Masonry degradation caused by aging of the material and consequent reduction in loadbearing capacity due to the lack of connection between masonry elements is commonly repaired by injecting. This technique is one of the less invasive ones and it preserves most of the characteristics of the original masonry, which is especially important in conservationrestoration of cultural heritage. While injection grouts based on cement can result in further degradation of masonry, lime-based grouts have become increasingly used in conservationrestoration practice. Although lime grouts are more compatible with historic materials, they are deficient in terms of consolidation of the structural elements. In order to overcome these shortcomings, different grout mixtures were prepared by mixing natural hydraulic lime and two different types of metakaolin at substitution rates of 10 %, 20 % and 30 % by mass. The test results of grout properties showed that higher metakaolin content increases the water demand
of the fresh mixtures as well as compressive strength and porosity of the hardened mixtures. The addition of metakaolin at constant fluidity slightly enhanced water retention, but had no significant impact on volume change and bleeding. When comparing two different metakaolins that were used, higher compressive strengths were measured in the case of the grouts which included the metakaolin with higher surface area and smaller quantity of unreactive mullite. The results show that the combination of natural hydraulic lime and metakaolin generates a stable grout, which has the potential to achieve considerable strengths, the necessary workability as well as use in conservation- restoration practice.