Macro-defect-free (MDF) cement pastes are being studied with the following objectives: (1) optimizing processing parameters, (2) improving resistance to moisture, and (3) developing alternative cement-polymer composites. Flexural strengths up to 300 MPa are achieved in the dry state, with a 0-20% decrease in strength occurring after immersion in water.

Evolution of microstructure and microchemistry as a function of processing parameters is being investigated using electron microscopy. It is shown that the processing history has a primary effect on the microstructure and microchemistry of the composite. In particular, the chemistry of the interphase that formed as a result of mechanochemical interactions between the highly sheared polymer and hydrating calcium aluminate was a strong function of the shear-mixing parameters. TEM analysis coupled with analytical microscopy studies showed that the mechanical properties of the composite were dependent on the fine microstructure and microchemistry of the interphase as well as the reacted polymer matrix phase.

This research focuses on synthesis and processing of novel organocement composites based on Al₂O₃-CaAl₂O₄ microcomposite powders, which contain inert ceramic cores and reactive cement-based coatings. We have studied the chemical and morphological evolution of these powders by NMR, DRIFTS, XRD, and SEM/TEM analyses. The final, as-synthesized powder consists of Al₂O₃ cores (about 5 micro-m diameter) with a thin CaAl₂O₄ coating (about 25 nm thick). Currently, we are fabricating novel organocement composites from these powders and investigating their microstructural development and properties.

This project investigates the development of new materials we have termed ``organoceramics,'' which are based on the concept of inorganic crystal growth in the presence of polymers and/or monomers. The products of interest are inorganic-organic intercalated structures in which poly mers and/or monomers may participate in setting reactions. We have been able to synthesize a number of organo calcium aluminate structures containing poly(alcohols) and cationic polyelectrolytes. Interestingly, we find that the intercalated polymers can modulate the crystal structure, crystal habit, and chemical composition of these cementitious materials.

Studies of the electric-field dependence of the shape of cement-based materials are being made using interferometric and strain-gauge techniques. These investigations have already revealed the presence of large unexpected electrically induced strains. Studies as a function of composition, pore parameters, water content, and ion content are being systematically performed. It is hoped that these investigations will reveal for the first time the nature of the complex nonlinear electrical, electromechanical, and mechanical properties of cement materials.

Structural studies of the calcium silicate hydrate gel phase of ordinary portland cement are being performed by transmission electron microscopy, high-resolution transmission electron microscopy, and micromechanical methods. These investigations have already revealed the presence of a mesostructure in calcium silicate hydrate gels for the first time. The nature of this mesostructure is currently being studied as a function of composition, heat treatment, and moisture history. It is hoped that these investigations might lead to a more comprehensive understanding of the complex structure property relationships in cement-based materials.

Strong, dense materials are formed by casting mixtures of calcium silicate (portland) cement and silica fume at very low water contents. The chemistry of the cementitious reactions and development of microstructure are being studied using small-angle x-ray scattering, trimethylsilylation, and thermal analysis. Improvements in processing and particle packing are also under investigation.