Inelastic Neutron Scattering Measurement of Pozzolan Performance in Portland Cement

Abstract

Pozzolanic materials such as silica fume or fly ash are added to Portland cement concrete mixes to reduce shrinkage and increase durability. One measure of a pozzolan’s performance is its consumption of calcium hydroxide, which is a product of the main cement hydration reaction[1]. Inelastic neutron scattering has been applied to make in situ measurement of the amount of calcium hydroxide in cement paste samples as a function of pozzolan mix fraction, composition and particle size distribution. This is a molecular vibrational spectroscopy technique that identifies calcium hydroxide by the major phonon mode of the OH group oscillations at 41 meV [5]. The method detects calcium hydroxide in both crystalline and amorphous forms. The measurements were made at the recently completed Filter Analyzer Neutron Spectrometer (FANS) at the Center for Neutron Research at the National Institute of Standards and Technology in Gaithersburg, MD, which has a resolution on the order of 1 meV. The nondestructive nature of the technique makes it possible to remeasure the same specimen over time and thus to determine the kinetics of the pozzolanic reaction.