Study on Firing Temperature and Material Properties of Ancient White Clay Brick of Nanjing City Wall

Ancient city walls hold immense scientific, artistic, and historical significance. In order to guarantee the precise and genuine conservation and restoration of ancient city walls, it is imperative to conduct an exhaustive examination of the masonry components. Although considerable interest has been devoted to the investigation of blue bricks in historical sites, relatively little is known about white brickwork. In this study, the white clay was calcined at various temperatures, and its physical properties, compressive strength, mineral composition, micromorphology, and pore structure were investigated. The results show that the original white clay is mainly composed of talc and quartz. When the firing temperature exceeds 1000°C, the talc phase disappears, and when the temperature exceeds 1200°C, the cristobalite phase appears. With an increase in firing temperature, the clay exhibits a decrease in the number of small pores and an increase in the number of large pores. Through the analysis of mineral composition and observation of micromorphology, it has been confirmed that the firing temperature of some white bricks from the Nanjing City Wall exceeds 1200°C. The findings of this study can provide valuable scientific insights for the careful restoration of the Nanjing City Wall.