When RHA/POFA used as a pozzolan in mortar/concrete, it will react with the hydration product calcium hydroxide. How can we measure the residual calcium hydroxide after reacting with RHA/POFA?
For cement pastes and perhaps mortars, you can use XRD, TGA or a combination of both techniques to check whether they give the same results. SEM is not as good, larger CH crystals would be readily segmented but you would miss the smaller CH.
For concrete, none of these methods works, because of the large amount of aggregates in the mixture (the signal to noise ratio would be too high and you will also have problems of sample size and homogeneity of samples).
In pastes, to check how much CH is consumed and when, we are comparing the residual amount of CH in mixtures with pozzolans to the residual amounts in mixtures with non-reactive quartz filler. You need to have the same amount by volume of pozzolan and filler in the mixture. This approach is useful for distinguishing the pozzolanic reaction from the filler effect, which is also influencing cement hydration.
Dear A. B. M Amrul Kaish,
The most proper methods to measure residual amount of portlandite (calcium hydroxide) in cementpaste are the thermogravimetric (TG), or the combibed (simultaneous) thermogravimetric (TG) and differential thermoanalytic (DTA) methods. TG measures the change in mass (of your sample). In most technics the first derivative of the TG is also obtained by analogue or computed way (DTG, derivative thermoanalytic data). Of course, you need to have a thermobalance (TG/DTG).
Using the TG/DTG measurement you are able to measure the change in mass due to the dehydration of Ca(OH)2. The peak is about 450oC. DTA curve indicates simultaneous endothermic reaction. In case of measurement without DTA you should check the presence of portlandite (qualitive analysis) by X-ray powder diffraction (XRD).
In case of mortar or concrete the most important step is the sample preparation to have average sample as well as to avoid the sample from the long contact with air. Portlandite in the powder sample easily reacts with the airborn CO2. In case of no additional CaCO3 in the sample (some cements contain ground limestone or limestone powder is used during mixing) the amount of calcium hydroxide calculted from the dehydration reaction should increase with the amount calculated by the decomposition of CaCO3.
In case of any other question I am at your service.
Regarding
Katalin Kopecsko
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