Radon exhalation from cement-based materials under accelerated carbonation.

Abstract

Cement-based materials manufactured from rocks and soils will release radon which is a carcinogen and affects indoor air quality. The alkaline cement-based material neutralizes with the acidic gas carbon dioxide in the air, reducing its pH value, known as carbonation. Carbonation of cement-based materials is an important environmental factor that can change the pore structure and effect radon release. In this study, test blocks of concrete, fly ash concrete, cement mortar, and cement paste were subjected to carbonation at 20 vol% CO2, 70% relative humidity, and a temperature of 20 ± 2 ℃ for 28days to explore the effect of material characteristics and carbonation age on the radon exhalation rate. Carbonation had a significant influence on the radon exhalation rate, but this effect showed positive (promoting/increasing) and negative (inhibiting/decreasing) fluctuations with carbonation age. Among the material characteristics, aggregate content had the most significant influence, followed by fly ash and cement variety. The radon exhalation rate was ordered as cement mortar > concrete > cement paste before carbonation, but was concrete > cement mortar > cement paste after carbonation. The radon exhalation rate of cement paste blocks without aggregate was ~ 1mBq/(m2·s) lower than that of cement mortar. The inhibition of radon emission by concrete was mainly observed in the early carbonation period (< 7days), while that by fly ash concrete was observed after 7days. The content of fly ash did not have a significant influence on the radon exhalation rate of materials. Radon inhibition by composite Portland cement concrete was mainly observed in the middle stage of carbonation (~ 14days), while inhibition by ordinary Portland cement concrete was mainly observed in the early (3-7days) and late (i.e., ~ 28days) stages. The water/binder ratio did not significantly affect the radon exhalation; concrete with a low water/binder ratio showed weak radon inhibition only when the carbonation age was long. These results will help to evaluate radon pollution in indoor or underground environments under long-term use.