This study investigates the enhancement of hydraulic lime mortar (HLM) using varying contents of metakaolin (MK) to improve its application in the restoration of historic buildings. Samples from historic structures were analyzed, and the effects of different MK contents on the physical and mechanical properties of HLM were examined. The reaction mechanism and microstructural changes were evaluated using XRD and SEM analysis. The results indicated that increasing MK levels in HLM led to a decrease in fluidity, with fluidity reducing by 4.8% at 12% MK. The addition of MK increased water consumption for standard consistency by 5.4% and shortened the final setting time by 10.2%. MK consumption promoted secondary hydration, enhancing compressive strength by up to 98.1% and flexural strength by up to 55.1%, and increasing bonding strength by 26.9%. The density of HLM improved with MK addition, slightly reducing moisture content by 4.5% and water absorption by 4.6%, while the water vapor transmission properties decreased by 50.9%, indicating reduced porosity. The elastic modulus of the mortar increased significantly from 2.19 GPa to 7.88 GPa with the addition of MK, enhancing rigidity and crack resistance. The optimal blend for restoration materials was found to be 9.0% MK and 25.0% heavy calcium carbonate and was characterized by moderate mechanical strength, enhanced early strength, commendable permeability, minimal risk of cracking, and ease of application. This blend is highly suitable for the rehabilitation of historic structures.

This study investigates the effect of polyvinyl alcohol (PVA) on the performance of natural hydraulic lime (NHL)-based repair mortars used in historical building restoration. Mortars were prepared with varying PVA dosages (0.25%, 0.5%, 0.75%, and 1.0%) to evaluate their impact on physical, mechanical, and durability properties, including setting time, strength, water absorption, shrinkage, and resistance to freeze–thaw and sulfate attack. The results demonstrate that PVA significantly enhances bonding strength, reduces water absorption, and improves durability, with optimal performance observed at a 0.5% PVA dosage. Microstructural analysis showed that PVA forms a cohesive film, reinforcing the mortar’s structure. These findings suggest that PVA can enhance the performance of NHL-based mortars, offering significant potential for historical building restoration, particularly under challenging environmental conditions.