Factors Influencing the Long-Term Properties of Fly Ash-Based Geopolymer Mortar

The primary goal of the paper was to study the effects of key factors on the long-term mechanical properties of fly ash-based geopolymer. The factors considered consisted of curing time, humidity, temperature, admixture, and water coefficient. In the study, the geopolymer paste and mortar specimens were prepared and cured at four different environmental conditions: (1) ∼24°C and approximately 45 % RH; (2) 95°C and 100 %R.H; (3) 80°C and 25 %RH; and (4) ∼24°C and 90 %R.H. The compressive and flexural strength tests were performed for the mortar specimens at the ages of 7, 28, 120, 360, 540, and 720 days. X-ray diffraction (XRD), X-ray fluorescence (XRF), and nuclear magnetic resonance spectroscopy (²³Na, ²⁷Al, ²⁹Si, MAS NMR) tests were performed for the paste specimens at the ages of 7, 28, 120, 360, 540, and 720 days. Selected specimens were also examined under scanning electron microscope (SEM). The results indicated that water evaporation from specimens was a major cause of the strength development of the geopolymer. In addition to SiO₄⁻⁴ anions in alkali activator, which accelerated the geopolymerization, the Na⁺, Ca²⁺, and Mg²⁺ ions in the system also played an important role in bridging -Si-O-Si-O-Al- chain, thus improving strength development of the geopolymer. The crystallization occurred only in the hydrothermal environment, and it had little influence on the strength development of the low calcium fly ash-based geopolymer mortar as a very small amount of crystalline phase was formed.