Our Pulse

Copenhagen in winter
16 September 2019

Fluorescence Microscopy: A Song of Ice and Light

DRP emerges from a polar vortex with fluorescence microscopy techniques firmly in hand

In the Spring of 2018, DRP added fluorescence microscopy to our quiver of petrographic tools. After more than a year of continuous research, development and refinement of potential applications, here are some of our experience and vision for the future.

What is Fluorescence Microscopy?

Fluorescence microscopy (FM) involves the use of epoxy mixed with a dye that is activated under ultraviolet light. The epoxy is used to impregnate a sample, which may range from a billet cut for a petrographic thin section to an entire core that requires stabilization, before it is sawed and polished for petrographic work. While FM has been around for a long time and is used in a number of fields including biological and material sciences, the use of FM in concrete began in earnest in the 1980s in Denmark. There a group associated with Dansk Beton Teknik A/S began using fluorescence microscopy to study deterioration mechanisms such as alkali-silica reaction (ASR) in concrete and to develop methods for using fluorescence to measure the water-cement ratio (w/c) and water-cementitious materials ratio (w/cm) of hardened concrete. Through the 1990s, the Danish group developed further the method for impregnating samples and for producing thin sections and deployed the method as an effective quality control tool for major infrastructure projects.

The Key to Thin Sections

In 2004, Peter Laugesen, formerly of the Dansk Beton Teknik concrete group, started PELCON. For the past 15 years Peter and PELCON have focused on developing a robust consulting practice and in the production of first-class precision machines for the preparation of samples for concrete petrography. Among these machines is the Automatic Thin Section machine (ATS). The ATS comprises a diamond saw, a water cooling system, vacuum pump, vacuum systems for cutting and grinding, sets of rollers with embedded diamonds, and automated thickness acquisition and adjustable grinding speeds. Years of continuous refinement have led to the development of a machine which is reliable, robust and incredibly efficient.

Photograph of PELCON Automatic Thin Section Machine

PELCON Automatic Thin Section Machine, see http://pelcon.dk/products/automatic-thin-section-machine/

DRP Experience with Fluorescence microscopy

In early 2018, DRP purchased the ATS and traveled to Copenhagen to receive training on the machine and attend a two-day petrographic training workshop. With thoughts of crisp but spring-like conditions, we jumped across the pond and landed in the middle of a record-breaking polar vortex. Undaunted and relatively un-frozen, we completed our training, enjoying the city, and great company with the PELCON crew.

Copenhagen in winter

Copenhagen in the midst of the polar vortex, March 2018. Biting cold and stinging winds can be effectively offset with rich pastries, lush chocolate and well-crafted beer (not necessarily in that order…)

Our return to the States was followed closely by the arrival of the ATS and a number of other accoutrements, such as a first-class vacuum impregnation system. For the past year, we have refined our methods in not only sample preparation, but in the analysis of images obtained from fluorescent thin section microscopy.
The basic premise behind the fluorescence method for understanding concrete microstructure is relatively straightforward. The degree to which epoxy can penetrate a porous medium while under vacuum is directly proportional to the capillary porosity of the that material. In our case, this is the cement paste. The capillary porosity is in turn proportional to the w/cm of the paste. By mixing a dye into the epoxy that is activated under UV light, one can see changes in the green tone of an image that relate directly to capillary porosity. An example for a concrete mixture of different w/cm is shown below. As the w/cm increases, the brightness of the green tone increases as well.

Collage of fluorescent transmitted light photomicrographs of concrete

Collage of fluorescent transmitted light photomicrographs of concrete prepared with same components at different water-cement ratios. The bright green circles are paste with 100% porosity while the dark, irregularly shaped features are aggregate particles that effectively have 0% porosity. The intermediate green tones between these end members reflect the capillary porosity of the paste. Note the steady progression in brightness with w/cm.

Fluorescence Microscopy—DRP Looks Forward

We’ve been using more and more advanced image analysis methods to deconvolute these images and obtain quantitative measurements of the capillary porosity of the paste. For example, we can plot distributions of pixel intensity and map pixel intensity to understand in a quantitative fashion how the microstructure and capillary porosity of the paste changes with w/cm. Examples are shown below.

Three-dimensional pixel intensity plots

Three-dimensional pixel intensity plots, maps of pixel intensity and different distributions of pixel intensity for 0.35 and 0.55 w/cm concrete mixtures.

We’ve also found interesting applications for fluorescence microscopy to study aspects of cement hydration and construction practices such as re-tempering, finishing, consolidation and curing. We’ll post soon on those…

Interested in learning more about Cements as porous materials? Check out this paper written by Hamlin Jennings and co-authored by DRP’s very own David Rothstein.