Effect of Dry and Wet Annealing on the Mechanical Properties of Limestones

Chemical and mechanical processes are coupled in many geological and geochemical environments. For example, reactive processes, from simple heating to replacement anneal defects and restructure grain boundaries, modifying their elastic properties, levels of internal friction, and other properties of interest such as wave propagation rates and fracture behavior. The nature of these changes is, however, contingent on the initial state of the rock. In this study, impulse excitation (IE) was used to noninvasively measure mechanical property changes with heating at 300 °C for three carbonates (Carrara marble, Carthage marble (Burlington Limestone) and Texas Cream limestone (Austin Chalk) with initial porosities about 1%, 3%, and 27%, respectively) under both dry and wet (steam) annealing environments. The IE equation for estimating shear modulus for overtone frequencies was improved based on measurements of 19 glass bars with varied lengths and widths. Young's and shear moduli of carbonates dramatically decreased during the first two hours of dry heating and the first eight hours of wet heating but changed little after that to 256 total heating hours. Mechanical property changes were qualitatively similar under dry and wet annealing conditions. Internal friction, a direct function of grain boundary motions during vibration, reflects bonding between grains and thus changes caused by annealing. When measured from fundamental frequencies in flexural and torsional modes this first increased and then decreased and was somewhat variable for the dry-heated samples. Conversely, internal friction in torsional mode increased with heating time for wet annealing. The increase could indicate mineral recrystallization under wet condition. Work is ongoing to further understand the microstructural mechanisms generating these macroscopic changes in mechanical properties.