Incorporating Cs and Sr into blast furnace slag inorganic polymers and their effect on matrix properties

Abstract

Minimizing harmful effects to the environment in waste-management practices requires continuous innovation. This is especially important in the field of radioactive waste management. Alternatives to the commonly used ordinary Portland cement matrices are being increasingly studied for improved immobilisation purposes. The development of inorganic polymers (IP) from industrial residues has been successfully studied for the immobilisation of caesium (Cs +) and strontium (Sr 2+). However, knowledge of the effect of these introduced elements on the IP-matrix is scarce, especially considering that studied effects are dependent on the IP-precursor characteristics and the form in which the Cs + and Sr 2+ are introduced. In this study, IPs containing varying amounts of CsNO3 and Sr(NO3)2 were developed to study the effect of the introduced elements on the IP-characteristics. IPsamples were developed from ground granulated blast furnace slag (GGBFS) and 6 M NaOH activating solution. Cs + and Sr 2+ were added to account for 0.5, 1 and 2 wt% of the total IP-mass. Throughout the entire study, Cs +-addition showed no significant effects on the studied parameters. Calorimetric results showed that Sr 2+ severely affects reaction kinetics, consuming hydroxide ions necessary for the alkali activation reaction. Sr 2+-addition also caused a severe decrease in compressive strength, increased calcium leaching, and decreased sodium and hydroxide leaching. Micro-chemical analyses showed that Cs + is almost fully incorporated in the formed IP-matrix, while Sr 2+ mainly precipitates as Sr(OH)2 in concentrated regions throughout the IP-structure. The findings presented in this paper give insights on the effect of contaminant elements on the immobilizing matrix.

Alkali-activated materials for radionuclide immobilisation and the effect of precursor composition on Cs/Sr retention

“…The composition of the GGBFS closely resembles that of S_3.4 (excluding the contribution of MgO, SO3 and other trace elements). With a 1 wt% Cs + loading, the GGBFS-AAM leached 66 % of the introduced Cs + [29]. This is much higher than the Cs + release observed in AAM_3.4 and AAM_3.4b which could be due to e.g.…”

“…AAM pastes were prepared by mixing the powdered slag precursors with a 2 M NaOH solution (prepared from NaOH pellets (Fischer Scientific, 98.44 % pure) and type II distilled water) using a laboratory mixer and maintaining a liquid to solid ratio (L/S) of 0.30. These parameters were chosen based on our earlier study [29] and adjusted for optimal workability and setting time. At this molarity and L/S ratio, the amount of Na + added is 1 wt% for each sample.…”

“…Cs + and Sr 2+ were added as nitrates (CsNO3, Alfa Aesar 99.8%; Sr(NO3)2, Emsure 99.0%) to account for 1 wt% and 0.1 wt% respectively of the final AAM-mass (solid precursor + activating solution). A lower wt% of Sr 2+ was chosen to avoid significant interference with the polymerisation kinetics, as reported in our earlier studies [29,30].…”

“…AAM_1.1 shows the best Cs + immobilisation, releasing only 7.8 ± 0.3 % of the introduced Cs + , which is almost twice as good as the second best (AAM_1). In our earlier study [29], GGBFS-based AAMs activated with 6 M NaOH were leached for 7 days under the same conditions. The composition of the GGBFS closely resembles that of S_3.4 (excluding the contribution of MgO, SO3 and other trace elements).…”

“…The release of introduced caesium and strontium, and of the structural elements silicon, aluminium, calcium and sodium was measured using a semi-dynamic diffusion test described in our earlier study [29] and based on the standards ASTM C1220-98 [31] and CEN/TS 15863:2015 [32]. The samples were demoulded, cleaned with a dry brush and measured for dimensions and weight before being submerged in 400 ml of pre-heated Milli-Q water (90 °C, polypropylene bottle and sample holder).…”