Variation of the emanation factor of 222Rn in different types of cementitious pastes

Abstract The determination of the activity concentration of 222 Rn is related to its emanation and the equilibrium reached with its short-lived gamma-emitting progeny, 214 Pb and 214 Bi. Previous studies conducted with hybrid and alkali-activated cements, both utilizing fly ash (FA), reflected 226 Ra/ 214 Pb ratios significantly greater than 1. This study further investigates this disequilibrium by determining the 226 Ra/ 214 Pb ratios in different cement pastes: (i) magnesium phosphate, (ii) blended with FA, (iii) hybrid with FA, (iv) alkali-activated with FA and metakaolin (MK) using various alkaline activators. The samples were measured by gamma spectrometry both in solid form and after grinding. The results showed 226 Ra/ 214 Pb ratios of 1.3 for the ground pastes manufactured with 100% FA and using NaOH as the activator at different molarities (8 M, 10 M, and 12 M). Measurement of the samples with the addition of 2% (w/w) activated carbon showed that this effect was neutralized, resulting in ratios of 1, indicating equilibrium between 226 Ra and 214 Pb. The measurement of the emanation coefficient (F e ) for the powdered solids obtained from the ground cubes showed that the emanations were higher than those of the anhydrous materials (cement (C), FA, and MK). This high emanation is primarily caused by NaOH activation, which induces microstructural changes in the cement pastes, leading to increased radon release. The highest emanations were 36.2% for samples derived from pastes alkali-activated with NaOH. Additionally, it was observed that 222 Rn was lost by 5% from the cylindrical containers used for gamma spectrometry measurements when the F e was 0.20 (20% emanation).