EFFECT OF RELATIVE HUMIDITY ON CARBONATION SHRINKAGE AND HYDRATION PRODUCT OF CEMENT PASTE

<p>It is important to assess the durability of concrete structures due to their long-term service. Carbonation, one of the dominant forms of deterioration of concrete structures, is influenced by various conditions, such as concrete materials, mixture, temperature and relative humidity （RH）. The effect of humidity on carbonation is well known；carbonation rate and carbonation shrinkage are most effective under intermediate humidity. The effect of carbonation on concrete involve the dissolution of hydration products, mainly portlandite and calcium silicate hydrate （C-S-H）, and the precipitation of CaCO₃ in pore solution. However, the effects of RH on the carbonation reaction of hydration products have not been clarified. <br/>The purpose of this study was to investigate the effect of RH on the carbonation of hydration products. Ordinary Portland cement-paste samples were exposed to accelerated carbonation under 3％-CO₂ concentration at different RH conditions （RH85％, RH66％, RH43％ and RH11％） and the temperature at 20℃. Thermogravimetric, differential-thermal and X-Ray-diffraction-Rietveld analysis were used to quantify the amount of portlandite and CaCO₃ polymorphs. Change in the carbonation shrinkage over time under different RH conditions, was also measured. Experimental results show that calcite was the dominant mineral among the CaCO₃ polymorphs under the condition of RH66％, whereas aragonite and vaterite were mainly present CaCO₃ polymorphs in carbonated cement paste under RH43％. Change in the carbonation shrinkage over time differed by RH conditions, although the amount of CaCO₃ was the same. There were major changes in carbonation rate of C-S-H and portlandite under different RHs. Results indicate that carbonation level of C-S-H under RH43％ is higher than that under RH66％ and RH85％. Thermodynamic modeling was performed to demonstrate how carbonation shrinkage is affected by carbonation behavior of C-S-H under different RH conditions.</p>