In the previous post, I introduced the protocol to measure the yield stress of cement paste. I would like to share the method how to measure the plastic viscosity of cement paste.
The principle determine the plastic viscosity of cement paste is using the relationship between shear rate and shear stress. Applying a serials of shear rate on the cement paste would result in the corresponding shear stress. After obtaining the set of shear rate and shear stress, the plastic viscosity of the paste is calculated by regression.
The measured shear rate and shear stress of a cement paste, Measuring device: HAAKE™ VT550 (ViscoTester VT550), CR (Controlled shear Rate): 0-100 1/s; t 120.00 s; T 20.00 °C.
The paste should be measured by increasing shear rate and then the decreasing rate. Typical shear rate applied on cement paste is linearly between 0 and 100 1/s. Since the rheological feature of most of cement pastes is a kind of Bingham model, we can choose the liner part of the decreasing shear curve from the date set (shear rate vs. shear stress) for the regression. The slope from the regression equation is the so-called plastic viscosity.
However, if the rheological cure of the paste behaves rather than Bingham model, selecting an appropriate model is necessary.
The rheology of cementitious material paste is complex and difficult to be investigated. One value of rheology is yield stress. The best method to measure the yield stress in laboratory is using the vane method. However, there is no much literature reporting an practical and detailed testing protocol to measure the yield stress of cement paste. Here I briefly introduce the protocol I am using at our laboratory.
To obtain stable and reliable results, we should choose the same container and the same mixing procedure. We normally use a plastic beaker with a volume 500 ml, for the beaker has enough diameter and depth for the test without the influence of boundary effect, i.e. the diameter is two times larger than the diameter of the vane. For each test, 270 g of cement is mixed with relevant water at the required water to cement ratio. For the mixing, a high-shear mixer is applied with a shear rate 800 RPM for 2 minutes.
As soon as the well mixed paste is ready, let the blades of the vane of rheometer immerse in the paste. Make sure the top surface of the paste staying between the two lines labeled on the shaft.
The principle to measure the yield stress is applying a very low fixed shear rate on the paste, e.g 0.02-0.001 1/s, then the shear stress linearly increases till yield stress. As shown in the recorded data (Fig 1), the shear stress drops remarkably when the paste is yield, thus the maximum stress is the yield stress, i.e. dynamic yield stress.
Fig. 1. Measuring device: HAAKE™ VT550 (ViscoTester VT550), CR (Controlled shear Rate): 0.01000 1/s; t 120.00 s; T 20.00 °C.
If the maximum stress does not occur within the testing period, longer time or higher shear rate may be tried. It is also worth to note that this method gives good result for paste with yield stress higher than 10 Pa.