dimensional stability under wet curing of mortars containing high amounts of nitrates and phosphates

Investigations were carried out in order to solidify in cement some aqueous streams resulting from nuclear decommissioning processes and characterized by a high salinity (300 g/L), as well as important concentrations of nitrate (150–210 g/L) and phosphate ions (0–50 g/L). Special attention was paid to the influence of these compounds on the dimensional variations under wet curing of simulated solidified waste forms. The length changes of mortars containing nitrate salts only (KNO3, NaNO3) were shown to be governed by a concentration effect which involved osmosis: the higher their concentration in the mixing solution, the higher the swelling. The expansion of mortars containing high amounts of phosphates (≥ 30 g/L in the mixing solution) was preceded by a shrinkage which increased with the phosphate concentration, and which could be suppressed by seeding the cement used with hydroxyapatite crystals. This transitory shrinkage was attributed to the conversion into hydroxyapatite of a precursor readily precipitated in the cement paste after mixing.