Carbonation in cement binders has to be thoroughly understood because it affects phase
assemblage, binder microstructure and durability performance of concretes. This is still not the case as
the reaction products can be crystalline, nanocrystalline and amorphous. The characterisation of the
last two types of components are quite challenging. Here, carbonation reactions have been studied in
alite-, belite- and ye’elimite-containing pastes, in controlled conditions (3% CO2 and RH = 65%). Pair
distribution function (PDF) jointly with Rietveld and thermal analyses have been applied to prove
that ettringite decomposed to yield crystalline aragonite, bassanite and nano-gibbsite without any
formation of amorphous calcium carbonate. The particle size of gibbsite under these conditions was
found to be larger (~5 nm) than that coming from the direct hydration of ye’elimite with anhydrite
(~3 nm). Moreover, the carbonation of mixtures of C-S-H gel and portlandite, from alite and belite
hydration, led to the formation of the three crystalline CaCO3 polymorphs (calcite, aragonite and
vaterite), amorphous silica gel and amorphous calcium carbonate. In addition to their PDF profiles,
the thermal analyses traces are thoroughly analysed and discussed.
