Rate of thixotropic rebuilding of cement pastes modified with highly purified attapulgite clays

• Purified attapulgite clays accelerate rate of thixotropic rebuilding of cements.
• Stress relaxation exhibit compressed exponential behavior, indicating jammed system.
• Accelerating effect of clays on rebuilding decreases over time as hydration governs.

This study investigates the influence of highly purified, nano-sized attapulgite clays on the rate of structural rebuilding of cement pastes. A shear rheological protocol is implemented that 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 immediately after placement. The rate of recovery for different resting times and preshear conditions are considered. The strain rate decay curves are fitted with a compressed exponential model to obtain relaxation time. The results show that the purified attapulgite clays significantly accelerate rate of recovery of pastes, especially at early ages. However, this accelerating effect diminishes at longer resting times as hydration mechanisms begin to dominate.