When historic buildings are restored, ancient „concrete“ often has to be replaced. To recreate its exact composition FTIR spectroscopy and microscopy are excellent tools.
Ever wondered why some ancient buildings such as Roman aqueducts or the Hagia Sophia in Istanbul are so well preserved? The secret often lies in Pozzolana a natural siliceous or siliceous-aluminous material named after Pozzuoli, a town near Naples. In the famous pozzolanic reaction it reacts with calcium hydroxide in the presence of water at room temperature, which leads to the formation of a very versatile cement.
FTIR analysis of ancient concrete
Still, every once in a while even the most resistant buildings need renovation. Unfortunately, a lot of mistakes have been made in their preservation. One of them is the use of modern cement. This can, for example, cause cracks that will trap mmoisture, risking severe damage of the buildings. Because of that, the composition of the old cement is now thoroughly examined, and the substitute cement is formulated based on this analysis.
For this application FTIR spectroscopy and microscopy are an excellent choices. They not only provide information about the exact mineral composition of the cement but also allow insights into the spatial association of these components through imaging.
Before we come to FTIR imaging let’s have a quick look at a typical FTIR spectra of ancient Roman cement.
The surprising FTIR spectrum of Roman cement
The FTIR absorption peaks indicate the presence of both inorganic and organic additives. The spectrum shows that this cement contains binder components like CaCO3, Pozzolan and quartz, which indicates the presence of sand as a major component. Furthermore, iron, as well as a small amount of gypsum can be detected in the sample.
With this information, restorer of old buildings are able to create an exact chemical and mineralogical copy of the original cement.
Cement science
However, sometimes a simple content analysis is not enough. That’s when FTIR imaging comes into play. It can assist in determining the spatial distribution of components that are relevant for the stability of the concrete. Some of these components are Calcium silicate hydrates (C-S-H phases). Let’s take a quick look at the science behind this.
C-S-H phases have excellent cementing properties. Basically they are the glue that keeps a concrete together. The phases form during the so called pozzolanic reaction where two reagents that lack the ability to cement by their own form something really durable. These reagents are silica-rich components such as natural glasses or Pozzolana and calcium hydroxide (see equation (1) below). These components are called latent hydraulic components or pozzolanic materials.
(1) Ca(OH)2 + H4SiO4 → CaH2SiO4·2 H2O
The C-S-H are usually analyzed in situ by a variety of different techniques such as electron microprobe (EPMA). EPMA can visually depict the different elements and their distribution through element mapping. However, it is challenging if not impossible to determine the modal amount and exact distribution of the C-S-H phases with this technique.
Thankfully ATR-FTIR imaging is up to the challenge!
Deformulating concrete by FTIR imaging
In the following example the distribution and origin of C–S–H phases in reaction rims of old concrete was determined by FTIR imaging. For this task Bruker’s HYPERION II FTIR microscope with a focal plane array detector (FPA) was used.
The colors in the FTIR image represent the Ca/Si ratio. Dark blue areas have a high amount of Si , whereas white indicates a high amount of Ca. The green color depicts C-S-H phases.
The fact that the C-S-H phases form in the transition zone between the silica rich areas and the calcium rich limestone indicates that the Si-rich glass act as pozzolanic material in this sample. A reaction according to equation (1) took place.
To obtain more detailed information about the reaction rim, individual spectra were recorded based on the color scheme. These showed that area (a) consisted of aragonite, C-S-H phases and SiO2 gel wheres area (b) additionally contained calcite. The white area (c) was dominated by calcite.
The occurrence of aragonite and SiO2 gel points to the carbonation of C-S-H phases. This reaction of atmospheric CO2 with Ca-bearing phases in cement can contribute to the densification of the material, improving its mechanical properties.
FTIR imaging also revealed highly polymerized C-S-H phases linked to aragonite and SiO2 gel. Polymerization may benefit strength, reduces permeability, and improves chemical resistance in concrete. Excessive polymerization however, can pose issues like shrinkage, internal stresses, and reduced workability. These factors strongly impact restoration practices and overall concrete performance.
Conclusion
In conclusion, FTIR spectroscopy and microscopy emerge as excellent tools in the analysis of historic concrete. They enable restorers to accurately create a chemical copy of the old concrete thus preventing issues arising from the usage of modern materials. On top of that FTIR imaging aids in the determination of the spatial distribution of crucial concrete constituents such as C-S-H phases. Here it can give valuable information about what processes occurred during ancient concrete production and how the concrete reacted with the environment over time.
The use of ATR-FTIR imaging with an FPA detector results in a much better S/N ratio. But that is not all, instead of needing 32×32 (1024) individual measurement points with a 5 μm aperture, the FPA can acquire the same information in a single, 2 second scan. Thus, maximum spatial resolution at minimum measurement times is feasible.
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