Influence of clay minerals addition on mechanical properties of air lime–metakaolin mortars

• Two sets of lime mortars with addition of clay minerals and metakaolin were analysed.
• Clay minerals and metakaolins cause microstructural changes of mortars.
• Lime with higher bulk density is more suitable to be substituted by additives.
• Clay minerals substituted air lime in 1st set clearly improved mechanical strengths.

Two sets of mortars differing in type of lime and metakaolin, with air lime:sand volumetric ratio 1:3 were prepared with the aim to be used for restoration of historic masonries. The first set involved air lime and 20 wt.% of metakaolin with more impurities (calcite and kaolinite, respectively) and with higher bulk densities compared to materials from the second set. Clay minerals (sepiolite, zeolite A, palygorskite and vermiculite) characterised by high specific surface areas and thus able to keep water in the structure and promote pozzolanic activity of metakaolins were used as additives in air lime and air lime–metakaolin mortars and their impact was evaluated in a view of mechanical strength at 28, 90 and 180 days. Substitution of air lime from the first set by clay mineral and/or metakaolin caused improvement of mechanical strengths predominantly at latter ages, while lime mortars from second set suffer by lack of binder when other additives are supplemented and just palygorskite incorporation improves flexural strength, while vermiculite and metakaolin create mixture with improved compressive strength than lime mortar alone.