| Abstract [eng] |
Carbonation of cement hydrates is one of the crucial aspects causing the deterioration of concrete structures [1]. It is known that ambient CO2 dissolves in the alkaline pore solution of concrete and interacts with the cementitious products causing deterioration of hydrates and formation of calcium carbonate (CaCO3). This process is influenced by various environmental factors such as carbon dioxide (CO2) concentration, relative humidity, temperature, etc. [2]. Despite numerous studies, the opinion of scientists differs on the carbonation mechanism, kinetics, and influence of external factors to date. One of the possible pathways to study carbonisation is the investigation of the behaviour of individual hydrates under different conditions [3]. Thus, this work aims to examine the influence of varying water vapour pressure (p/p0) on the carbonation mechanism of cementitious hydrate, namely calcium monosulfoaluminate 12-hydrate (Ms12). For the study, synthetic Ms12 was produced using a mixture of analytical grade materials (CaO, Al2O3, CaSO4·2H2O), referring to ye`elimite (Ca4Al6O12(SO4)) stoichiometry. The synthesis approach consisted of the mechanochemical treatment (dry grinding at 900 rpm in a vibrating cup mill with 3 on-off cycles of 10 min) followed by the hydrothermal synthesis (for 8 h at 130 °C). After hydrothermal treatment, the suspension was decanted, rinsed with acetone, filtered, and dried in an oven at 50 °C ± 0.2 °C for 24 h. Meanwhile, for carbonation examination, prepared samples were placed in 5 desiccators with different relative water vapour pressures (p/p0 varied from 0.355 to 1) in the presence of the room CO2 concentration (668 ± 3 ppm). The experiment was carried out at a temperature of 21 °C ± 0.5 °C for up to 90 days. The results indicate that the water vapour pressure conditions are an essential factor that influences the stability and carbonation rate of Ms12. It is determined that Ms12 interacting with CO2 at p/p0 of 0.355-0.565 gradually disintegrates into calcium carbonate, aluminium hydroxide, and calcium hemihydrate. Meanwhile, gypsum is identified at a higher p/p0 (>0.565) instead of calcium hemihydrate. Additionally, above 0.565 p/p0, Ms12 becomes metastable and gradually recrystallises into ettringite. Referring to the rate of carbonation, it is seen that an increase in the water vapour pressure leads to accelerated carbonation of the sample (Fig. 1.). Thermodynamic and apparent kinetic parameter calculations also were applied to verify the obtained experimental data and the theoretical hypothesis. The phase changes in the samples were confirmed by X-ray diffraction analysis and simultaneous thermal analysis methods. [...]. |
