IntroductionThe brick-carved cultural relics is a precious cultural resource. The uniqueness of brick carving causes a high requirement for the technique and materials of in-situ repair. The difficulty is to preserve the original features of brick carving while restoring its microstructure. The inorganic reinforcing agents are beneficial for maintaining the physical properties of the historical material and have been considered the optimal choice for restoring the cohesive strength of the historical material in recent years. The restoration material prepared by basic magnesium sulfate cement (BMSC) has the advantages of high tensile strength, high resistance to weather, high viscosity, early strength, and low cost. It has a good effect in reinforcing the weathered sandstone of the cultural relics. The hydraulic lime is widely used due to its waterproof and breathable properties. But it is very expensive. The Silicon-based repair material has good permeability. But it has poor stability in the environment with the changing humidity. Three types of strengthening agents are selected as the alternative options for the restoration of brick carving based on the need for the brick repair and in-situ protection of the ancient mortar during the process of the restoration of brick carving.MethodsThree strengthening agents are Ca-based hydraulic lime, Si-based silica sol, and Mg-based BMSC. Three steps are implemented in order to select the most effective strengthening agent in the restoration of the cultural relics of brick carving. That is the preparation and research of performances of three kinds of imitation bricks, infiltration reinforcement of the ancient material, and the verification of practical engineering. The powder of green bricks is used as the main material for replica bricks. The hydraulic lime, silica sol, and BMSC-based reinforcing agent are firstly mixed with the powder of green brick to form replica bricks, which are the Ca-bricks, Si-bricks and Mg-bricks. The multiple experiments were conducted to investigate the compatibility of replica bricks. The studied properties include the composition of crystal phase, compressive strength, porosity characteristics, and the hydraulic performance of the replica bricks, and so on. WW/T 0065—2015 “Code for Investigation of Stone Cultural Relics Protection Engineering” is referenced. Secondly, the adhesive properties and infiltration reinforcement of the strengthening agents were taken into consideration to evaluate the feasibility of application of these material to the restoration of the brick-carving cultural relics. Finally, the verification of the referred BMSC-based strengthening agent used in actual engineering was conducted because the laboratory environment is different from the complex environment where the cultural relics are located.Results and discussionCa-based and Si-based reinforcing agents only serve to adhere the powder of green brick. The BMSC-based reinforcement agent continues to hydrate after being added to the powder of green brick to form the replica bricks. The main hydration product is the 5·1·7 strength phase. So the BMSC-based reinforcing agent has the strongest adhesion among three types of reinforcing agents. The Mg-brick has the best performance among three kinds of replica bricks, which is similar to the green brick. But Mg-brick has the slight hydrophobicity. The properties of Ca-bricks and Si-bricks are close to the ancient bricks. The bending strength of the masonry of bonded Mg-bricks is 10 times that of the Si-bricks. Only the BMSC-based strengthening agent has no carbonization and has good aging resistance. The infiltration solution with a content of 30% BMSC-based reinforcement agent and a moisture content of 44%-50% has the best effect of reinforcement, maintaining the original physical properties of brick-carved cultural relics while restoring the original cohesion. The referred BMSC - based reinforcing agent can penetrate and bond the slag of brick carving of 9.5 times the mass of agent.ConclusionsThe BMSC-based reinforcing agent demonstrates higher penetration enhancement ability than reinforcing agents based on hydraulic lime or silica sol. The volume stability of the referred BMSC-based strengthening agent meets the requirements. It also can be found that the repair effect of the referred BMSC-based strengthening agent is good after two years. In summary, the optimal ratio of BMSC - based reinforcing agent shows good repair effect at low cost. The Mg-brick and BMSC - based reinforcing agent can be used for the restoration of brick carving.