Calcareous sands are consisted of marine biogenic granular materials with carbonate content often greater than 50%. From the microscopic viewpoint

calcareous sand grains demonstrate highly angular and irregular in shapes and contain intra-pores. This unique intra-pore structure of calcareous sand particle is prone to crush under stress. Therefore

the calcareous sand has a relatively low strength. The intra-pore structure has a significant effect on the geotechnical properties of calcareous sands. Calcareous sand samples from the lagoon of Yongshu Reef atoll

Nansha Islands

were cut by femtosecond laser for the optical microscopic studies. The intra-pore structures were quantitatively analyzed throughout the MATLAB digital image process tool. The results indicate that

larger porosity can be found on the breakage section of larger size calcareous sand and a larger number of small pores with small total volume are in the intra-pore structure. For sand grains with a size greater than one millimeter

the number of slit shape intra-pores is far less than the number of intra-voids with equiaxed or unequal axed pores. The amount of the equiaxed and unequal axed pores are approximately the same.