Use of geopolymeric composites for the restoration of a Roman mosaic fragment.

Thanks to a collaboration among the Archaeological Survey of Restoration Center for the Fine Arts and the landscape for the metropolitan city of Florence and the provinces of Prato and Pistoia, the Institute for Art and Restoration Palazzo Spinelli of Florence and CNR -ISTEC Faenza, a support panel made in geopolymeric composites for containing a fragment of Roman mosaic it was possible to realize. The mosaic is a dichromate tessellated pavement dated back to the Imperial age and called "A 2016" This innovative solution was chosen in order to improve and facilitate a safe and adequate outdoor exposure as well as a general correct reading of the mosaic. The geopolymer is an inorganic material, obtained through a chemical reaction between reactive aluminum-silicate raw material and an activating alkaline solution; the reaction is called geosynthesis or geopolimerization and takes place at temperatures below 300 ° C. This material, for various reasons (i.e. compatibility with original materials and reduction of the restoration work times) was chosen as a possible alternative, to common movable supports such as Aerolam, mortar, concrete etc. In addition this material has high mechanical properties, high workability and its hardening is at low temperature, even at room temperature. Before proceeding to the intervention, numerous laboratory tests were made with the aim to obtain the suitable formulation. Two geopolymeric mixtures were realized by introducing sand, perlite and micronized calcite in the binder based on metakaolin and potassium alkaline solution (mixture of KOH and KSiO3). Micronized calcite was used in only one mixture in order to obtain two final products different in colors. This solution has permitted to create a sacrifice surface able to guarantee the principle of complete and total reversibility of conservative intervention. Firstly, a suitable mold (60x70x10 cm), able to contain the two geopolymeric mixtures and the mosaic fragment, still placed on concrete layer and already partially restored, was realized. Then, the first mixture, darker in color, was casting into the mold to compose the bottom of the support panel. Before the second pouring, a stainless steel net was inserted with the purpose of reinforcing the entire structure. After this operation the second mixture was poured. At last, the mosaic fragment was placed onto and into the geopolymeric bed. The use of this Roman mosaic has been possible and necessary because it in the first half of the 900's was detached and re-placed directly on the concrete, suffering over the years a strong degradation with consequently breaking. The geopolymeric envelope therefore aims to give solidity and a more comprehensible and adequate form to the fragment, coming only into contact with the concrete bottom and not with the Roman mosaic tesserae. After a suitable period of maturation (more than the usual 28 days, taking into account the size of the object), it was decided to proceed with the integration of the mosaic. Black and white geopolymeric tesserae were used to fill the warp internal gaps and also outside, so as to obtain a mosaic of rectangular form, where it was kept and followed the original warp. The result was to have a correct reading of the geometric design in black and white tesserae, confirming that conservation and experimentation can and must work together to achieve advanced and satisfactory results.