Composition and Performance Evolution of the Ming Great Wall Ancient Mortar in Dazhuangke of Great Wall Built at Ming Dynasty

The Great Wall is one of the greatest construction projects in the history of human civilization, and the traditional lime mortar is an important guarantee standing for centuries without falling. Exploring the phase and composition ratio of ancient mortar used in the Great Wall through modern scientific and technological means and analyzing its performance evolution process, we could effectively understand the characterizations of traditional technology. The use of the past for the present is of great significance in guiding and promoting the conservation and restoration of endangered ancient buildings. Therefore, four representative locations were selected in the Dazhuangke of the Great Wall. The physical properties, phase composition and ratios of the ancient mortar samples were systematically analyzed by X-ray fluorescence spectroscopy, X-ray diffractometry, thermogravimetric-differential thermal analysis, Fourier transform infrared spectroscopy, iodine-starch test, mercury-pressure test and scanning electron microscopy, respectively. The comparison with the modern repair mortar reveals the evolution pattern of the performance of ancient mortar. The results show that the ancient mortar is a pure air-hardened lime mortar. The type of lime is a magnesian lime. No water-hardening components occur in the mortar, and no aggregate components or organic additives appear. The long-term carbonation process can inhibit the crystallinity of calcite crystalline calcium carbonate, refine the granularity of calcium carbonate, and effectively improve the denseness of the mortar hardening body. This could be one of major reasons for the superior performance of ancient mortar.