In context of carbon capture and storage in cement and concrete industry, there is a strong demand for fast, reliable, and low‐cost CO ₂ quantification methods. Attenuated total reflection infrared spectroscopy (ATR‐IR) in conjunction with multivariate calibration via partial‐least‐squares regression was applied to quantify CaCO ₃ in carbonated hardened Portland cement pastes, as this method shows great potential in the field of process control. Thermogravimetric analysis coupled with infrared spectrometry for the detection of the evolving gases was used as a reference for quantification. Three methods for the quantitative analysis with different partial‐least‐squares parameters were developed on a series of ground physical mixtures of slightly carbonated and highly carbonated hydrated cement pastes that had absorbed up to 77% of the theoretical capacity for CO ₂ . Additional samples for optimization and validation of the method were prepared by accelerated carbonation of cylindrical slices of hardened cement paste as a function of exposure time. In these experiments, the major CO ₂ uptake occurs in the first 60 min until the formation of CaCO ₃ layers limits the diffusion of CO ₂ and Ca ²⁺ ions. The developed partial‐least‐squares models provided low estimation errors of max. 1.5 wt% and high correlation coefficients above 99.5%. The validation covers a concentration range of 20–48 wt% of CaCO ₃ . Limitations of the method are discussed.

This study provides an insight into possible recycling processes for autoclaved aerated concrete (AAC) at low temperatures (<1000 °C). Belite binders were synthesized from wastes of AAC by the addition of CaCO₃ and adaption of the molar CaO/SiO₂ (C/S ratio) in the range of 2 to 2.5. An in situ XRD study performed during heating up to 1000 °C and subsequent quenching to room temperature evidenced the formation of ternesite besides C₂S in sulfate-containing systems. Several factors influencing the reaction kinetics and the evolution of the phase composition were investigated thoroughly. Increased sulfate content and dwelling time during heating increase the ternesite content and promote the formation of highly crystalline α’H-C₂S. The C/S-ratio of the starting materials has to be adjusted to the sulfate content in order to prevent the formation of ternesite at the expense of C₂S. Ternesite remains stable during quenching to room temperature or even increases in amount, except in cases of very low cooling rates or high residual quartz contents (C/S-ratio ≤ 2). Temperature and range of α’H-C₂S to β-C₂S phase transition on quenching strongly depend on the cooling rate. The onset temperature for β-C₂S formation varies between 540 °C (slow quench) and 450 °C (fast quench). Thermal expansion coefficients of ternesite are calculated showing similarity with C₂S. The incorporation of CaSO₄ modules in the structure switches the direction of maximum compression.