High-speed neutron radiography for monitoring the water absorption by capillarity in porous materials Veerle Cnudde a,b, * , Manuel Dierick b , Jelle Vlassenbroeck b , Bert Masschaele b , Eberhard Lehmann c , Patric Jacobs a , Luc Van Hoorebeke b a Department for Geology and Soil Science, Ghent University, Krijgslaan 281/S8, B-9000 Ghent, Belgium b Department for Subatomic and Radiation Physics, Ghent University, Proeftuinstraat 86, B-9000 Gent, Belgium c Department of Spallation Neutron Source (ASQ), Paul Scherrer Institut, Villigen, CH-5232, Switzerland Received 21 March 2007; received in revised form 15 October 2007 Available online 7 November 2007 Abstract Fluid flow through porous natural building stones is of great importance when studying their weathering processes. Many traditional experiments based on mass changes are available for studying liquid transport in porous stones, such as the determination of the water absorption coefficient by capillarity. Because thermal neutrons experience a strong attenuation by hydrogen, neutron radiography is a suitable technique for the study of water absorption by capillarity in porous stones. However, image contrast can be impaired because hydrogen mainly scatters neutrons rather than absorbing them, resulting in a blurred image. Capillarity results obtained by neutron radi- ography and by the European Standard 1925 for the determination of the water absorption coefficient by capillarity for natural building stones with a variable porosity were compared. It is illustrated that high-speed neutron radiography can be a useful research tool for the visualization of internal fluid flow inside inorganic building materials such as limestones and sandstones. Ó 2007 Elsevier B.V. All rights reserved. PACS: 83.80.Nb; 83.85.Hf Keywords: Capillarity; Water absorption; EN 1925; Neutron radiography; Fluid flow 1. Introduction Moisture transport in porous media plays a role in a wide variety of processes of environmental and technolog- ical concern. Moisture inside building stones can lead to their degradation due to freeze/thaw cycles, causing the formation of cracks and spalling. Moreover, the solvent action of water and its dissolved impurities like sulphate, carbon dioxide and nitrate chemically attack building stones. Therefore, an understanding of moisture transport in natural building stones is important for their conservation. A parameter that characterizes the tendency of liquids to spread spontaneously over solid surfaces is capillary absorption [1]. A test that directly measures the rate of cap- illary absorption is the European Standard EN 1925 [2], which determine the amount of water uptake per square meter as a function of the square root of time. However, the determination of water absorption by this gravimetric method does not give a very good picture of the distribu- tion of the absorbed liquid inside the building stones. Although there is probably a linear dependence between the capillary rise (m) and the capillary coefficient ([g/m 2 ]/ p s)), the correlation factor strongly depends upon the combination of liquid and solid phases [3]. Since the pore network is not shaped like parallel tubes but is a complex 0168-583X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2007.10.030 * Corresponding author. Address: Department for Geology and Soil Science, Ghent University, Krijgslaan 281/S8, B-9000 Ghent, Belgium. Tel.: +32 (0)9 2644580; fax: +32 (0)9 2644943. E-mail addresses: veerle.cnudde@ugent.be (V. Cnudde), manuel. dierick@ugent.be (M. Dierick), jelle.vlassenbroeck@ugent.be (J. Vlassen- broeck), bert.masschaele@ugent.be (B. Masschaele), eberhard.lehmann@ psi.ch (E. Lehmann), patric.jacobs@ugent.be (P. Jacobs), luc.vanhoor- ebeke@ugent.be (L. Van Hoorebeke). www.elsevier.com/locate/nimb Available online at www.sciencedirect.com Nuclear Instruments and Methods in Physics Research B 266 (2008) 155–163 NIM B Beam Interactions with Materials & Atoms