Using the Orbis Micro-XRF Spectrometer to Study the Microstructure of Ancient Roman Seawater Concrete

Sep 01, 2014

Ancient Roman builders designed maritime concrete harbor structures to remain intact in the aggressive seawater environment for very long periods of time. Modern concrete engineers continue to deal with the challenges of exposure to chlorine ions, via either seawater or road salt. When chlorine ions penetrate into modern cement-based concrete they corrode the steel reinforcement, which ultimately can cause failure of the concrete structure. In contrast, the ancient Roman maritime structures have retained their structural integrity for 2000 years, despite having been partially or fully immersed in seawater since their construction. The concrete is composed of a volcanic ash-hydrated lime mortar that binds decimeter-sized chunks of volcanic tuff or limestone, shown by cores drilled by the ROMACONS research group from 11 different Roman harbors (1). There is no steel reinforcement. One key raw material is pumiceous volcanic ash quarried around Pozzuoli Bay in the Gulf of Naples, which reacts pozzolanically with lime and seawater to produce enduring cementitious hydrates in the mortar. Romans mastered the formulation and installation of maritime concrete in harbor structures in Pozzuoli Bay and the central Italian coast, most of which remain durable and intact to this day. The formulation of the concrete is theorized to have then spread through the Mediterranean area by the movement of Roman engineers and engineering manuals, which facilitated construction of concrete harbors. Analyzing these concrete samples for their elemental distribution with modern techniques like micro-X-ray fluorescence (micro-XRF) can give insight to the unique properties and processes that contribute to its longevity.

Characterization Technique

Micro-XRF is an elemental analysis technique that focuses an X-ray beam to generate characteristic elemental lines from the sample, with detection starting at sodium (Na). The X-rays are focused by means of total internal reflection through a series of glass capillaries, allowing high flux to be maintained at very small beam sizes. An X-Y-Z moveable stage allows for elemental distribution maps. For the measurements in the Roman mortar from Pozzuoli Bay, the EDAX Orbis PC-SDD spectrometer was used, which utilizes a 50 W Rhodium anode X-ray tube, a 30 µm diameter (FWHM at MoKα) X-ray beam (normal to sample), and a silicon-drift detector.

Using micro-XRF provides a number of advantages, particularly with historical and potentially high-value items. It is nondestructive, meaning the exciting X-ray beam does not damage the sample and the sample can be measured "as is." To contrast with some common characterization techniques for concrete, micro-XRF does not require coating the sample to prevent charging as with scanning electron microscopy-energy-dispersive spectroscopy (SEM-EDS). Techniques that utilize laser ablation, such as laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and laser induced breakdown spectroscopy (LIBS), will damage the sample, and are not acceptable in a case where the analyst wants to make high resolution compositional maps. Keeping the sample undamaged and intact as much as possible is especially important if the analyst intends to use multiple analytical techniques.

Figure 1: Orbis micro-XRF imaging panel of ancient Roman marine mortar submerged in Pozzuoli Bay since ~55 BCE (1,2,3). The color video image is in the lower left corner and the gray-scale video image used for elemental overlays is in the lower right corner. Elemental X-ray maps are shown in the top 6 images.
Another advantage to micro-XRF is that the sensitivity is 10 to 100 times superior to EDS for higher energy elements (that is, transition metals and above), typically reaching at least 50 ppm with a 30 µm beam diameter. Detection of chlorine, a key element to failure analysis of modern cement-based concrete with steel reinforcements, can reach about 15 ppm with optimized measuring conditions. Because lighter or organic elements, such as C, N, and O are not of interest in this analysis, EDS is not ideal. Another factor is the large sampling area, which in this example (Figure 1) is approximately 4.6 × 3.6 mm. Micro-XRF mapping lends itself well to larger mapping areas (mm scale or larger) as contrasted with EDS, which maps on a much smaller scale (µm to mm) and requires extremely long mapping times for satisfactory spatial resolution.

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