The carbonation of portlandite , calcium silicate hydrate (C-S-H), and ettringite was investigated at 57% RH and 91% RH using X-ray diffraction, thermogravimetric analysis , infrared spectroscopy , and the phenolphthalein spray test. The experiments show that the carbonation of portlandite, ettringite , and C-S-H with Ca/Si = 0.7 is significantly faster at 91% RH than at 57% RH. Little effect of RH is observed for C-S-H with higher Ca/Si. Portlandite and C-S-H with Ca/Si = 0.7 carbonate only partially at 57% RH; complete carbonation is observed if the relative humidity is increased to 91% RH. In contrast, the carbonation of C-S-H with Ca/Si = 1.2 and 1.5 is complete at both relative humidities. The carbonation rate of C-S-H decreases with decreasing Ca/Si ratio, both at 57% and 91%RH. Carbonation at 57% RH promotes the formation of vaterite and aragonite over calcite ; the precipitation of amorphous calcium carbonate is observed for C-S-H with Ca/Si = 0.7.