In order to gain further insight into changes in the properties and microstructures of granite due to temperature changes, two series of granite samples with different granularities from Aji, Takamatsu City, Kagawa Prefecture, Japan, were subjected to a one-cycle heating and cooling test with maximum temperatures ranging from 200 to 600℃ at heating and cooling rates of 50℃/h. Properties reflecting the internal microstructures, such as the size, weight, effective porosity, and P-wave velocity, were measured before and after the thermal cycle, and microstructures were visualized and observed using the fluorescent approach. The development of microcracks induced by the thermal test was then investigated with an image analysis technique. No significant differences in the changes in size and weight were identified within samples heated to 500℃, but a marked change was identified in the samples heated to 600℃. Increases in the effective porosity and decreases in the P-wave velocity were clearly identified. The effective porosity changed markedly from 500 to 600℃, while the P-wave velocity changed gradually to 600℃. These suggest that changes in the inner structure occur within samples. Microcracks developed in the tested samples, and this tendency was marked in samples treated at higher temperatures. These microscopic observations agree well with the observed changes in properties. Furthermore, a difference in the crack growth pattern between the two series of granite was observed. In the fine-grained sample, grain boundary cracks strongly developed between 500-600℃, while, in the medium-grained one, this tendency was not identified. It was inferred that the phase transition of quartz at 573℃ markedly influenced crack development. On the other hand, no marked differences due to the granularities were identified from 200 to 500℃.
