Experimental and Numerical Investigation of Sodium- and Potassium-Based Alkali Activator on the Mechanical Properties of Geopolymer-Mortars Using Lebanese Kaolin

Abstract

This paper aims to study the use of Lebanese raw kaolin in the formulation of geopolymers (GP) mortars based on an experimental and numerical approach. Two classes of GP mortars were prepared: the first contained sodium (Na)-based alkali activator, while the second was prepared by adding potassium (K)-based activators. A microstructural characterization was realized before and after the calcination of the raw material. Mechanical characterization of the mortars was conducted by highlighting the impact of the two alkaline solutions used, on tests performed on days 1, 3, 7, 28, 90, and 120. Simultaneously, a three-phased heterogeneous numerical modeling (matrix, aggregates, pores) was used to obtain specific characteristics not accessible experimentally. As main results, the experimental work showed that the binding solutions’ nature and samples’ aging affected the microstructure and, consequently, the mechanical properties. K-based geopolymer mortar presents higher values of mechanical strength than Na-based geopolymer mortar. Moreover, an asymptotic behavior is observed in mechanical characteristics at about 90 days. The numerical results confirmed that micro-defects and porous network have an important impact on Young’s modulus values as well as the presence of the sand phase. The mechanical characteristics of pure binder samples were also obtained numerically. These samples contain natural defects and pores leading it impossible to obtain experimentally the desired characteristics. This work and the associated results confirm that Lebanese raw material can be considered as a sustainable construction material solution for Middle Eastern countries.