Study on modification of natural hydraulic lime historical building repair mortar

Abstract

Qingdao boasts a wealth of historical buildings, yet they have sustained varying degrees of damage due to environmental erosion over decades. Traditional cement-based repair materials can exacerbate the damage to these structures and often fall short of the stringent requirements for historical building restoration projects. Natural Hydraulic Lime (NHL) offers excellent compatibility and breathability, making it suitable for restorative work on historical buildings. However, its low early strength and durability pose limitations in Qingdao's complex environment. Consequently, this study investigates the enhancement of NHL mortar using Ground Granulated Blast Furnace Slag (GGBFS/BFS) powder. Nine mix formulations were developed to investigate the impact of varying BFS contents on the physical properties and microstructure of the repair mortar. The results indicate that, compared to the reference sample, the modified mortar exhibits reduced water requirement for normal consistency, shorter setting time, lower water absorption, and decreased shrinkage. Regarding mechanical properties, the incorporation of BFS significantly enhances the mechanical strength of the repair mortar. When the proportion of BFS is 60 %, the flexural and compressive strength of the mortar samples increase by 304 % and 611 % at 28 days, respectively. Upon incorporating BFS, a pozzolanic reaction occurs in the alkaline environment, leading to the formation of calcium silicate hydrate (C-S-H) gel. This gel fills the pores within the mortar, enhancing the densification of the system, which in turn improves the mechanical properties and microstructure of the repair mortar. SEM analysis corroborates these findings. XRD analysis confirms that BFS does not introduce harmful substances.