Comparative study on material properties of ancient fired clay bricks of China

The city wall is a military defense building of an ancient city and a unique landscape architecture of a modern country that has extremely important historical, artistic, and scientific values. Most ancient Chinese walls were built of bricks and have been severely damaged and degraded by environmental and human influences. To protect these historical building monuments, it is necessary to determine scientifically the material properties of the bricks used in the ancient walls. In this study, the physical and mechanical properties, chemical element composition, mineral composition, micromorphology, and pore structure of 20 ancient fired clay bricks from four different regions, including white and blue wall bricks from Nanjing, blue wall bricks from Xi'an and Linqing, and the Great Wall bricks from Beijing, were compared using various modern analysis and testing methods. The results show that talc could be one of the raw materials to produce white bricks and the firing temperature of some white bricks could exceed 1200 ℃. The ancient blue bricks were mainly composed of quartz and feldspar, whereas the ancient white bricks were mainly composed of quartz and long prismatic and rod-like enstatite. The solid density and compressive strength of the white bricks were higher than that of the blue bricks. The proportion of pores larger than 10 μm in all ancient bricks is less than 10%. The pores smaller than 0.1 μm is less than 6% at higher firing temperatures. The results of this study can provide scientific guidance for the restoration and conservation of fired clay bricks of ancient city wall.