Study of alkali-silica reaction occurring in cemented waste packages based on simplified model and concrete medium approaches

Abstract

International audienceNuclear power production generates radioactive waste, the management of which is an important industrial and environmental issue. Low - or intermediate - level radioactive aqueous waste can be concentrated by evaporation, stabilized and solidified with Portland cement before being sent to dis-posal. Interactions can however occur between the waste and the cement phases or aggregates, and decrease the stability of the final waste forms.The formation of a gel-like product, which results from an alkali-aggregate reaction, has been recently observed at the surface of cemented drums of evaporator concentrates. Its properties differ however from those usually reported for alkali-silica gels in many aspects: (i) very low calcium concentration, (ii) significant presence of Zn$^{2+}$, Cl$^-$, B(OH)$_4$$^-$ and SO$_4$$^{2⁻}$ ions, (iii) high formation rate, (iv) rather limited damage of the cementitious matrix considering the amount of gel produced.This work investigates the progress of alkali-silica reaction in the cemented drums, by studying the deterioration rate of the aggregates in model systems and in simulated concrete specimens.A synthetic alkaline solution, which mimics the pore solution including the waste, was used to de-grade the siliceous aggregates under controlled conditions. The extent of degradation caused by alkali-silica reaction was determined by weighing the residual flint aggregates and quantified by ther-mogravimetric analysis (TGA), specific surface area measurements, and gas pycnometry.A concrete specimen, formulated to be representative of the concrete embedding the radioactive waste, was cast and submitted to a thermal cycle, before being cured at 20°C and 90% relative humidity. Samples were taken from the concrete specimen and were observed at the micrometric scale using a scanning electron microscope coupled with energy dispersive X-ray analysis (SEM-EDX). The aggregates contained in the concrete were then mechanically and chemically retrieved, before being characterized by TGA.The model medium allows highlighting the advanced dissolution of the flint constitutive of the aggregates, leaving intact the most crystalline fraction of the grains. These results comply with the observations led on aggregates extracted from concrete samples, using SEM-EDX. Using this model medium,high degradation extent of the aggregates can be achieved. Indeed, the degradation progress of aggregates in concrete medium after 18 months could be reached in only a few hours using the model mediu