Thermogravimetric Study of Dehydration and Thermal Degradation of Gypsum Board at Elevated Temperatures

Abstract

Over the past few years a growing number of investigators have used parameter estimation techniques based on genetic algorithms to determine material properties for pyrolysis modeling from small-scale test data. Southwest Research Institute is exploring the use of these techniques to determine material properties needed for computer modeling to predict the behavior of structures in a fire. This paper focuses on the kinetic parameters that are needed to predict dehydration and thermal degradation of gypsum board using a set of Arrhenius equations. Specimens of a Type C board were tested in a TGA apparatus in nitrogen at four heating rates; 5, 10, 20 and 60 °C/min. A more conventional approach was used instead of a genetic algorithm to determine the parameters from the TGA data. This approach is implemented on a spreadsheet and is much easier to use than optimization techniques based on genetic algorithms. In many cases it may provide a good, albeit less versatile, alternative. Initially, reactions that occur in temperature ranges that overlap were lumped into a single reaction. At the end of the paper an attempt is made to separate overlapping reactions and estimate the kinetic parameters for each of the overlapping reactions separately.KEYWORDS: gypsum board, kinetic constants, parameter estimation, modeling, TGA. NOMENCLATURE LISTING

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“…Models 2Aa and 2Ba, that include the combustion of the organic content without limiting effect, result in the peak temperature to occur faster and at a higher magnitude than observed in the tests. On the other hand, Model 2Ab with limit 1, limit 2 and limit 3 (46,90 and 180 kW/m 3 respectively) are not capable of reproducing the peak either, because they over-estimate the temperature. However, Model 2Bb that accounts for the passage of hot air through the specimen and limits the amount of energy released is capable of capturing the peak, and the best fit is retrieved when the released energy is limited to 180 kW/m 3 .…”

“…The reactions occurring in gypsum plasterboard have been characterised using Arrhenius formulation (Eq. 12) by several authors [14,46,55,56].…”

“…10) around 200°C [11,44], depending on the heating conditions and composition of the board [45]. These endothermic reactions release water in the liquid phase and there is extra energy required to evaporate the water (Q 3 ) [14,46]. A third exothermic reaction (Eq.…”

“…11) occurring at higher temperatures is linked to the transformation of calcium sulfate from its soluble form to insoluble form (around 400°C [44,47,48]). Some authors report a fourth reaction due to the decarbonation of calcium carbonate, present in the gypsum as an impurity [13,44,46,49]. This reaction occurs between 650 and 700°C, and it is not always detected due to the low content of calcium carbonate in gypsum plasterboards.…”

“…Kukuck [55] considers two dehydration reactions and a third exothermic reaction corresponding to the transformation of CaSO 4 soluble to the insoluble form. Janssens [46] initially considers two reactions, the first one being the dehydration, and the second one being the decarbonation which occurs at 700°C. Later he divides the first reaction into two overlapping reactions.…”