Stabilisation/Solidification of soils contaminated by mining activities: Influence of barite powder and grout content on γ-radiation shielding, unconfined compressive strength and 232Th immobilisation

In this work, γ-ray shielding, unconfined compressive strength (UCS) and contaminant immobilisation performances of barite (Ba)-Portland cement (PC) grouts in Stabilisation/Solidification (S/S) treatment of 232Th-contaminated soils were studied. This is with the view to investigating the influence of the Ba and grout (G) content used in S/S mixes. Results show that the replacement of PC with Ba (up to 50%) leads to a significant increase, up about 2-fold, of the γ-ray shielding performance, but a higher Ba content in Ba-PC mixture decreases the S/S shielding performance due to the reduction of the final density of the S/S products, caused by the inhibition of cement hydration process. The highest γRS index of ~ 37% was found in the case of the maximum G to soil content of 50%. Ba and G content significantly influences the UCS performance of the S/S products. A marked UCS increase up to ~ 20% was found replacing PC with Ba with a Ba content of 50%. All the investigated S/S samples have UCS values that fulfil the quality criteria requirements. The use of Ba in PC grout allows the achievement of the almost total 232Th-immobilisation, even in extreme-acidic environment conditions (pH: 3, 1), also due to the Ba fine texture and high surface area able to reduce the final porosity of S/S products. Overall, the γRS index, observed jointly with the excellent immobilisation performances, reveals the mixture PC:Ba = 1:1 with S:G = 2:1 as the optimal choice in order to achieve the remediation targets and, at the same time, limit the costs, highlighting the possibility to successfully treat real 232Th-contaminated soils. However, a S:G = 1:1 can be considered if a higher γ-ray shielding of 37% is required.