Proceedings of the 2006 International Radon Symposium September 17 - 20,2006 PREDICTION OF RADON CONCENTRATIONS AT CONCRETING IN CONSTRUCTION SITES WITH LIMITED VENTILATION Konstantin Kovler National Building Research Institute - Faculty of Civil and Environmental Engineering Technion - Israel Institute of Technology, Haifa 32000, Israel INTRODUCTION It is known that radon exhalation rate can achieve extremely high values during cement hydration. Radon contained in trace amounts in hardening material, as other inert gases, does not interact with the surrounding media chemically, and hence does not interfere with chemical reactions accompanying cement hydration. Therefore, the unique properties of radon as a noble gas can be used for monitoring cement hydration and microstructural transformations in cementitious system. The first attempt to measure radon exhalation in hardening cementitious materials and use radon as an indicator of microstructural changes occurring during cement hydration was made in Czechoslovakia by Balek and Donhhlek (1983). Balek and Donhhlek attributed the changes in radon exhalation rate, mainly, to the changes in surface area and microporosity occurring in the hardening material during hydration. The authors suggested labeling the sample of cement powder before hydration by the parent isotopes of radon. They impregnated cement powder with a non-a ueous (acetone) solution containing 2 2 8 ~ h or 2 2 4 ~ a in the concentration of 1011 Bq m ' . The radionuclides were adsorbed on the surfaces of cement particles and the non-aqueous solution was evaporated. The specific radioactivity of the labeled cement was 106 - 107 Bq kg", i.e. higher by 5-6 orders of magnitude than that before labeling, therefore the sensitivity of the method was tremendously enhanced. The disadvantage of the labeling is that working with very high radioactivity concentrations requires specific safety precautions. In addition, labeling cement particles for a certain depth, which is independent of cement particle size, leads to uncertainty with the interpretation of the results, especially when the effect of cement fineness is studied. The more detailed review of the publications on using radon exhalation method for monitoring cement hydration and microstructural transformations in hardening cementitious materials is available in the State-of-the-Art Report of RILEM (International Union of Laboratories and Experts in Construction Materials, Systems, and Structures) Technical Committee TC-185 ATC "Advanced Testing of Cement-Based Materials during Setting and Hardening" (Kovler, 2005). Copyright @ 2007 by the American Association of Radon Scientists and Technologists, Inc. 30 www.aarst.or2