Palygorskite as an admixture to air lime–metakaolin mortars for restoration purposes

• Metakaolin and palygorskite were chosen as additives for lime mortars.
• Both, palygorskite and metakaolin influence mechanical properties of mortars.
• Mortar with metakaolin:lime mass ratio 1:4.75 seems optimal.

As mortars for rehabilitation purposes should have a similar composition to the historical materials, this work focussed on the preparation of mortars with binder:aggregate 1:3 volumetric ratio (vr) traditionally used in old buildings in central parts of Portugal. Mortars based on air lime binder with additions of 5 mass% of palygorskite (Pal) and 20 mass% of metakaolin (MKaol) were prepared to provide 1:3 vr, in which lime binder or sand was substituted.Needle-like Pal particles are functional as reinforcing fibres both in lime–Pal mortars and in air lime–MKaol–Pal systems. Chemical and mechanical properties of mortars were analysed at 28, 90 and 180 days of curing. Pozzolanic activity of MKaol was revealed by the presence of tetracalcium aluminate hydrate C4AH13 and monocarboaluminate C4A C¯ H11 at early ages of curing. Top value of flexural strength at 180 days shows lime–Pal mortar, in which sand was substituted by Pal, while the highest value of compressive strength was reached by lime–MKaol–Pal mortar, in which MKaol:lime mass ratio was 1:4.75. It was confirmed that differences in MKaol:lime mass ratios, even very small, cause considerable variances in the obtained mechanical properties.