On the Carbonation Calcium Auminate Compounds

Abstract

The carbonation of several calcium aluminate compounds, constituting hydrated cement, was investigated individually. The samples of C4AH13, C2AH8, C3AH6, C3A⋅3CaSO4⋅H32, C3A and CA, synthesized from pure chemicals, were carbonated 100% CO2 gas atmosphere (gas reaction), or by blowing 100% CO2 gas into 100% water slurry (liquid reaction). Reaction rates, the fraction of the produced calcite in CaCO3, and species and crystallinity of the cabonation products were examined by X-ray diffraction method and infra-red absorption method. The results obtained were as follows:1) Carbonation of the hydrates is influenced by crystal type of hydrates, and the reaction rate is concerned with properties of reacted layer, which is determined by species of hydrates and crystallinity of the carbonation products. The reaction proceeds rapidly with C4AH13 and C2AH8 of meta-stable type, very slowly with C3AH6 of stable type, and rapidly with C3A⋅3CaSO4⋅H32 of double salt type. The reaction products are calcium carbonate and hydatred alumina gel, and the calcium carbonate is characterized as amorphous.2) Carbonation of the anhydrous compounds proceeds very slowly as compared with the reaction of hydrates, and even slowly with the anhydrous calcium silicates. Reactivity of the anhydrous compounds in carbonation is very small in contrast to the large reactivity in hydration. The difference of reactivity of anhydrous compounds is significant in analysing mechanism of carbonation.