141 International RILEM Conference on Materials, Systems and Structures in Civil Engineering Conference segment on Concrete with Supplementary Cementitious Materials 22-24 August 2016, Technical University of Denmark, Lyngby, Denmark EFFECTS OF W/P RATIO AND LIMESTONE FILLER ON PERMEABILITY OF CEMENT PASTES Quoc Tri Phung (1) , Norbert Maes (1) , Diederik Jacques (1) , Geert De Schutter (2) , Guang Ye (3)  (+6 ,QVWLWXWH %HOJLDQ 1XFOHDU 5HVHDUFK &HQWUH 6&.&(1 %HOJLXP (2) Magnel Laboratory for Concrete Research, Ghent University, Belgium (3) Microlab, Delft University of Technology, The Netherlands Abstract Because of environmental and economic benefits, a fraction of cement is increasingly replaced by limestone fillers raising a question on to what extent limestone fillers affect the durability of cementitious materials. This work aims at understanding the effects of water/powder (w/p) ratio and limestone filler replacement on water permeability of cement pastes. A newly proposed technique using a controlled constant flow concept was applied to measure permeability of hardened cement paste samples following a factorial experimental design. It was observed that both limestone filler and w/p ratio significantly influence the water permeability. At a given w/p ratio, adding limestone filler made the microstructure coarser, especially for high w/p ratio. Nevertheless, if the comparison is based on a given water/cement (w/c) ratio instead of w/p ratio, the limestone filler replacement refined the microstructure in terms of capillary porosity and pore size distribution, resulting in permeability decreases of cement pastes. Furthermore, a modified Carmen-Kozeny relation was established which enables prediction of the permeability from capillary porosity and the critical pore diameter. 1. Introduction Durability and performance of concrete are affected by a number of chemical degradation phenomena such as carbonation [1], calcium leaching [2], sulphate attack, chloride attack, and corrosion of the reinforcement bars [3]. Diffusivity and permeability of concrete determine how fast aggressive substances penetrate into concrete, thereby influencing almost all mentioned degradation mechanisms. Therefore, these are fundamental parameters for characterizing long-term performance of concrete rather than the standard compressive/tensile strengths. There are many factors influencing the permeability of cement-based materials. Several factors are classified as extrinsic related to experimental conditions such as applied pressure,