Abstract: The long-time weathering causes significant deterioration of earthen building materials, which poses a great threat to the durability of earthen monuments. This process, however, keeps unknown under the real weathering conditions because of their complexity and large time scale. Soil micro-structure changes with the natural exposure and its evolution is an important feature characterizing the deterioration process. The loess road cuts, which were exposed to similar weathering for different years, were selected as analogy model to capture the evolution of micro-structures of undisturbed soil samples from surface and internal parts, respectively. Scanning electronic microscope (SEM) was used to observe the fabric and texture of soil samples. Mercury intrusion porosimetry (MIP) was used to measure the pore size distribution (PSD) quantitatively. The experimental results show that the micro-structure of surface soil changes much more significantly with weathering time compared to that of internal soil. The fabric of surface soil is in lamellar form and its PSDs are bimodal (i.e., inter-aggregate pores and intra-aggregate pores). The inter-aggregate pores volume accounts for a larger proportion in the surface soil and shows a decreasing trend with weathering time. Fractal analysis of pore surface indicates that building age has a tendency to decrease the irregularity and roughness of micro-pore walls in the surface soil samples, which could lead to a reduction in friction forces between soil aggregates or particles. Consequently, surface weathering, like crusts or peeling off, or other diseases would be engaged at the macro-scale.

Key words: earthen monument, micro-structure, weathering, fractal dimension, scanning electronic microscope (SEM), mercury intrusion porosimetry MIP