Processing and microstructure of standard and modified macro-defect-free cements

Abstract

Evolution of macro-defect-free microstructure and microchemistry was studied by TEM/EDS and SEM/EDS. MDF samples were prepared through an alternative, Banbury mixing route. Experiments with varying mixing time at constant mixing rate showed that an intimate distribution of constituents in the composite was assured at the early stages in the shear mixing step. With increasing mixing time, mechanically induced chemical reactions triggered an interplay of network formation and simultaneous network degradation.Analytical electron microscopy studies at the TEM level indicated that there was a progression of microchemistry of the interphase zone with mixing time. At early stages of processing, the interphase was richer in Al ions than the contiguous cement grain. With increasing mixing time, the interphase chemistry approached equilibrium and the composition of the adjacent cement grain.Experiments with varying mixing rates showed that the time constants associated with the window of processibility were strongly affected by the mixing activity. A quiescently prepared calcium aluminate polyvinyl alcohol couple did not form an interphase layer between the polymer and the cement grains. It was deduced that the microstructure and microchemistry of MDF cements were a direct result of mechano-chemical interactions between the polymer and the hydrating calcium aluminate cements.Different amounts of a titanate chelate cross coupling agent were added to MDF paste during shear mixing. A titanate chelate modifier was shown to cross link polyvinyl alcohol at room temperature. XPS studies established that a C\rm\sb{PVA}-O-Ti-O-C\rm\sb{PVA} type bonding was responsible for the cross linking.These cross linking reactions further reduced the time constants of processing with increased amounts of the cross coupling agent. Analytical electron microscopy studies by TEM and STEM revealed that the modifier did not alter the MDF microchemistry significantly.Water durability tests showed that upon prolonged immersion in still water, composites were leached and degraded. The cross coupling agent could not eliminate the moisture sensitivity problem of HAC, MDF cement.U of I OnlyETDs are only available to UIUC Users without author permissio