Metakaolin as a precursor of materials for applications in Cultural Heritage: Geopolymer-based mortars with ornamental stone aggregates

Potentialities and suitability of metakaolin-based geopolymers in Cultural Heritage have been explored. In particular, in order to evaluate their possible use as restoration materials in conservation of historic manufactures, mortars have been prepared by adding aggregates of Italian ornamental stones to alkali-activated metakaolin with binder/sand ratio of 1:1. To improve workability, geopolymer binders have been synthesized from metakaolin and sodium silicate solution with water/solid weight ratios between 0.33 and 0.66 and SiO2/Al2O3 and Al2O3/Na2O molar ratios of 3.70 and 1.04, respectively, and characterized by several techniques, including mechanical strength tests according to UNI EN 196-1. All binders display good mechanical properties, with compressive and flexural strength values as high as 72 MPa and 6 MPa, respectively, and decreasing with increasing water/solid ratio. The increase of water in geopolymer formulation has little negative effect on the aluminosilicate gel development and on the strength of these materials. Mortars display a homogeneous and compact matrix, bonded (silicoaluminate aggregates) or interlocked (carbonate) with aggregates. Their compressive strengths fall in the masonry mortars class M20 range. Their pore size distribution guarantees good breathability and adaptability to the substrate. The final materials mimic the original stones, with good aesthetic compatibility.