Volcanic raw materials for the low environmental impact production of pozzolanic mortars

The aim of this study was to select a series of glassy volcanic rocks for the production of pozzolanic mortars with a low environmental impact to be used in the field of Cultural Heritage restoration and, more in general, in the construction field. To achieve this objective, we selected a series of natural pozzolans (i.e., pyroclastic rocks) from the Sardinia territory (from the Monte Arci and Fordongianus areas) as raw materials, which share a high reactivity with lime due to the pronounced fraction of the amorphous phase (up to 90% in volume). Furthermore, a silica-rich sand as aggregate ("inert") and two types of binder were used: a slaked lime for plaster, along with a natural hydraulic lime-based (deriving from the use of a marly limestone) for bedding mortars. In order to experimentally evaluate the performance of the mortars, and to define the best mix between the raw materials, several mortar specimens have been tested from a physical-mechanical point of view (by means of He-porosimetry, real and apparent density, vapour permeability, PLT test) at set times of 1 week (only in the case of hydraulic lime), 1 and then 2 months. At the same times, the various samples were analysed by means of optical and electron microscopy (SEM), in order to describe: the microstructural and textural aspects of the mortars during the setting and hardening stages, the chemical reactivity between the pozzolans and the binder, the carbonation reaction of the hydrated lime and the formation of the hydraulic phases (CSH, CAH), responsible in both cases for the mechanical resistance. The results indicate that bulk density and porosity, in response to the hydration process of calcium silicates and aluminates (with formation of CSH and CAH), experience respectively an increase and decrease of their values, leading to a consequent improvement of the mechanical properties. On the basis of the physical-mechanical and compositional features measured after 2 months, the mortars made by pozzolan+slaked lime and pozzolan+hydraulic lime proved to be suitable in the field of Cultural Heritage restoration and, in general, in construction field.