Some perform TGA under nitrogen environment and some under air. Also, some dry their samples before performing TGA and some do the TGA without drying the powdered hardened paste samples. If we dry at elevated temperature before performing TGA, wouldn't it increase hydration?
Hi,
First, here are some references you can read:
-Galan et al. Journal of thermal analysis and calorimetry, Springer, 2012
-Villain et al. Cement and Concrete Research, 2007
Air vs Nitrogen: the problem is carbonation.
For example, if your TGA is coupled to a mass spectrometer, the CO2 in the air can modify the signal you will get for CO2 due to potential carbonation of your cement paste.
Drying or not drying: It depends on what you want to do. Do you want to quantify your hydration degree ?
People dry their sample because they want to:
-stop hydration
-stop carbonation
-get rid of free water that is not chemically bound
If you do not dry you sample, the mass loss between 25-105C will be much higher due to the free water in the pores that evaporates. If you dry your sample, it will depend on the drying technique. Oven drying (105C) is not good since chemically bound water will evaporate from Aluminate phases and CSH. You can use freeze-drying or water-solvent exchange.
You can read Mounanga et al. Cement and Concrete Research, 2004 for the quantification of the hydration degree using TGA and Zhang & Scherer. Cement and Concrete Research , 2011 in order to compare the drying methods.
Hi,
First, here are some references you can read:
-Galan et al. Journal of thermal analysis and calorimetry, Springer, 2012
-Villain et al. Cement and Concrete Research, 2007
Air vs Nitrogen: the problem is carbonation.
For example, if your TGA is coupled to a mass spectrometer, the CO2 in the air can modify the signal you will get for CO2 due to potential carbonation of your cement paste.
Drying or not drying: It depends on what you want to do. Do you want to quantify your hydration degree ?
People dry their sample because they want to:
-stop hydration
-stop carbonation
-get rid of free water that is not chemically bound
If you do not dry you sample, the mass loss between 25-105C will be much higher due to the free water in the pores that evaporates. If you dry your sample, it will depend on the drying technique. Oven drying (105C) is not good since chemically bound water will evaporate from Aluminate phases and CSH. You can use freeze-drying or water-solvent exchange.
You can read Mounanga et al. Cement and Concrete Research, 2004 for the quantification of the hydration degree using TGA and Zhang & Scherer. Cement and Concrete Research , 2011 in order to compare the drying methods.
Dear Antoine, thanks heaps for providing a very detailed answer. You have resolved most of my queries. I'll go through the papers which u have suggested.
The information the previous poster has provided is all good.
You are definitely better off drying your samples prior to analysis. This is because there is an overlap of the evaporation of free water with the loss of chemically-bound water, and it is not easy to deconvolute these when you are analysing the data.
I have tried various techniques to rapidly dry pastes prior to thermal anlaysis, such that hydration is arrested rapidly and only free water is removed. I have had some success with freeze-drying, but I would say that the best results I have obtained were simply by grinding the paste with acetone and then drying under vacuum at 45oC. As stated by the last poster, you want to avoid any higher temperatures, as ettringite starts to lose water quite a bit below 100oC. I have only encountered one issue with this techniques, which is that it seems to affect the structure of Friedel's salt (which will be present where exposure to chlorides has occurred), but it does not affect the results from thermal analysis.
I use nitrogen during analysis, but I don't think air would be a major problem with cement, as everything should be wholly oxidised anyway.
I've been using solvent exchange with isopropanol.
1. I'm grinding the sample under isopropanol
2. I next filter it with underpressure by putting the sample on the nulon filter (0.45 micrometer) and sucking the isopropanol from below the filter.
3. Next step is flushing the sample with diethyl eter to remove all possible isopropanol left.
4. I then again filter the diethyl ether from the sample.
The isopropanol-diethyl eter procedure is repeated twice. Note that during the whole procedure, the sample stays in the same glass container on the same filter, you just exchange the fluids and apply underpressure after each step.
5. I place the sample previously dried with underpressure in the oven at 40degC for about 10 min.
@Tom Dyer - the oxidation is not an issue, but carbonation surely is. This is why we use nitrogen atmosphere with cement samples.
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