Determination of the Neutron Scattering Contrast of Hydrated Portland Cement Paste using H2O/D2O Exchange

Accurate measurements of the surface area of cement based materials using small-angle neutron scattering (SANS) require determination of the neutron scattering contrast between the C-S-H gel and H₂O. Because the C-S-H gel has a poorly understood structure and a variable composition, calculated values of the C-S-H:H₂O neutron scattering contrast based on an assumed C-S-H chemical composition and density are subject to error. The C-S-H:H₂O neutron scattering contrast was determined experimentally by measuring the change in apparent surface area as the H₂O in a hydrated cement specimen was replaced with D₂O, resulting in a new value of 6.78 × 10²⁸ m⁻⁴. This new contrast value increases some previously reported SANS surface area values by 21%. The results also confirm that C-S-H interlayer water should be included in the solid phase whereas gel pore water should be excluded.