New insights into aluminosilicate gel from acetic acid attack of hydrated Portland cement: Experimental and thermodynamic characterization

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 Q4 and AlIV (obtained by 29Si and 27Al NMR analyses respectively). Existing databases (MINTEQ 3.0, Thermoddem, Cemdata) were first used in the thermodynamic simulation of the experiment using PhreeqC, which showed discrepancy with the experiment in the early stages of the attack. They were then improved by the addition of the Si-Al gel identified experimentally. The incorporation of the new phase enabled to better fit the experimental data. The calculated equilibrium constant of the gel at 20 degrees C (log Kgel = 0.37) was consistent with the equilibrium constants of similar Si-Al phases already explored in the literature.