Clinker mineral formation and thermal decomposition of calcium carbonates in carbonated tobermorites: Mechanism of CO2 release in low-temperature ranges

Thermalgravimetric analysis (TGA) has been widely used to quantify the amount of fixed CO₂ in cementitious materials. It was well-known that calcium carbonates in cementitious materials decomposed not only around 700–800 °C but also in lower-temperature ranges (around 300–600 °C). However, the mechanism of CO₂ release in low-temperature ranges was not clarified. This study investigated the semi-dry carbonation of synthesized tobermorite under different relative humidities and a 1.0 % CO₂ concentration to reveal the mechanism of CO₂ release in low-temperature ranges. TGA revealed that the decomposition of calcium carbonates occurred at three different temperature ranges. The peak around 700–800 °C was attributed to the decomposition of calcite transformed from vaterite and aragonite. The peak around 400–600 °C was attributed to the solid-state reaction of vaterite and aragonite with silica gel, resulting in CO₂ release. The peak around 300–400 °C was attributed to the decomposition of amorphous carbonate minerals.