Many concrete structures in aqueous environments suffer leaching, affecting their microstructure and durability. The resulting chemical and mineralogical degradation are difficult to predict over the long term and for environments of varying chemical composition, especially for severely degraded cement matrices. This is mainly because of the lack of chemical and thermodynamic data on the degraded phases formed during these attacks. In this context, this study aims to evaluate the chemical changes of leached ordinary Portland cement (OPC) paste by combining experimental (batch experiments) and modelling (thermodynamic equilibria calculations) approaches. The ground OPC paste was gradually added to an acetic acid solution. pH and chemical compositions of the solution were monitored throughout the experiment. The solid fraction was characterised over time, with particular attention paid to the phase obtained during the first additions. The latter was found to be an amorphous aluminosilicate gel (Al/Si = 0.3), with major contributions from Si Q 4 and Al IV (obtained by 29 Si and 27 Al NMR analyses respectively).