Structure, properties and decay of ancient Chinese blue brick masonry in the Bell Tower of Xi’ an

The internal structure of the ancient blue bricks masonry at the base of the Xi'an Bell Tower was investigated by core drilling, and the constituting building materials and decay products/mechanisms were studied using a multianalytical approach. Results show that the masonry structure (from outside to inside) included a surface (paint) coating (∼0.5 cm thick) - modern blue brick masonry (∼22 cm thick) - ancient blue brick masonry (∼140 cm thick) - internal rammed earth. The rammed earth included quartz, feldspars, calcite, and clay minerals. Blue bricks were made up of quartz, K-feldspar, and high- T (>1000 °C) phases (anorthite, augite/enstatite) with a highly vitrified aluminosilicate matrix, consistent with calculated firing T for modern (1126 °C) and ancient (1057 °C) blue bricks. The high compressive strength of surface bricks (10.5 MPa) and inner bricks (9.5 MPa) was consistent with high- T enhanced vitrification under reducing conditions after addition of water at the end of the firing process in traditional "Yin shui" blue bricks making. Saline solutions migrated from the rammed earth core and lime mortars (containing calcite, and minor gypsum,) to the bricks, inducing precipitation of secondary calcite (lublinite) along with soluble salts, such as halite and nitratine, the latter originated from atmospheric NO x . Crystallization of soluble salts caused extensive damage to the surface bricks. An impervious polyethylene-acrylic resin coating was applied to the surface of the powdery bricks, which further aggravated salt damage. These results provide new insights into the internal structure of ancient Chinese masonry buildings and the sources and effects of salt crystallization in blue bricks, offering valuable guidance for the conservation of the base of the Xi'an Bell Tower and similar ancient masonry buildings. • Layered structure (paint coating→modern/ancient blue brick→rammed earth) of Bell Tower base revealed. • Blue bricks have a high strength from "Yin shui" firing leading to enhanced vitrification. • Migration of saline solutions induces soluble salt precipitation, damaging surface bricks. • Impervious resin coating exacerbates salt-related damage.