Title: How Admixtures Affect Yield Stresses of Cement
Author(s): Chandrasekhar Bhojaraju, Malo Charrier, and Claudiane M. Ouellet-Plamondon
Publication: Materials Journal
Appears on pages(s): 311-324
Keywords: admixtures; calcium silicate hydrate (CSH)-seed; creep recovery method; thixotropy; yield stress
A factorial experimental design plan was used to compare admixtures (high-range water-reducing admixture, viscosity-modifying agent, accelerator, calcium silicate hydrate [CSH] seeds, and nanoclay) in a cement and silica fume blend. Two methods were tested to measure the structuration rate: the constant velocity method and the creep recovery method. The measurements were performed with a rotational rheometer with a double-helical spiral geometry to reduce slippage. The evolution of yield stress and thixotropy of the mixtures at four resting times was evaluated, providing insight into the stress that the recently printed structure can withstand. The creep recovery method generally provides a higher static yield stress than the constant velocity method, except for the stronger mixtures, raising additional questions on the effect of the paste history on microstructural buildup mechanisms. When the extruder begins, shear is applied and the microstructure is broken, causing the dynamic yield stress to be lower than the static yield stress. The effect of the admixtures on thixotropy is discussed.