Improving the fire and mechanical performance of geopolymer composite
Wijaya, Stephen Franseda
Date of Issue2016-05-12
School of Civil and Environmental Engineering
Several parameters such as Na/Na+K, Si/Al, Al/Na, H2O/Na2O and Specimen’s size were studied to determine the thermal effect on mechanical and microstructure of fly ash geopolymer(before and after 900oC). For this study, the geopolymers were made from Class F fly ash, sodium/potassium Hydroxide, and silicate solution. Sodium aluminate or silica fume was used also to control certain parameter. Geopolymer specimens were heated to 900oC to evaluate its strength loss and its cracking behaviour. A compressive strength test was conducted at ambient and after exposure to 900oC and its microstructure was studied using SEM and XRD. Alkali cation was observed to have an important role in determining the thermal properties of geopolymer. Potassium based geopolymer possessed better thermal resistance compared to sodium based geopolymer. Improvement in the strength was also observed when Na was added into K-geopolymer. The optimum ratio of these two combined alkali cations was found between 0.4-0.45. Si/Al ratio has an important role also in the strength development. Lower Si/Al(Si/Al <1.8) showed high initial strength gained and showed an increasing strength gained after 900oC due to sintering reactions of unreacted fly ash at high temperature. At Si/Al >2.3 higher compressive strength was gained and expansive behavior were observed due to high content of silica in the system. The optimum Al/Na ratio was found between 1.8-1.9. In this range high compressive strength was obtained. Increasing sodium (Na) content led to lower compressive strength gained and showed more severe cracking after exposure to 900oC. Decreasing the amount of sodium (Na) in the system also showed lower compressive strength gained and lesser cracking was formed. Similar to OPC, water plays a significant role in the development of compressive strength and thermal properties of geopolymer. Adding more water than what it required will form a porous structure and lower compressive strength will be gained. Build-up pore pressure caused by evaporated excess water will cause an internal damage to the specimen and will deteriorate mechanical properties of specimen. Two possible causes of the development of crack in the geopolymer specimen were observed, which were the thermal difference between the core and specimen surface and destruction of specimen matrix. Depending on the geopolymer formulation, either one of these two will play a major role in the development of the crack. In this experiment, destruction in the geopolymer matrix was more likely play a significant role in the development of the crack. This was support by the compressive strength result obtained and expansive behavior showed by the specimens.
Final Year Project (FYP)
Nanyang Technological University