Evaluation of two-atmosphere thermogravimetric analysis for determining the unburned carbon content in fly ash

The unburned carbon (UC) content of fly ash impacts the performance (e.g., air entrainment, rheology) of concrete mixtures. The loss on ignition (LOI) test is commonly used to estimate the UC content; however, this may be inaccurate, as the weight change upon igniting fly ash is also due to calcination of carbonates, desorption of bound water, and oxidation of sulfur and iron minerals. In this study, a two-atmosphere thermogravimetric analysis (2A-TGA) coupled with mass spectrometry (MS) was performed to evaluate the chemical reactions that occur upon heating of fly ash and to measure the true UC content. 2A-TGA was performed under two distinct atmospheres: (i) in non-oxidizing helium gas, to measure weight loss due to decomposition of carbonates and loss of bound water and (ii) in oxidizing air, to measure weight loss due to conversion of UC to carbon dioxide (CO₂). The method was applied to five class F fly ashes with LOI in the range 1.89 % to 7.66 %, a class C fly ash (LOI = 3.86 %), and a fluidized bed combustion (FBC) ash (LOI = 8.10 %). The results were also compared with the total carbon (TC) of each fly ash measured using infrared (IR) spectroscopy via a commercial Laboratory Equipment Corporation (LECO) carbon analyzer. The results show that there is no one-to-one relationship between the LOI and the TC or UC contents of fly ash. LOI overestimated TC by up to 2.5 times and overestimated UC by up to 6.4 times.