Material characteristics of ancient Chinese lime binder and experimental reproductions with organic admixtures

• Organic-inorganic binders were widely used in ancient Chinese construction.
• Representative samples from four important ancient Chinese walls were investigated with diverse analytical methods.
• Three series of lime binder reproductions with sticky rice, tung oil and pig blood were fabricated and investigated with similar methods.

The application of traditional construction materials, such as lime binders containing organic additives, has attracted a great deal of attention in recent years for the sustainable conservation and restoration of ancient buildings. Here, representative samples from four important Chinese city walls from Hebei, Jiangsu, and Gansu provinces, constructed from 563 to 1381 CE, are comprehensively investigated with micro- and macro-scale analytical techniques. Traces of organic additives were detected, and according to the Song Yingxing text (1637 CE), these should be sticky rice, tung oil or pig blood. To clarify the potential influences of additives on the physical properties of ancient lime binders, reproductions of the lime paste binders were fabricated according to the Song Yingxing formulas with sticky rice slurry, tung oil and fresh pig blood, and evaluated with standard engineering testing experiments. Continuous observation of the early stage of microstructure formation used an innovative ultrasonic monitoring apparatus. Interactions between the lime and organic additives, analyzed via powder X-ray diffraction and environmental scanning electron microscopy, show gains in specific performance capabilities. Sticky rice slurry at 5 wt% accelerates setting and hardening and increases compressive strength and bonding capability, although unit weight and water resistance are slightly reduced. Tung oil at 5 wt% strongly improves water resistance through dramatically reducing water sorption. Pig blood at 1 wt% accelerates setting and hardening, and improves water resistance. The archeological record and Song Yingxing text indicate that Chinese builders recognized these influences and selected the specific organic additive for lime binders in wall constructions, caulking, wall facings, and groundwork foundations.