Engineering Mechanics Institute Conference 2013

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Rate of thixotropic rebuilding of fresh cement pastes modified with highly-purified, nano-sized attapulgite clays

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Shiho Kawashima
Columbia University
United States

Abstract:
All cement-based materials, in the fresh state, exhibit some level of thixotropy – structural breakdown under shear followed by structural recovery over time upon removal of shear. And controlling thixotropy can be an effective means to control constructability. In this study, the level of thixotropy of cement pastes modified with highly-purified, nano-sized attapulgite clays is quantitatively captured through a “breakdown-recovery” shear rheological protocol. The protocol measures the rate of rebuilding of pastes after being broken down under shear and maintained under stress corresponding to the weight of the material. This simulates a real casting situation during which the concrete is initially in motion, then cast in place and measures how quickly it gains green strength (fresh-state stiffness) immediately after placement. Results indicate that the clay-modified pastes exhibit significantly higher rate of recovery compared to the plain cement pastes, especially at early ages. And strain rate relaxation curves are found to be fitted well with a compressed exponential model, indicating “jamming” in the system.

 

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