Advanced Cement Based Materials最新文献

Relationships among mixing conditions, extensional viscosities of the formed paste, and the morphology and flexural strength of hardened composites have been studied for a calcium aluminate cement-phenol resin composite. Mixing torque was monitored as a function of temperature and resin pH. The behavior of the torque time curves as well as ¹³C nuclear magnetic resonance and differential scanning calorimetry measurements suggest that paste formation is characterized by an induction period of flat torque, corresponding to dissolution of cement ions followed by a styffening period of rapid torque rise, corresponding to an ionic interaction between the resin and cement particles. The viscoelastic nature of the paste was studied using a biaxial squeeze flow device. The steady biaxial extensional viscosity trends with extension rate are consistent with the formation of an increasingly cross-linked bulk organic phase in the paste. Flexural strengths of hardened material processed under varying degrees of resin pH suggest that an optimal structure forms when the resin is allowed to simultaneously polymerize and ionically interact with cement particles. This conclusion is supported by evidence of scanning electron microscopy, which shows structure formation for a given cut of missing conditions.

Diffusion of ethanol into water-saturated white cement pastes has been investigated by carbon and proton nuclear magnetic resonance (NMR). The diffusion of ethanol was shown to be Fickian, assuming one-dimensional diffusion under perfect sink boundary conditions. Derived diffusion coefficients were found to increase with increasing water/cement (w/c) ratio from (2.7 ± 0.5) 10⁻⁸ cm²/s at w/c = 0.30 to (59 ± 5) 10⁻⁸ cm²/s at w/c = 1.0. At the end of the exchange process, only a fraction of the total volume of water is exchanged with ethanol, varying from 60% for samples containing mainly micro- and mesopores to about 80% for samples where additional capillary pores are present. Time needed to reach 90% and 95% exchange of the total intrudable amount of ethanol in cylindrical samples with diameter of 5.5 mm varied from 1 day to nearly 3 weeks. This has importance for exchange in larger samples with typical diameters of 10 mm or more (as used in mercury intrusion porosimetry), which may require on the order of months for 90% exchange to take place. The mole fraction of ethanol and water in the pore system was determined from sampled carbon and proton NMR spectra vs. exchange time by comparing H₂O-saturated and D₂O-saturated samples. At the end of the exchange process, water was found to occupy the remaining volume not accessible to ethanol. In the tested w/c ratio range, the water content in all samples is below the value where damage to the pore structure normally occurs due to internal tension when exposed to drying. An empirical relationship between chemical shift of the CH₃CH₂OH/H₂O peak and mole fraction of ethanol is derived, enabling the mole fraction of ethanol from the NMR peak to be estimated.

The effects on the microstructural development of adding silica fume to cements and concretes during cement hydration have been studied using small-angle neutron scattering and ultrasmall-angle X-ray scattering. A previously developed fractal based microstructural model has been applied to extract representative microstructural parameters from the small-angle scattering data. A link has been established between the existence of coarse or agglomerated particles in the silica fume particle size distribution and possible deleterious microstructural evolution during cement hydration.