The Concrete Truth

Dr. Bruce Scruggs, Micro-XRF Product Manager EDAX

In the modern age of nanomaterials, concrete appears rather mundane and antiquated having been in existence for millennia.  However, concrete has enabled man to build amazing structures and modern reinforced concrete has allowed us to build the tallest buildings, the largest dams and the longest bridges in history.  Simply put, modern society could not exist without it.

1- pantheon 2 - Empire_State_Bldg
The Pantheon, A.D. 125 Empire State Building, 1931

So, in my travels, I’m always a bit amazed and rather concerned when I see damaged concrete.  As solid as it seems, environmental factors can reduce concrete to rubble in a matter of years.  The chemistry of concrete can be quite complex.  Fueled by water and other environmental chemicals permeating into the concrete matrix, reactions continue to take place over time even after the concrete has fully cured.  Much of this chemistry involves inorganic reactants which makes micro-XRF an ideal tool to study the ongoing chemistry of concrete.

Typical environmental nemeses of reinforced concrete are chloride ions from deicing salts and seawater.  Chloride ions diffuse into the concrete matrix which can lead to corrosion of the steel reinforcements.  This corrosion is a volume expansion reaction which leads to cracking and loss of adhesion to the steel reinforcement.  The next time you pass some older damaged concrete structure, see if the reinforcements are exposed and corroded.  The gray scale micro-XRF mapping images show a measurement of the Cl ion diffusion into a concrete matrix (mapping image on right).

3 - Rinforced-concrete_rusty-iron-bars__DSCF3748-360x480 4 - Cl ion diffusion
Concrete damage and exposed
reinforcements
Cl ion diffusion
Data courtesy of J.M. Davis, Research Microanalysis Research Group – NIST

As this is such a critical problem, a great deal of research is being expended on methodologies and concrete formulations which will lead to more durable and lasting concrete structures.  One such effort involves the study of Roman maritime concrete.  The Romans built harbor structures out of concrete along the coast of Italy which provided ports to supply Rome and project Roman power throughout the empire.  These harbor structures have maintained their structural integrity for 2000 years despite continuous exposure to seawater.

Micro-XRF provides a means to study this concrete chemistry non-destructively by imaging inorganic elemental distributions in sample cross sections which were prepared for other analytical techniques.  Micro-XRF does not require any treatments such as sample coating for SEM-EDS analysis.  Also, the X-ray beam does not damage the sample whereas laser ablative analysis does.  The Roman concrete sample mapped here was collected as part of a systematic study of the concrete in several ancient Roman harbors [1].  A micro-XRF mapping panel (below), collected with EDAX’s Orbis micro-XRF spectrometer, shows several elemental maps of a 16 mm2 area of the concrete thin section.  The mapping panel shows characteristic mortar microstructures of Roman marine mortar [2, 3] including volcanic ash, Al-tobermorite crystals and chlorine sequestered in hydrocalumite crystals.  Sulfur was also found to be concentrated in ettringite crystals.  The sequestration of Cl- and SO42- anions in crystalline microstructures in Roman maritime concretes is in contrast to modern concrete formulations, where these anions typically participate in secondary expansion reactions which lead to the loss of structural integrity in modern concretes.  Perhaps this is a clue in formulating longer lasting concretes?

5 - Orbis micro-XRF mapping panel

Orbis micro-XRF imaging panel of ancient Roman Marine mortar submerged in Pozzuoli Bay since ~55 BCE [1, 2, 3]. Elemental X-ray maps are shown in the top 6 images.

 One of the difficulties incurred in this initial mapping study was that Cl was also found in the epoxy binder used to maintain the integrity of the concrete thin section.  This can be seen in point A of the Cl map above where a void is filled with epoxy.  However, image scaling can be used to distinguish the Cl background signal originating from the epoxy from the more intense Cl signal originating from concrete microstructures using a multiple color image scaling.

6 - Orbis micro-XRF Cl map in a multi-color scaling 7 - Sum of spectra from Cl hot spot
Orbis Cl map in multiple color scaling Spectral sum from Cl hot spot

The brighter Cl spots in the epoxy-filled void are now readily highlighted in the multiple color scale image and correspond with the locations of bright spots in the Ca and Al maps.  Full spectral data is collected at each map pixel and can be used to show the corresponding Al, Ca and Cl signals from one of the hydrocalumite crystals in the cavity.  Lesson learned.  Find a more suitable epoxy for this application!

References
[1] Brandon C., Holhlfelder R. L., Jackson M. D., Oleson, J. P. Building for Eternity: the History and Technology of Roman Concrete Engineering in the Sea. Oxbow Books, Oxford (2014)
[2] Jackson M.D., et al.:  Cement Microstructures and Durability in Ancient Roman Seawater Concretes.  In:  Historic Mortars:  Characterisation, Assessment and Repair.  RILEM Book series 7, (2012)
[3] Jackson M.D., et al.:  Unlocking the Secrets of Al-Tobermorite in Roman Seawater Concrete.  American Mineralogist. 98, 1669-1687 (2013)

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