EFFECTS OF CARBON DIOXIDE GAS CONCENTRATION ON CARBONATION OF C-S-H

Abstract

Not only the change of pH in pore solution but also the decomposition of cement hydrates has to be considered to accurately estimate the carbonation progress. A large part of cement hydrates consists of C-S-H. Therefore the carbonation characteristics of C-S-H have to be investigated in detail. In this paper a carbonation test of C-S-H was carried out. Synthesized C-S-H and the C-S-H with calcium hydroxide powders in sodium hydroxide solution that is a simulated pore solution were carbonated using atmospheres containing 0.06, 0.5 and 5.0 vol.％ CO₂ gases. From the results of experiments, the effects of CO₂ gas concentration on carbonation of C-S-H were particularly discussed. A part of C-S-H was not decomposed especially in an atmosphere containing 0.06 vol.％ CO₂ gas, while the pH of solution decreased to the equilibrium value. It is thought that carbonation changed the composition of C-S-H to a low Ca/Si ratio. The experimental results suggested that the lower the CO₂ gas concentration of an atmosphere, the more the C-S-H including a low Ca/Si ratio was not decomposed even though the pH of the solution decreased. The decomposition of C-S-H was delayed by the existence of calcium hydroxide. It is because the carbonation rate of C-S-H was lower than that of calcium hydroxide.