The influence of different drying methods on cement paste microstructures as reflected by gas adsorption: Comparison between freeze-drying (F-drying), D-drying, P-drying and oven-drying methods A. Korpa * , R. Trettin Institute of Building and Materials Chemistry, University of Siegen 57068, Germany Received 28 November 2005; accepted 30 November 2005 Abstract Drying of cement pastes is required prior to microstructure investigation by means of gas adsorption technique. An ideal drying method, which would give reproducible results that could perfectly remove only the non-bound water and, at the same time, preserve the microstructure, unfortunately does not exist. The different drying methods used affect the microstructures in different ways. However, an effective water removal and, less damaging drying method between the common methods used would be still of outstanding importance for sample preparation. Many drying methods have been investigated in the past for such a purpose, and a good agreement for the best drying method does not exist. The so- called D-drying method is being used in many laboratories as the ‘‘best’’ method for drying cement pastes. The surface areas and pore size distributions results of the current work confirm that D-drying (D-Drying C  t ) is a relatively good preservation and effective drying method, and that Freeze-drying gives slightly better results compared to D-drying (C  t ) and other methods. However the short time versions of some of these methods indicate the presence of very few ‘‘micropores’’, which are not present with prolonged drying times. The outgas level is also a very important variable affecting the gas adsorption measurements especially in the case of short duration drying conditions, as indicated by the results of this work. D 2005 Elsevier Ltd. All rights reserved. Keywords: Cement paste; Drying; Pore size distribution; Microstructure 1. Introduction Cement pastes have a characteristic pore structure which depends primarily on the w/c ratio, the extent and conditions of hydration, and, to a lesser extent, on the specific cement used, the presence or absence of admixtures and certain other factors [1]. The pores stem mostly from what were originally water-filled spaces between cement particles, and finer pores, below around 2 nm, are believed to be characteristic internal features of C – S – H gel: the main hydration product of cement paste [1]. The pores form an interconnected network, although at low water/cement ratios the connections may become largely plugged by hydration products. Because of this interconnection, it is often not possible to delineate the boundaries of a given individual ‘‘pore’’ [1]. The high surface area of hydrated PC is primarily due to the porous microstructure of the main hydration product, CaO – SiO 2 –H 2 O (C–S–H) [2]. To measure the surface area by the gas adsorption technique, the water stored in the pores must be removed. It is thought that two general types of pores are present in C–S–H gel, both of which contain water; capillary pores are larger pores that hold water in saturated conditions but lose their water on exposure to air, and gel pores are nanometer-sized pores (<2 nm). The water that is present in gel pores is more strongly bound than capillary water and can be removed only by drying the paste. Any gel pores which remain filled with water will not be accessed by the adsorbate molecules and will not be included in the surface area measurement [2]. The smallest pores are the most difficult to empty of water, but they also have the highest relative surface area, and the stresses related to surface tension of the receding water menisci generate a collapse of some of the fine pores, which are the most sensitive to these capillary effects [3]. It has often been suggested by other authors that the drying 0008-8846/$ - see front matter D 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.cemconres.2005.11.021 * Corresponding author. E-mail address: korpa@chemie.uni-siegen.de (A. Korpa). Cement and Concrete Research 36 (2006) 634 – 649