Microstructural Evolution and Radon Emission Dynamics in Class F Fly Ash-Blended Cement Pastes

Abstract

Radon release from cementitious building materials is a major indoor air quality concern due to the presence of naturally occurring radionuclides in raw materials. This study investigates the influence of partially replacing Portland cement with Class F coal fly ash (0–50% by weight) on the pore structure and radon emission of hardened cement pastes. Cement paste specimens with varying fly ash content were analyzed using mercury intrusion porosimetry (MIP) to quantify porosity and pore size distribution, and an open-loop radon concentration setup (using a DURRIDGE RAD7 detector) to measure radon exhalation. The results reveal that increasing fly ash content leads to a pronounced increase in total porosity (from 14.23% at 0% fly ash to 20.22% at 50% replacement) and a corresponding rise in radon concentration (steady-state radon concentrations increasing from 20.8 Bq/m³ to 32.1 Bq/m³ for the same range). Microstructural analysis indicates that high fly ash substitution coarsens the pore network – the volume-based median pore diameter expanded from ~102 nm to ~381 nm – while also nearly doubling the internal surface area, reflecting the development of both larger capillary voids and fine pores. These changes suggest enhanced radon transport pathways at higher fly ash levels. The findings underscore a mechanistic link between fly ash-induced pore structure modifications and radon diffusion behavior. High-volume fly ash use, while beneficial for sustainability and reduced clinker usage, can thus inadvertently increase radon release. Therefore, optimizing the replacement ratio is essential to balance sustainability goals with indoor air quality considerations and to minimize potential health risks associated with indoor radon exposure.

Keywords

• Radon emission
• Fly ash
• Cement paste
• Microstructure
• Porosity
• Sustainability

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