Effect of diagenetic variation on the static and dynamic mechanical behavior of coral reef limestone

Coral reef limestone at different depositional depths and facies differ remarkably on the textural and mineralogical characteristics, owing to the complex sedimentary diagenesis. To explore the effects of pore structure and mineral composition associated with diagenetic variation on the mechanical behavior of reef limestone, a series of quasi-static and dynamic compression tests along with microscopic examinations were performed on the reef limestone at shallow and deep burial depths. It is revealed that the shallow reef limestone (SRL) is classified as a porous aragonite-type carbonate rock with high porosity (55.3±3.2)% and pore connectivity. In comparison, the deep reef limestone (DRL) is mainly composed of dense calcite-type calcium carbonate with low porosity (4.9±1.6)% and pore connectivity. The DRL strengthened and stiffened by the tight grain framework consistently displays much higher values of the dynamic compressive strength, elastic modulus, brittleness index, and specific energy absorption than those of the SRL. The gap between two types of limestone further increases with an increase in strain rate. It appears that the failure pattern of SRL is dominated by the inherent defects like weak bonding interfaces and growth lines, revealed by the intricate fracturing network and mixed failure. Likewise, although the preexisting megapores in DRL may affect the crack propagation on pore tips to a certain distance, it hardly alters the axial splitting failure of DRL under impacts. The stress wave propagation and attenuation in SRL is primarily controlled by the reflection and diffusion caused by plenty mesopores, as well as an energy dissipation in layer-wise pore collapse and adjacent grain crushing, while the stress wave in DRL is highly hinged on the insulation and diffraction induced by the isolated megapores. This process is accompanied by the energy dissipation behavior of inelastic deformation resulted from the pore-emanated microcracking.