The secret of ancient Roman hydraulic mortar. The lesson learnt from the past for future cements

Aqua Traiana is an ancient Roman aqueduct, built to supply water to the ancient city of Rome and still working nowadays. This work is focused on the characterization of its mortars, and in particular the binders, to identify the reasons of such a durable resistance. Samples collected inside the aqueduct were analysed by Optical Microscopy (OM) in thin sections, X-ray Powder Diffraction (XRPD), Scanning Electron Microscopy with Energy-Dispersive X-ray Spectroscopy (SEM-EDS), High Resolution Field Emission Scanning Electron Microscopy (HRFESEM-EDS), Electron Microprobe (EMPA), Differential Scanning Calorimetry coupled with Thermogravimetric Analysis (DSC-TGA), Fourier-Transform Infrared spectroscopy (FTIR), X-ray Absorption Near Edge Structure (XANES) spectroscopy and N2 adsorption/desorption measurements. The presence of an amorphous gel-like C-(N,K)-A-S-H binder and the complete absence of calcite in Trajan Age samples constitute a unique case, and may be the reason of the high hydraulicity of the mortar. Establishing how these features were produced could allow them to be replicated in high-durability and high-compatibility restoration mortars.