Ex-situ mineral carbonation via direct aqueous process at a low CO2 partial pressure (pCO(2)) is a motivating factor for promoting industrial application. The carbonation extent meets a plateau around CO2 supercritical pressure, making carbonation at the lower part of the plateau attractive in an energy saving prospective. This paper investigated the mechanism of olivine carbonation using a buffer solution with NaHCO3 and NaCL, at 185 degrees C and 6.5 MPa of pCO(2). The results obtained show that when the dissolution of magnesium silicate is effective in the solution, carbonation at relative low pCO(2) is preferred due to the slight enhancement of aqueous CO32- concentration and pH in the solution. The optimum NaHCO3 concentration is not 0.64 mol/L at 6.5 MPa of pCO(2), and further addition of sodium salts (NaCI and NaHCO3) is necessary to enhance the carbonation extent. The optimum NaHCO3 concentration at low pCO(2) could be reduced through increasing the carbonation time and reducing the particle size. The passivation layer was not formed under the experimental condition. Agitation is necessary to be optimized to prevent settlement of solids in the solution. The successful sequestration of CO2 using olivine at a pCO(2) of 6.5 MPa can aid in reducing energy requirements. (C) 2019 Elsevier Ltd. All rights reserved.