This research focuses on the effects of waste loading and composition (specifically Ca(NO3)2 , Ba(NO3)2 and NaNO3) on the early age properties and mechanical strength of nuclear cementitious waste forms, which are subjected to accelerated alkali-silica reaction (ASR) test. A laboratory-prepared acidic liquid waste simulating a high active raffinate (HAR) generated from the PUREX process is neutralized to a pH of 10 with a mixture of Ca(OH)2 and Ba(OH)2. The neutralized waste is then immobilized at 50, 53 and 56% by volume waste loadings (WL50, WL53 and WL56, respectively), with water/binder ratios of 0.33, 0.38 and 0.45, respectively. The immobilization matrix contains CEM I and supplementary cementitious materials such as blast furnace slag and silica fume, along with other components such as limestone fillers and lime. The mixture is cast in 40 x 40 x 160 mm3 molds and subjected to the accelerated ASR test using 1M NaOH solution at 80°C for 28 days. Afterwards, the compressive strengths are measured to check if they conform to the Waste Acceptance Criteria for homogenous cementation process (ACRIA) of ONDRAF/NIRAS in Belgium. Hydration heat and setting time are measured for the three different mixes using an isothermal calorimeter and automatic Vicat apparatus, respectively, to investigate the effect of waste loading on the early age properties. The initial heat of hydration is found to be the highest for WL50, which may be attributed to the highest percentage of cement present in the designated volume of mix. Although at a later stage, the hydration heat of WL53 and WL56 become similar to WL50, which may be due to the accelerating effect of Ca(NO3)2 in the waste sludge on the hydration kinetics.
This effect is proportional to the waste loading. Vicat data shows that the initial setting time is the lowest for WL50 followed by WL53 and WL56. One possible explanation to this is the proportional increase of Ba(NO3)2 within the designated volume with respect to waste loading. It may progressively retard the set of the mix by decomposing ettringite. However, this may also be a combined effect of increased Ba(NO3)2 and water content. A mineralogical and microstructural analysis of the samples is needed in order to examine these assumptions. Compressive strength tests after 28 days reveal average values of 42, 35 and 23 MPa for 50, 53 and 56% waste loadings respectively, which are well above the ACRIA criteria.
|Proceedings of ASME 2023 International Conference on Environmental Remediation and Radioactive Waste Management, ICEM 2023
- Waste Management and Disposal
- Environmental Engineering