ANCIENT POZZOLANICITY REVISITED: NOVEL INSIGHTS ON MATERIALS AND REACTIVE PROCESSES OF ROMAN HYDRAULIC BINDERS

The use of pozzolanic materials to trigger low-temperature hydraulic reactions in calcium-based inorganic binders was the most relevant technological evolution in the field of construction materials since the beginning of pyrotechnology. After the first applications by ancient Mediterranean societies of the Second and First Millennium b.C., the potential of pozzolanic binders was fully exploited by ancient Romans, which used them for large-scale structural purposes through the formulation and development of Roman concrete.

Driven by the necessity of parametrizing its production techniques, the Roman technology of pozzolanic binders pushed since the beginning to the systematic quarrying of the most suitable geological sources of pyroclastic materials from the Central Italy volcanic districts (Phlegraean Fields, Monti Sabatini/Alban Hills). Such marked standardization led to remarkable trades of raw materials throughout the Roman Empire. Nevertheless, several pieces of evidence indicated that such established supply criteria were often overcome by the employment of non-standardized local pozzolanic materials, both natural and anthropogenic, with obvious geographic and social advantages.

In this study, a combined mineralogical-spectroscopic-microstructural analytical approach has been adopted for the characterization of the mineralogical and crystal-chemical features of Roman pozzolanic binders, considering several materials collected from various archaeological sites in the Italian peninsula and around the Mediterranean region.

The study demonstrated that, besides the import of traditional pyroclastic compounds, several alternative materials were used by Roman craftsmen, including natural products such as volcanic breccias, microcrystalline sedimentary silicates, and artificial compounds such as ceramic by-products and combustion residues. The information obtained on the microstructural features, reaction interfaces, short range atomic structure of hydration products, and extent of hydraulic reactions allowed a better understanding of the advanced levels of technical knowledge and the exceptional physical and engineering performances of the Roman structural materials. A new model for the reaction products of Roman pozzolanic materials is presented.