A novel citric acid-modified magnesium oxysulfide cement for the preservation of weathered sandstone relics

Due to the relatively low mechanical strength and high porosity, sandstone cultural relics show poor resistance to weathering and are vulnerable to significant deterioration. In this work, for the first time, a novel inorganic cementitious compound was introduced for sandstone preservation by combining magnesium oxysulfide (MOS) cement with citric acid (CA). Accordingly, the effect of CA on MOS cement was investigated in terms of mechanical properties, phase analyses, micropore structures, and microtopography. MOS cement with 0.5 % (wt.) CA exhibited the best performance, with excellent infinite compressive strength (20.92 MPa) and softening coefficient (125.06 %). Due to the CA addition, the X-ray diffraction (XRD) analysis represented the remarkable appearance of the 5 Mg(OH)2·MgSO4·7H2O (5-1-7 phase), the primary strength-contributing phase in MOS cement. Furthermore, the investigations on MOS mortar (mixing MOS cement with silica sand) showed that MOS cement effectively functioned as a binder for silica sand. Finally, the excellent compatibility between the MOS cement and sandstone particles highlighted the feasibility of the cement as a novel material for sandstone preservation. These findings demonstrated the MOS's promising role in sandstone cultural relics conservation with significant potential. •The mechanical properties of MOS can be effectively improved via adding CA, resulting from more 5-1-7 phase.•Both CA-modified MOS cement and MOS mortar show high mechanical strength and softening coefficient.•After 10 freeze-thaw cycles, both the mass and compressive strength of CA-modified MOS mortar do not decrease much.