Microstructure-Property Relationships in Macro-Defect-Free Cement
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polymeric phase serves many functions: (1) it acts as a rheological aid facilitating particle packing, (2) it fills the pore space between cement particles, and (3) it reacts chemically with cement hydration products to form an integral microstructural element.4'710 During the high-shear mixing step (refer to Figure 1), the water plasticizes the polymer, giving the paste a workable
MI>K Cement Processing Methodology
I'olVITKT/PlilStk-izi
Premixing (low sheiirl
High Shear Mixing (twin roll mill)
Calendaring (to form sheets)
Drying/Curing
(sirci
MDF Cement
Figure 1. Process flow diagram for MDF cement fabrication.
viscosity. The temperature must be controlled during mixing to prevent the paste from stiffening due to premature setting (or hydration) reactions. On complete mixing, the volume fraction of cement particles in the paste approaches 0.70. This high-volume loading is obtainable due to the broad particle size distribution (e.g., 0.3-70 /xm) of the cement powder. In subsequent pressure curing and dry curing steps, water reacts with the cement particles to form hydration products.31" The chemical composition and morphology of these products will be discussed in detail in the next section. MDF cement-based composites have many potential applications (e.g., substrates for electronic packaging, structural, or acoustic-damping materials), since they may be filled with secondary phases, metallized, coated, or even stacked and laminated to incorporate fiber-reinforcing layers or other materials between MDF sheets.7 However, their use is currently limited to low-humidity environments. The hydrophilic nature of the polymeric constituent in MDF cement causes these materials to be highly moisture sensitive. Moisture absorption has a negative impact on most of their performance properties (e.g., flexural strength, dielectric constant, dimensional stability).""15 However, with a thorough understanding of microstructureproperty relationships in MDF cement, one can envision both an emergence of commercial applications as well as the development of a generic class of MDF materials based on alternative inorganic/ polymer systems. In this article, we present an overview of our current understanding of microstructure-property relationships in the calcium aluminate/PVA system. We begin by highlighting recent microstructural studies that include both the microstructural analysis and microhydration behavior of MDF cement composites. We then discuss the organic phase composition and its percolative nature within the microstructure. Finally, we relate these microstructural observations to their influence on the bulk properties of MDF cement, and discuss their implications for the further development of MDF materials systems. Microstructural Analysis and Microhydration Behavior The significant improvement in strength and toughness of MDF cements relative to conventional cement pastes is attributed not only to their low porosity, but also to the chemical reactions that occur between the PVA polymer and the inorganic ions p
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