Comparison and reliability assessment of CO2 quantification methods for carbonated cementitious materials

Abstract

Various CO₂-absorbing concretes have been developed; however, an accurate assessment of their CO₂ content is critically important. CO₂ has been quantified by releasing it from concrete and measuring the sample mass loss and/or the released CO₂; however, the differences between these methods remain unexplored. In this study, cementitious materials were carbonated and evaluated using five analysis methods. The presence of free water in the samples and moisture adsorption significantly affected the sample mass. Therefore, all the samples were ground and dried by heating in vacuum before analysis. Heating above 980 °C or immersion in HCl was effective for CO₂ release, and non-dispersive infrared spectroscopy and coulometric titration were effective for quantifying released CO₂, with measurement variance within ± 5 %. However, CO₂ release using H₃PO₄ was insufficient, leading to an underestimation of up to 12 %. The combination of temperature-programmed desorption and non-dispersive infrared spectroscopy had the best reproducibility among the five methods and clearly showed that CO₂ and H₂O were released simultaneously at the moderate temperature range. This overlap causes difficulties in the popular thermogravimetric analysis which attributes the mass change below/above a threshold temperature to H₂O/CO₂, respectively. CO₂ contents were underestimated due to the unaccounted CO₂-derived mass loss for CO₂-rich samples and/or high threshold temperatures while overestimated due to misincluded H₂O-derived mass loss for H₂O-rich samples and/or low threshold temperatures. This study presents a highly reliable approach for assessing CO₂ fixation, which is essential for calculating the carbon footprints of various cementitious materials and concrete.