Microstructural evolution of a sodium metakaolin-based geopolymer paste in neutral and CEM V basic environment

During their service life, geopolymers may be in contact with Portland cementitious materials. As their porosity is open and connected, resaturation by the cementitious pore solution is possible, and may lead to the material destabilization with durability issues. This paper investigates the evolution of a sodium geopolymer after immersion tests in a neutral and basic environment (deionized water, and CEM V pore solution) for 18 months. A chemical equilibrium takes place between the immersion solution and the geopolymer pore solution, leading to a decrease in alkalinity which may destabilize the geopolymer. A leaching of aluminum ions indicates a degradation of the geopolymer by a slow dissolution process. Apart from that, no major change in mineralogy or porosity was evidenced. However, in contact with a CEM V pore solution, part of charge-compensating cations (Na+) were substituted by potassium ions (K+), which should not impact negatively the geopolymer paste.