An accelerated carbonation test (at 5 % CO2, 22 A degrees C and 55 % RH) was carried out on bulk and powdered cement paste samples incorporating two different supplementary cementitious materials: microsilica and nanosilica. Extensive characterization using thermogravimetric analysis, scanning electron microscopy with X-ray microanalysis, and image analysis was performed at various degrees of carbonation, to study the fundamental effects of nanosilica on carbonation of main hydration products, calcium hydroxide (CH) and calcium silicate hydrate (C-S-H). Results at every stage were compared with the effects of microsilica addition to cement paste. Results suggested that incorporation of small amounts of micro or nanosilica can decrease the total carbonation of cement paste. In particular, nanosilica could reduce carbonation (decalcification) of C-S-H and the depth of carbonation significantly.