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?
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.