Methods for Porosity Characterization of Cement Based Materials
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Methods for Porosity Characterization of Cement Based Materials 1
Canut, M.M. and 1,2Geiker, M. R. Department of Civil Engineering, Technical University of Denmark, Kgs. Lyngby, DK 2 Department of Structural Engineering, Norwegian University of Science and Technology, NO 1
ABSTRACT Important pore structure parameters related to mechanical properties and durability of cement-based materials can be determined by techniques such as scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP), and low temperature calorimetry (LTC). The methods provide information on porosity characteristics as pore volume, pore thresholds and/or pore size distribution in different size ranges and do therefore to a large extent supplement each other. Pastes of w/b=0.4 with 0%, 40% or 70% slag by volume were cured saturated at 20ºC for up to two years. The porosity was characterized by LTC, MIP, and SEM. Higher volume of pores was obtained by MIP compared to results obtained using LTC and SEM. Measured porosity was correlated with predicted porosity using information on the density and degree of hydration of the cement and slag. Porosity estimation showed best agreement with the porosity data measured by MIP. The use of slag showed the same trend for all tests: a higher total volume of pores, but a lower threshold pore size when compared with Portland cement paste. The findings illustrate the importance of measuring not only pore volume but also threshold pore sizes when characterizing porosity of cement-based materials with different binder compositions. INTRODUCTION Engineering properties are related in some way to the amount, size and distribution of pores and characterization of the pore system is often used to predict the performance of hydrated cementitious materials. Volume of pores may be related to the mechanical properties of the concrete while the threshold pore size is relevant for the assessment of the resistance to transport of substance in the material. Determination of pore structure may be complex and based on physical relationships and geometrical models that simplify the complex structure of the pores. Different mechanisms are used on the porosity measurements: some involve intrusion of different liquids (water, solvent, mercury) with particular properties (surface tension, density); others direct measurements by images, e.g. scanning electron microscopy (SEM). Assumptions are made using the equations to calculate pore size and total volume of pores. For instance, the Kelvin Equation uses parameters (e.g. surface tension, density, heat of fusion) to calculate the diameter of a cylindrical pore size [1] . Apparently, the parameter's values are not well established in the literature and may give large difference on the porosity data, also cylindrical pores may not represent the pores of cementitious materials [2]. Moreover, porosity data given by MIP and SEM may be affected by the prerequisite of drying; such drying may change the microstructure of cement based materials. Limited impact of drying was found wh
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