Reducing Radon Gas Emissions in Concrete
Abstract: A multifold of compulsory regulations and recommendations regarding ionizing radiation for building products has in recent years been introduced. Also, industry-affiliated aggregate and concrete companies have implemented environmental goals (Green Council Building 3.0) that shall be fulfilled regarding building materials. One of these environmental goals is the levels of radon within the indoor environment. During the decades it has also become more usual that the concrete industry uses combinations of different Supplementary Cementitious Materials (SCMs) in concrete in order to reduce carbon dioxide emissions of the cement production. Insert of SCMs and different admixtures can also improve the properties of concrete, such as increased strength and durability. However, the knowledge of ionizing radiation and radon is still limited. How do SCMs and hydrophobic admixtures contribute regarding properties such as radon gas exhalation from concrete? Are there any advantages? Disadvantages? Can one make use of specific properties in specific indoor climate environments? Effect of moisture? The main part of the Thesis has embraced these concerns. Twelve different concrete recipes were cast where the radon exhalation rate was investigated. Ten recipes consisted of different mixes of binders and hydrophobic admixtures containing a crushed rock with slightly enhanced 226Ra-activity concentration (Bq/kg). Two recipes included a crushed rock with low levels of radioactivity. As a reference cement and binder a CEM I, 52.5 R (Skövde cement factory) was used. The concretes´ composition had a water binder ratio (w/b) of 0.55.For radon gas analysis and radon diffusion measurements a method using the decay rate of alpha energies from 222Rn and 218Po was employed. The amount of decay per unit time were calibrated in relation to a well-defined radon gas level. The readings or the output from the radon gas monitor were then displayed as 222Rn content in air in the unit Bq/m3. Diffusion measurements included an instrument named RAD 7 from Durridge Inc. The instrument´s measuring technique uses a solid state detector. The results imply that SCMs and hydrophobic admixtures (liquid) have a moderate to fairly large impact on the radon exhalation rate a humidity of 75 % and 60 %. The largest impact at a relative humidity of 75 % is shown by micro-silica (SF-30), that reduces the radon exhalation rate by up to 57 %. However, at a relative humidity of 45 %, the radon exhalation of the reference concrete is in line with most other concrete mixes regarding their radon exhalation rates. One need to separate between radon gas exhalation and radon gas diffusion. They both affect the radon rate within a building. In the study, the radon gas exhalation rate generally decreased with decreasing relative humidity. The radon gas diffusion, however increased in general as the relative humidity decreased. Also, the natural process of carbonation affects the radon exhalation rate. The study performed as part of the Thesis, relating to carbonation and its influence, generated different results depending on the concrete recipe, but can be summarized as: (i) concrete with only CEM I or CEM I combined with an hydrophobic admixture indicated a reduced radon exhalation rate as for (ii) a concrete recipe containing CEM I as a binder combined with slag or fly ash, the radon exhalation rate increased.Another study, as part of the Thesis, embraced induced cracks and their influence upon the radon exhalation rate. The study showed that the influence of cracks can be quite large. In two cases an increase of 200-250 % was calculated compared to the radon exhalation rate of the same concrete without cracks. In the other cases, the increase was proportional to the increase of the concrete surface.Several factors influence the final rate of radon being exhaled from a building material. The radon exhalation in the examined building materials can also be addressed as the production rate of radon (exhalation of radon per unit volym) for the investigated concrete mixes. The production rate is mainly governed by the emanation coefficient, the content of radium in the materials and the material´s density (volume and mass). Since the investigated concrete mixes have a similar density and radium content, these variables are of less importance assessing the differences between the concrete mixes exhalation rates. Consequently, the influence of the radon emanation becomes a major parameter, when comparing the different concrete mixes. The radon emanation has in the ongoing assessments been shown to show a substantial variation, due to the influence of the relative humidity. Initially in a water filled system (100 % RH), the water acts as a barrier and radon is accumulated in the pores (e.g. the recoil theorem). When the moisture level decreases, the initially high radon levels in the pore system are enabled to diffuse to the free air. The initially high concentration of water molecules also act as carriers for a part of the radon atoms. This promotes that when the relative humidity successively is reduced in the concrete samples, the amount of radon atoms reaching the concrete surface is also diminished, consequently reducing the radon exhalation rate. In other words, the most important factor for differences in the radon exhalation rate can be dedicated to its radon emanation, meaning the number of radon atoms being released from the material itself to the free air. As a consequence, the tightness of the concrete, or its permeability is very important. This is in part reflected in the diffusion coefficient or the radon length being assessed for the different concrete samples. That the radon gas diffusion increases with a lower relative humidity in the concrete is reasonable since the diffusion rate in water is markedly lower than in air. The diffusion rates in the investigated concrete samples have however a subordinate role, when one evaluates the final exhalation rate. The high radon exhalation rate in this study is foremost due to (i) the material´s high radium content and (ii) a higher emanation coefficient at higher relative humidities. It is of importance to note that the materials´ slightly elevated radium content has a large influence on the high radon exhalation. Comparing the concrete recipe C, with a recipe being replaced with an aggregate with low radioactive content (low amount of radium), the production rate is very limited, meaning low radon exhalation rate, even though a moderate emanation coefficient can be shown. Conclusively, this implies that the relation between the relative humidity (RH), the radon concentration and diffusion within a concrete wall, ceiling or floor is a complicated interaction. In practice, the influence of relative humidity is the dominating factor for the final radon exhalation rate from a building material. Consequently, the radon exhalation rate, in general, decreases over time as the concrete is drying out, and the relative humidity decreases. Some essential conclusions of the Thesis that can be derived are that SCMs and hydrophobic admixtures effectively can reduce the radon gas exhalation rate and specifically at higher relative humidities as well as that fractures in concrete may generate substantial radon concentration increases. Depending upon choice of binders, the carbonation of concrete may give a positive or negative effect upon the radon exhalation rate.
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