Dimensional Changes in DSP Cement Paste
J. F. Young,* H. Ai
NSF Center for Advanced Cement-Based Materials (In conjunction with the Department of Civil and Environmental Engineering)

Low-porosity cement pastes containing silica fume undergo considerable autogenous shrinkage, due both to hydration and the pozzolanic reaction. The amount of drying shrinkage of the hardened paste depends on how much shrinkage has occurred during curing. Thermal expansion and contraction accompanying temperature change is also being studied. On heating, initial expansion is followed by a time-dependent thermal contraction which is attributed to moisture redistribution within the pore system. Both shrinkage and thermal change are being correlated with microstructural parameters.

High-Performance Concretes for Airport Pavements
J. F. Young,* D. A. Lange,* L. J. Struble,* H. Shin
FAA Center of Excellence for Airport Pavement Research (In conjunction with the Department of Civil and Environmental Engineering)

Low-porosity concretes are being evaluated for use as overlays on existing airport pavements. These materials use high additions of silica fume and superplasticizers to provide dense concretes of low permeability, good bonding characteristics, and improved wear resistance. However, cracking caused by thermal and drying shrinkage is a concern. This project examines experimentally the properties of these concretes and models their cracking tendencies. Development of bond with the existing pavement will also be considered.

Nanostructure of C-S-H
J. F. Young,* R. J. Kirkpatrick* (Geology), G.-K Sun
NSF Center for Advanced Cement-Based Materials (In conjunction with the Departments of Geology and Civil and Environmental Engineering)

The environment of substituent atoms, such as Al and Na, in the structure of quasi-crystalline calcium silicate hydrate (C-S-H) is being studied by solid state NMR spectroscopy. At low Ca/Si molar ratios of C-S-H, Al is in tetrahedral coordination, but at high Ca/Si ratio octohedral coordination is also observed. The effects of concomitant substitution of both Al and Na are also being examined.

Polyelectrolyte Effects on Cement Dispersions
J. A. Lewis,* J. F. Young,* G. Kirby
W. R. Grace

This research focuses on understanding the role of polyelectrolytes (commonly referred to as superplasticizers) on the rheological behavior and dispersion of cement-based dispersions. Model cement powders, beta- and gamma-C2S, have been synthesized for this study. The influence of several acrylate-based polyelectrolytes, obtained commercially, with varying molecular weight, branch lengths, and ionizable side groups have been studied. Experimental observations will be compared to their theoretically predicted stabilization behavior to elucidate the mechanisms by which stability is imparted by this important new class of superplasticizers.

Thermal Stability of Ettringite
J. F. Young,* Y. Shimada
University of Illinois

The stability of ettringite at temperatures up to 120?C is being studied. At 70?C, the pure solid dehydrates to an amorphous form, which regains its crystallinity on rehydration. When heated in suspension ettringite remains stable in pure water, but partial transformation to the 'U-phase' occurs in the presence of sodium hydroxide.