The normal practice of adding fine silica to portland cement grouts for geothermal wells to overcome strength regression due to the increased temperature has been shown to result in a greater rate of carbonation of the hardened grout. Carbonation is controlled not by permeability but by calcium hydroxide content at 150°C. It is concluded that the optimum silica addition for geothermal fluids which contain high levels of CO2, is about 15–20% to obtain the longest life, although actual lifetimes will have to be determined by downhole exposure.