[en] A simulated Low Level nuclear Waste (LLW) solution was tested for long term solidification in a cement-based matrix. The waste is characterized by high pH and high concentrations of sodium, aluminum, nitrate, nitrite, phosphate and carbonate. The effect of diluting the waste with additional water was studied. The cementitious matrix was composed of cement, fly-ash and clay (21%, 68% and 11% respectively) with high solution to solid ratio (1 liter /1 kg.). Mixes were prepared at 45 degrees C and cured at 90 degrees C for 28 days. Maximum 28 day compressive strengths and early age heat evolution were achieved by diluting the LLW solution to approximately 67% of its original concentration. More dilution led to a lower heat evolution and compressive strength. No dilution was found to give lower compressive strength, and a heat evolution that was delayed, and lower in intensity. XRD spectra showed formation of zeolites and tobermorite at the higher concentrations (67, 85, and 100% of the concentration of the undiluted simulated LLW), with a change from Na-Pl zeolite for 67% of the undiluted concentration to a sodalite at 100%. SEM observations showed a porous system for the low and high dilution rates but a less porous one for an intermediate level of dilution