Deterioration Mechanism of Lime Used in the Restoration of Qufu San Kong Ancient Architectural Complex

In recent years, a decline in the binding strength and frost resistance of lime used in the restoration of the Qufu San Kong ancient architectural complex in Shandong Province has been observed, which has significantly affected the quality of restoration. In this study, lime samples were collected from the Kuiwen Pavilion roof, limestone in the vicinity of the Confucius Temple, and modern industrial lime from various provinces in China. Analytical methods including X-ray diffraction (XRD) and Fourier-transform infrared (FTIR) spectroscopy were employed. Results indicate that the local limestone in Qufu primarily comprises calcium limestone with impurities such as Si or Mg. XRD patterns reveal that the main phase of the Kuiwen Pavilion roof lime sample is calcite. Using FTIR and Scanning Electron Microscope- Energy Dispersive Spectrometer (SEM-EDS), the presence of hydrated calcium silicate (C-S-H) gel is identified in the sample. This suggests that the traditional lime-production process in Qufu yields a multiphase composite system. Modern industrial lime, aged for two months, showed no presence of C-S-H gel in the FTIR spectra and SEM-EDS results, indicating that it is a comparatively pure system primarily comprising CaO. The study suggests that traditional lime-production processes typically involve burning limestone with higher Si content, leading to the generation of a significant amount of dicalcium silicate. Additionally, the use of high-ash coal as fuel introduces reactive silicon dioxide into the lime product. These factors contribute to the formation of C-S-H gel in the lime after aging. The presence of C-S-H gel enhances lime properties such as binding strength and frost resistance. Modern lime production, which selects limestone with lower Si content and employs electricity and low-ash coal as fuel, aims to minimize or avoid the formation of C-S-H gel in aged lime. High-purity CaO lime products align with the principles of modern fine processing; however, they compromise the binding strength and durability of lime required for traditional architectural restoration.