Assessing the impact of phosphogypsum composition on radon exhalation from alkali-activated materials

• Activation of ground granulated blast furnace slag and phosphogypsum • Incorporation up to 30 wt% phosphogypsum • Presence of vaterite increased radon exhalation • Presence of P and Sr influences radon exhalation This study assessed the influence of phosphogypsum (PG) composition on the immobilisation potential of 222 Rn in alkali-activated materials (AAMs) made with ground granulated blast furnace slag (GGBFS). The elemental composition of PG varies according to the origin of the ore and the process used. To improve recycling of PG, the effect of varying elemental composition on binder properties and radon exhalation needs to be assessed. The matrix properties were altered by incorporating different types and concentrations of PG as dry precursor as well as by varying the molarity of the sodium hydroxide alkali-activator. The resulting alkali-activated binders were characterised using gamma-ray spectrometry, X-ray diffraction (XRD), radon exhalation, mercury porosity, and compressive strength measurements. The massic activities of 226 Ra, 228 Th, 228 Ra, and 40 K were used to determine the Activity Concentration Index (ACI) of the dry precursor mixtures. Qualitative analysis of XRD diffractograms revealed the presence of mainly ettringite, bassanite, vaterite, aragonite, gypsum, and calcite with varying peak intensities in the different binders compositions. Radon exhalation rates ranged from 8 mBq/(kg·h) to 182 mBq/(kg·h), and these values were used to calculate the radon emanation coefficient. An increase in the PG/GGBFS ratio was associated with a reduction in total mesoporous and microporous surface area. Additionally, higher PG content in the dry precursor mix resulted in a more consistent stabilisation of compressive strength with values ranging from 15 MPa to 20 MPa. These findings highlight a novel route to reduce radon exhalation while valorising an industrial by-product. By showing that PG incorporation both densifies the AAM matrix and sustains strength, this work provides a practical framework for developing safer, radiation-aware building materials.