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Use Vaseline to cover the surface of alkali activated cement paste during setting test

Setting of cement paste is an important property to be measured. So how to measure it? The most common method is using Vicat needle instrument. The European Standards and ASTM both document the detailed method. One of the deferences of the two methods lies on the treatment of the surface of sample.

The “Methods of testing cement — Part 3: Determination of setting times and soundness” says “Place the filled mould and base-plate in the container, add water so that the surface of the paste is submerged to a depth of at least 5 mm, and store in the temperature controlled enclosure at (20,0 ± 1,0) °C”.

While the ASTM “C191 − 13 Standard Test Methods for Time of Setting of Hydraulic Cement by Vicat Needle” shows that “Immediately after molding, place the test specimen in the moist cabinet or moist room and allow it to remain there except when penetration measurements are being made.”

We could see the point here is trying to keep the constant water to cement ratio of the surface of the sample, otherwise the dried surface would make trouble for the penetrating of the needle and give fake setting time point.

However, it is difficult to apply European Standards to measure the setting of alkali activated cement paste. As the alkali ions dissolve into the water and significantly change the chemistry of the surface layer. If there is no moist curing cabinet, how can we test the setting of alkali activated cement paste?

vaseline-cement

Here we share the trick that we have been using at our lab with you. Use Vaseline (petroleum jelly)! After pouring the paste into the mould, cover the surface with a thin layer of melting Vaseline (about 100 °C), the Vaseline is then immediately cooled into a layer of gel.

Why choose Vaseline instead of water?

  • The Vaseline is easy to be melted and cover the surface and soon returns to soft gel status.
  • The soft gel functions as the water layer preventing the surface losing water due to drying.
  • What is more, the Vaseline does not have chemical reaction with alkali activated cement, nor do the alkali ions dissolve into Vaseline.
  • And the price of Vaseline is cheap.

So ladies use Vaseline for makeup, we use it for cement research.

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The method to measure the plastic viscosity of cement paste

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.

plastic-viscosity-cement-paste
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.

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The right protocol to measure yield stress of cement paste

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.

rheology-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.