The Nanostructure of Low-CO2 Concrete for a Sustainable Infrastructure

Executive Summary The objective of this program is to measure the changes in the properties of the C-S-H gel phase in cement paste as a function of the Ca/Si ratio. The motivation is to better understand the hydration products that form in blended cements with lower Ca/Si ratios than portland cement. The main contribution from Northwestern was to characterize specimens using small-angle neutron scattering (SANS). In particular, a new method of measuring the neutron scattering length density, composition, and mass density of the nanoscale hydration products was employed. For samples made at Lafarge, the general microstructure was characterized with SEM, the degree of hydration was measured with QXRD, and the local Ca/Si of the hydration products was measured using EDX. This report first provides an overview of the SANS analysis method and the information it provides (Section 1), and then a description of the types of specimens that are discussed in this report (Section 2). Sections 3 and 4 contain results for pastes that are not part of the present project, but are included because they are highly relevant. These are unpublished results that should be treated as confidential. In Section 5, the results from the C 3 S pastes with varying amounts of colloidal SiO 2 , made and characterized by LCR, are summarized. Finally in Section 6 recommendations for further collaborative work are made. The principal findings of this report are as follows. The hydration of C 3 S in the presence of colloidal silica, while scientifically quite interesting, is not representative of pozzolanic blended cements in general due to the high reactivity of the SiO 2. The results obtained during the past year shed light on the hydration of C 3 S and on the nanostructure of C-S-H, but also raise important new questions. The main recommendation is to perform similar experiments using silica fume or other pozzolanic materials to control the Ca/Si of the paste and of the C-S-H. These results will be of considerable practical use and will also help interpret the results obtained with colloidal silica.