Research on microstructure and porosity calculation of rammed soil modified by lime-based materials: the case of rammed earth of pingyao city wall, a world heritage site

Lime-based amendment materials, typically consisting of lime and hydraulic lime, are commonly used in the restoration of rammed earth heritage sites. To compare the modification characteristics of these two types of lime-based materials and to further elucidate the repair mechanisms of hydraulic lime-modified materials, this study utilized the rammed earth ramparts of Pingyao—an ancient city and World Heritage Site. Microstructural scans were conducted on rammed earth treated with both lime and hydraulic lime. The study comparatively analyzed the changes in the pore structure of the soil after treatment with these two lime-based materials. Using Image-Pro Plus (IPP) 6.0 image analysis software, the study obtained the change curve of two-dimensional porosity by adjusting various thresholds. Based on principles of calculus, a three-dimensional porosity calculation method was employed to establish the change curve of three-dimensional porosity relative to the threshold value. Finally, the study performed a comparative analysis of the porosity of the rammed soils modified with the two lime-based materials, integrating X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS) patterns. The modification mechanisms of the lime-based materials were analyzed. The results indicate that the three-dimensional porosity calculation formula applied in this study is applicable for the porosity calculation of rammed soils modified with lime-based materials. The calculation error of the three-dimensional porosity of rammed soils modified with lime is greater than that of rammed soils modified with hydraulic lime. Moreover, due to differences in the curing mechanisms of the two types of lime-based materials, the two-dimensional and three-dimensional porosity change curves of rammed soils modified with hydraulic lime are less steep compared to those of rammed soils modified with lime.