The influence of metakaolin on the properties of natural hydraulic lime-based grouts for historic masonry repair

This paper investigates the properties of injection grouts with different amounts of natural hydraulic lime (NHL) replacement by metakaolin MK1 or MK2. The focus is dedicated to the fresh properties of the NHL-based grouts, as well as to a parallel study of the evolution of compressive strength and phase composition of hardened grouts, through tests at 28, 56, 90 and 900 days. Additionally, at 90 days, the influence of the MK1 and MK2 incorporation on the porosity characteristics of hardened grouts was determined. Tensile bond strength, being one of the key hardened properties of the injection grouts, was for the time being determined only at an early age of 28 days. The results showed that by adding metakaolin at constant fluidity, the water demand of the fresh mixture increases with the increase of the metakaolin content. At the same time the water retention capacity of the grout is enhanced. In a hardened state the addition of metakaolin increases early age and long term compressive strength of the grout. Also, its bond strength can be considerably increased, by applying MK1. Mixtures with metakaolin resulted in the formation of hydration products such as ettringite, hemicarboaluminate and monocarboaluminate, whereas stellerite was present only in the case of higher proportions of metakaolin with a higher BET surface area and Al2O3 content.