Effect of binder-to-sand ratio for dicalcium silicate mortar on its restoration properties for sandstone cultural relics

Controlling volumetric stability of the repair material itself, along with its bonding performance to the sandstone cultural relic substrate, are critical aspects for achieving effective restoration. In our previous study, a home-made dicalcium silicate (Ca₂SiO₄, C₂S) clinker exhibited very high hydraulic activity was recommended as a potential candidate conservation material for sandstone cultural relics. Here, the hydration process and related composition of dicalcium silicate pastes were monitored for a long period over one year. By examining the microstructural evolution of the interfacial transition zone with varying binder-to-sand ratios and curing times, the relationship between the intrinsic strength of the repair materials, bonding strength, and the location of fracture surfaces was established. The results indicated that the incorporation of aggregate can effectively addressed the problem of volumetric stability during the curing of neat paste. Furthermore, by varying binder-to-sand ratio the bonding strength and fracture location could be regulated, which was important for avoiding restorative damage risks to cultural relics. It was found that the particles on the sandstone surface could actively promote the hydration process of dicalcium silicate and further lead to increase the strength of the interfacial transition zone overpassing that of the dicalcium silicate paste itself. Even though the incorporation of sand hinders the effective bonding between dicalcium silicate cementitious material and sandstone initially, the subsequently formed hydration products during curing, especially C-S-H gel, could densify the interfacial transition zone and thereby significantly improve bonding strength, which was the reason for explaining the continuously change and migration of the fracture position over time.