Measurements of radon exhalation rate for monitoring cement hydration

The paper deals with one of the physical methods, which can be used for monitoring hydration of cementitious materials: the radon exhalation method. Experiments with two types of hydrating cement paste (made with water to cement ratios of 0.25 and 0.33) are described. The kinetics of shrinkage and hydration heat development are discussed. Different mechanisms influencing the radon exhalation rate E from cement and hydration products are considered. The initial E-values determined in the beginning of the tests were 0.01–0.02 mBq  kg⁻¹ s⁻¹ for the cement pastes made at water/cement ratios of 0.25 and 0.33, respectively. In 3 days both pastes showed E = 0.04 mBq  kg⁻¹ s⁻¹. However, the most important finding seems to be the dramatic increase of the radon exhalation rate up to the maximum observed a few hours after mixing with water (0.66 and 0.58 mBq  kg⁻¹ s⁻¹ for 0.25 and 0.33 pastes, respectively). This was registered in the radon chamber within the time period usually classified as set. The test results showed a strong correlation between radon exhalation rate and liberation of hydration heat. Peaks of the radon exhalation rate coincide with those of temperature measured on the surface of the cement paste. Analysis of the literature data shows that heating of the materials weakens physical adsorption of radon gas atoms on newly formed solid surfaces and can enhance the radon exhalation rate by several times. However, the performed experiment shows that the radon exhalation rate drastically increases (by dozens of times), and then decreases again. Such a dramatic growth can be explained by a synergy between temperature effect and two more phenomena: (a) intensive formation of microstructure with an extremely high specific surface area, when cement sets and while porosity is still high and (b) intensive flow of water, which traps radon from the newly formed solid surfaces of C-S-H and brings it to the sample surface, enhancing the radon flux.