States of water in hydrated C 3 S (tricalcium silicate) as a function of relative humidity
- PDF / 253,879 Bytes
- 8 Pages / 585 x 783 pts Page_size
- 66 Downloads / 165 Views
Dan A. Neumanna) NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8562
Richard A. Livingston Office of Infrastructure R&D, Federal Highway Administration, McLean, Virginia 22101 (Received 24 March 2006; accepted 1 June 2006)
Quasi-elastic neutron spectroscopy was used to study the changes in the water content of hydrated tricalcium silicate cement paste with decreasing relative humidity (RH). The structurally bound water was divided into water bound in Ca(OH)2 and in calcium-silicate hydrate, or C–S–H-gel, utilizing the inelastic vibrational modes of Ca(OH)2. The quasi-elastic line was analyzed in terms of free and constrained water, and both were observed to decrease as the pores empty of pore-water with drying. An inelastic line related to translational vibrations of immobile water molecules was also extracted from the spectra, and its intensity was found to decrease with lower RH.
I. INTRODUCTION
Cement is the most widely used construction material by volume. Nevertheless, the basic chemistry governing the hardening process is not completely understood. The pore structure and microstructure of the calcium-silicatehydrate (C–S–H)-gel determines many of the important mechanical and corrosion-related properties of cement. The pore structure and hydration kinetics are strongly linked, but the link is not well understood. One obstacle to gaining a better understanding of the microstructure is that water is present in the gel in several states: free or unreacted, absorbed on the surface, and chemically bound in the gel or associated calcium hydroxide crystals. Moreover, the distribution of the water among these states varies with the relative humidity because of capillary condensation and the dehydration of the solid phases. The conventional method of handling this variability is to dry the material to some reference state and then to apply such methods as gas absorption Brunauer– Emmett–Teller (BET) surface area, mercury intrusion porosimetry, or electron microscopies for studying the microstructure. Because the drying methods are very
a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0332 2516 J. Mater. Res., Vol. 21, No. 10, Oct 2006 http://journals.cambridge.org Downloaded: 16 Mar 2015
harsh, there is concern that they can modify the fragile C–S–H pore structure. Consequently, microstructure observed under these conditions may not be representative of that under normal service conditions. Moreover, disagreements in observed values of parameters such as surface area among different microstructural measurement methods may be due to the moisture reference state.1 There are four major approaches to drying a sample. One is to heat it in an oven to a temperature slightly above the boiling point of water, usually 105 °C. The second is known as the dessicator method.2 It involves equilibrating the sample at a specified relative humidity, which is usually selected by the availability of a saturated solution of a part
Data Loading...
