Early exploitation of Neapolitan pozzolan (pulvis puteolana) in the Roman theatre of Aquileia, Northern Italy

Abstract

The paper reports the results of the analyses on mortar-based materials from the Roman theatre of Aquileia (Friuli Venezia Giulia, Northern Italy), recently dated between the mid-1st Century BCE and the mid-1st Century CE. Samples were characterized by Polarized Light Microscopy on thin sections (PLM), Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM-EDS) and Quantitative Phase Analysis by X-Ray Powder Diffraction (QPA-XRPD). Pyroclastic aggregates (mainly pumices and scattered tuffs), incompatible with the regional geology, were found in two samples from the preparation layers of the ground floor of the building. Their provenance was determined by means of QPA-XRPD, SEM-EDS, X-Ray Fluorescence (XRF) and Laser-Ablation Inductively-Coupled-Plasma Mass-Spectrometry (LA-ICP-MS). Mineralogical and geochemical analyses demonstrated their provenance from the Bay of Naples, thus recognizing them as pulvis puteolana, a type of pozzolanic aggregate outcropping around the modern town of Pozzuoli and prescribed by Vitruvius (De Architectura, 2.6.1) in mortar-based materials to strengthen masonries and produce hydraulic concrete for harbor piers. This evidence represents the oldest analytically-established case of pulvis puteolana exploitation in Northern Italy up to now, and an early use of the material out of Campania adapted for civil constructions in a non-strictly maritime-related environment. Indeed, the theatre was built in the low-lying Aquileia's deltaic plain, prone to water infiltrations that are typical in lagoon-like environments. The data highlight the craftsmen's resilience in adapting and reinterpreting the traditional use of the Neapolitan volcanic materials to deal with the geomorphological challenges of Aquileia's lowland.

Figure 1

The Roman theatre in Aquileia. (a) Reconstructive plan of the building with indication of the excavated sectors (in dark grey); (b) Reconstructive cross section of the theatre, from the ima cavea to the scaenae frons wall, with stratigraphic sketches of the floor preparations of the orchestra and hyposcaenium.

Figure 2

Cross sections of representative samples of the three groups of mortars. (a) WM_9 (Group 1); (b) PREF_13B (Group 2); (c) PREP_25 (Group 3).

Figure 3

Detailed micrographs of representative samples by Polarized Light Microscopy (PLM), both in crossed (XN) and parallel (PN) nicols. (a) WM_11 (XN). The lime matrix displays high birefringence colors, indicating a complete carbonation of the binder. The aggregate is mainly represented by medium-to fine clasts of limestones or dolostones and silicate sand, with chert prevailing over quartz; (b) PREF_11 (XN). The lime matrix displays low birefringence colors, indicating an incomplete carbonation of the binder; (c) WM_15 (XN). Unburned fragment of dolomitic limestone, with relicts of rhombohedral crystals of dolomite. The lime matrix displays low birefringence colors; (d) WM_12 (XN), reacted clast of dolostones (rDL) having low birefringence due to dedolomitization phenomena; (e) PREF_8. Reaction rims around clasts of chert (rSL), having low birefringence; (f) PREF_13B (PN). The terracotta component of the sample is abundant, with scattered coarse clasts (at the corners of the picture) and diffused terracotta powder, intimately mixed with the lime matrix; (g) PREP_13A (PN). On the right, a coarse fragment of recycled mortar; (h) PREP_25 (PN). A micrometric clast of pumice, with a K-feldspar (sanidine) phenocryst. The reacted rim is detectable by the low birefringence; (i) PREP_25 (PN). Micrometric clasts of pumice, with biotite phenocrysts.

Figure 4

SEM–EDS analyses on representative samples of Group 1, showing reacted dolostones and chert clasts displaying the development of M-S–H gels. Backscattered electrons (BSE) acquisitions. (a) WM_3, altered clasts of dolostone; (a1) EDS spectrum of the unreacted core of a dolostone clast; (a2) EDS spectrum of the Mg-depleted rim of the clasts, with local enrichment in M–(A)–S–H; (a3) EDS spectrum of the unreacted core of another dolostone clast; (a4) EDS spectrum of the Mg-depleted rim of the clasts, with local enrichment in M–(A)–S–H; (b) WM_3, altered clasts of chert; (c) magnification of the dashed area at fig. (b); (c1) EDS spectrum of a feebly altered core of a clast of chert; (c2) EDS spectrum of a reacted area of the clast, indicating a local development of M–(A)–S–H and a likely occurrence of C–S–H through reaction with the lime component; (d) PREF_12, lime matrix of the sample with a lime lump on the right; (e) magnification of the dashed area in fig. (d); (e1) EDS spectrum of an area of the binder matrix displaying M–S–H development; (e2) development of M–S–H within the micropores of a lime lump.

Figure 5

XRPD patterns of binder-concentrated fractions from representative samples of the three mortar groups, with indication of the main mineral phases (mineral abbreviation labelled according to). (a) sample PREF_12 (Group 1); (b) sample PREF_13B (Group 2); (c) sample PREP_25, lower layer (Group 3).

Figure 6

SEM–EDS analyses on representative samples of pumice in sample PREP_25. Backscattered electrons (BSE) acquisitions. (a) pumice clast c4; (b) magnification of the dashed area in fig. (a); (b1) EDS spectrum of a K-feldspar phenocryst (sanidine); (b2) EDS spectrum of unaltered volcanic glass; (c) pumice clast c5; (d) magnification of the dashed area in fig. (c); (d1) EDS spectrum of a Ti–rich biotite phenocryst; (d2) EDS spectrum of unaltered volcanic glass; (e) pumice clast c6; (f) magnification of the dashed area in fig. (e); (f1) EDS spectrum of an apatite phenocryst; (f2) EDS spectrum of unaltered volcanic glass; (g) pumice clast g; (h) High-resolution magnification of the dashed area in fig. (g); (h1, h2) EDS spectra of aphyric volcanic glass; (i) pumice clast a; (j) High-resolution magnification of the dashed area in fig. (i); (j1, j2) EDS spectra of aphyric volcanic glass.

Figure 7

SEM–EDS analyses of reacted pumices in samples PREP_25 and PREP_53. Backscattered electrons (BSE) acquisitions. (a) A reacted clast of pumice in sample PREP_25; (b) magnification of the dashed area in fig. (a); (b1, b2) EDS spectra of a reacted area of the volcanic glass, with vesicles filled of calcic compounds, likely related to calcium carbonates (calcite or vaterite) from the binder; (c) A reacted area of a clast of pumice in sample PREP_53; (d) magnification of the dashed area in fig. (c); (d1, d2) EDS spectrum of a C–A–S–H enriched zone, developed from the leached volcanic glass and filling the vesicles; (e) zeolitized volcanic glass in a clast of pumice in sample PREP_25; (e1, e2, e3) EDS spectra likely referred to anthropogenic phillipsite formed by pozzolanic reaction.

Figure 8

XRPD spectrum of the sub-centimetric clast of tuff (w) in sample PREP_53, with indication of the main mineral phases (mineral abbreviations labelled according to).

Figure 9

TAS (Total Alkali vs Silica) scatterplots of pumices (volcanic glass) in samples PREP_25 and PREP_53. (a) Samples’ distribution according to volcanic rocks’ chemistry (after); (b) samples’ distribution according to rock chemistry of the Phlegraean volcanic products regarding the main eruptive events of the pre- and Campanian Ignimbrite (pre-CI/CI), pre- and Neapolitan Yellow Tuff (pre-NYT/NYT), post-NYT (Epoch I, II, II, according to), and Phlegraean-correlated volcanoes of Ischia and Procida-Vivara (compositional fields edited from^,,,); (c) samples’ distribution in relation to the three main eruptive facies of the Somma-Vesuvius volcanic activities (compositional fields edited from^,,); (d) samples’ distribution in relation to the fields occupied by the products of the Roman and Tuscan Magmatic provinces (compositional fields edited from^,,); (e) samples’ distribution in relation to the fields occupied by the pyroclastic products of the Aeolian Arc Isles (compositional fields based on raw data from).

Figure 10

Trace elements’ scatterplots of the volcanic grains in samples PREP_25 (pumices) and PREP_53 (tuff); (a) Nb/Zr vs Th/Ta scatterplot of clasts’ samples in relation to the fields occupied by the Roman, Tuscan and Campanian magmatic provinces (compositional fields edited from^,,,,,,), and Aeolian Arc Islands’ volcanic products (compositional fields based on raw data from^–); (b) Nb/Y vs Zr/Y scatterplot of clasts’ samples in relation to the fields occupied by the Roman, Tuscan and Campanian magmatic provinces (compositional fields edited from^,,), and the Aeolian Arc Island's products (compositional fields based on raw data from^–); (c) Nb/Y vs Zr/Y scatterplot of clasts’ samples in relation to the fields occupied by volcanic products of the Phlegraean Fields main eruptions (according to; compositional fields edited from^,,), and Phlegraean-correlated products (pumices and scorias) of Ischia/Procida-Vivara (compositional fields based on raw data from^,); (d) Nb/Y vs Zr/Y scatterplot of clasts’ samples in relation to the fields occupied by volcanic products of Somma-Vesuvius main pre-79CE eruptions (according to, compositional fields edited from).