Radon diffusion coefficient of cement pastes made with recycled thermal carbon fly ashes

The use of supplementary cementitious materials (SCMs) is one of the most used ways to reduce CO 2 emissions associated with the production of Portland cement. However, many of these SCMs are Naturally Occurring Radioactive Materials (NORM), which present naturally occurring radionuclides. Radon emission from these types of materials, together with radon infiltration from the ground through them, is one of the main sources of indoor radon. These pathways through the building materials are regulated by the radon diffusion coefficient, D . In this work, the radon properties of 7 cement pastes (at 2 and 28 days) were studied in function of the technical properties to determine their suitability as building and insulating materials against radon. Two main results were found: The D of the pastes changed by an order of magnitude between those with a 2-day curing time and those with a 28-day curing time, the latter being less permeable to radon, and there was also a decrease in D as the percentage of superplasticiser admixture increased, due to the reduction in the porosity of the final pastes and the reduction in radon permeability. The lowest D value obtained was of (4.3 ± 0.4) ·10 - 11 m 2 s - 1 for the paste with a 70% of cement, 30% of fly ash, 0.6% of superplasticiser admixture and 28 days of curing. In conclusion, it was found that different characteristics of the paste production, which influence the final microstructure, can produce cements containing NORM with permeabilities to radon low enough to be used as a radon barrier. • The diffusion of cement pastes containing fly ash (at 2 and 28 days) was measured. • The higher the number of curing days of the paste, the lower the diffusion. • The higher the percentage of superplasticiser admixture, the lower the diffusion. • The variations in diffusion were found to be related to changes in porosity. • Cements containing NORM with low diffusion resulted from changes in production.