Degradation and nano-scale structural evolution of geopolymers: effect of temperature, stress and mine process solutions

Abstract

Waste containment barrier systems commonly employ geosynthetics to protect human health and the environment against contaminant release. An example of a robust, composite liner system used to contain mine waste includes a textured polyethylene geomembrane (PE GMX) and a geosynthetic clay liner with two layers of polypropylene geotextiles (PP GCL). A key challenge in barrier system design is forecasting long-term performance of geosynthetic materials in various waste containment applications. A design objective in engineering practice is to predict material stability and lifespan of geomaterials used in waste containment infrastructure. With that objective in mind, this research focused on understanding the degradation mechanisms of polymers used in GMXs and GCLs (i.e., polyethylene and polypropylene), with particular emphasis on micro- and nano-scale changes to chemical and mechanical properties. A preliminary study was conducted to characterize oxidation behavior of geopolymers at room temperature. The analysis confirmed the formation of free radicals and oxidation degradation in the geopolymers. Subsequent studies on the same materials were conducted using in situ small/wide angle X-ray scattering (SAXS/WAXS) experiments with simultaneous tensile tests at elevated temperatures. These novel experiments revealed that chemical and mechanical treatments led to the degradation of the geopolymer, causing it to transition into a more crystalline state and lose its original elasticity. This understanding is vital for the effective use of geopolymers in applications like waste containment. Accelerated exposure experiments were conducted on samples of PE GMXs and PP GCLs in three different solutions (i.e., de-ionized water, bauxite mine process solution, and copper mine process solution) and at three temperatures (i.e., 20 °C, 50 °C, and 80 °C). Differential scanning calorimetry (DSC) and tensile tests were conducted on geopolymers at different durations of exposure up to 300 d. The study found that PE GMX and PP GCL degrade differently under various pH environments and temperature gradients. The degradation is influenced by factors such as the type of polymer, temperature, specific transition metals present, their concentration, and the conditions of the liquid medium. A model to predict the lifetime was developed, and the activation energies for the high and low exposure solutions were extracted.