The role of metakaolin in pore structure evolution of Portland cement pastes revealed by an impedance approach

Abstract Metakaolin has been shown to play a significant role in the pore structure evolution of cementitious materials due to its high pozzolanic reactivity and small fineness. In this work, the early-age evolution of pore structure of moderate heat Portland cement (MHPC) pastes incorporating metakaolin is studied using an innovative impedance approach. Based on the fractal modeling, the pore structure parameters of MHPC blended with metakaolin of various mass percentages up to 20%, including porosity, pore size distribution, pore tortuosity and its fractal dimension, and mean and maximal pore diameters, are determined. The results from the impedance measurements show that the porosity of metakaolin-blended MHPC at both initial hydration and pozzolanic reaction stages increases with the hydration time, which is, respectively, attributed to the dissolution of cement and metakaolin grains. The maximal porosities and the saturation states of MHPC-metakaolin blended pastes occur concurrently due to the dissolution behavior of cement grains. It is concluded that metakaolin incorporation at the 10% replacement ratio is optimal in terms of pore structure refinement of blended cement pastes.