GEOCHEMISTRY, MICROSCRUCTURE AND PHYSICAL PROPERTIES OF THERMALLY STRESSED METAMORPHIC ROCKS

Metamorphic rocks are useful in many heat-based geotechnical facilities since they are believed to be strong and durable due to their crystalline nature. However, rocks get degraded under repeated thermal stress. This work evaluates the effect of repeated heating and cooling on the geochemistry, mineralogy, microstructure and physical properties of some metamorphic rocks from the Nigerian Basement Complex. Effect of cooling condition was also investigated. Marble, granite gneiss and banded gneiss were subjected to fifty cycles of heating and cooling in air and water using heating temperature between 100 oC and 700 oC. Marble samples used in the study completely disintegrates at the 35th cycle of heating and air-cooling at 500 oC. They could not withstand more than 26 cycles of heating and air-cooling at higher temperature of 700 oC. The porosity, water absorption and degradation degree are highest in marble and least in granite gneiss. The crushing strength of both samples of granite gneiss is higher than that of banded gneiss and marble. Similar trend was observed in the residual values of these physical properties after fifty thermal cycles. Microstructural images showed micro-cracks in the thermally stressed rocks. However, no changes in mineralogy and geochemistry were recorded in the rocks. The water – cooled samples exhibited greater degradation than the air-cooled ones. Granite gneiss is more resilient than banded gneiss while both gneisses are more durable than marble. Metamorphic grade appears to play a major role in the resilience of the rocks to degradation.