Energy Dispersive X-ray Spectrometry for Determining Chloride Concentration Profiles in Concrete
Liam Holloway1 and Nimue Pendragon2
1Principal Engineer, MEnD Consulting
2Lead Consulting Scientist, Microanalysis Australia
Abstract: Assessing the condition or durability performance of existing reinforced concrete infrastructure can involve a broad range of investigation techniques. Investigations during the life of the structure can be used to inform the asset management process. A typical test reg ime will involve visual inspection, cover meter surveys, electrochemical testing, strength assessments, and the collection of core samples for a range of analytical chemical testing. In cases where there is a perceived risk of Alkali ‐ Silica Reaction occurr ing, the testing regime may also include uranyl acetate fluorescence screening, or petrography. Petrography can be done using optical microscopy or Scanning Electron Microscopy (SEM). While petrography can be used to determine the likelihood of ASR, it is also able to give an indication of other concrete properties including, porosity, cement content, and strength. SEM petrography with the combination of Energy Dispersive X – Ray Spectroscopy (EDS) can also provide semi quantitative assessment of other proper ties including but not limited to; aggregate cement ratio, and chloride content. In this paper, we propose that the use of SEM petrography and EDS can provide a more efficient and cost effective way to gather most of the information required to perform dur ability assessments of existing structures. While the use of SEM petrography is not a new concept , using EDS to determine chloride concentration profiles is a novel approach. In this paper, we compare chloride profiles from SEM – EDS against those gained fr om conventional wet chemistry. The results from both assessment techniques were fit to estimate the apparent chloride diffusion coefficients .
For a full copy of the paper please use the following link:
Customisable Mineral Liberation
From lithium to heavy metals, Microanalysis offers customisable mineral liberation and association analysis for ores and concentrates of all kinds. Complementary to our petrographic capabilities, Microanalysis can analyse a range of particulate sizes up to 3 mm and provide additional manual analysis for particles >3 mm.
INCA Mineral by Oxford Instruments (product information available here) is a software package allowing for automated mineralogical analysis including mineral liberation, association data, size, morphology, deportment and more. INCA Mineral can emulate QEMScan data outputs, and be customised so you get only the information you need.
Usually QEMScan is recommended for particles up to to 500 µm – using a custom setup of INCA Mineral, our technique can analyse particles up to 3 mm. Still too small? Microanalysis has developed an equivalent, manual, technique for particles >3 mm using a combination of correlated optical and electron imaging, EDS and X-ray mapping.
Speaking of small – light elements (H, He, Li, Be) are always problematic from an analytical point of view. Too small for XRF and even EDS, indirect detection methods are required to ‘see’ lithium in mineral samples. Using image contrast and Si:Al ratio we differentiate spodumene and petalite from non-lithium aluminosilicate minerals, allowing us to determine the liberation and association of lithium bearing minerals. Lithium-bearing micas are even trickier, with a wide range of elemental compositions, so we combine our geological and petrographical expertise to characterise the lithium bearing micas and create a ‘fingerprint’ for the lithium bearing species, using the trace substitutions and specific elemental ratios to classify these minerals. We cross check our data with XRD results to give a comprehensive mineralogical assessment.
Whether it’s characterisation of light element minerals, characterising impurities in high purity sands, or simply gaining an idea of what your sample is, Microanalysis can help you get the most from your samples using our technology and expertise.
Petrographic Analysis & Thin Sectioning
Thin sections are 30 µm (0.03 mm) thick slices of rock, minerals, concrete, mortar, or other materials that are mounted to a glass microscope slide with epoxy and topped with a glass coverslip. They are typically analysed by geologists and petrographers by using a petrographic microscope (sometimes referred to as a polarizing light microscope). Petrographic microscopes allow experienced analysts to investigate the sample in great detail to collect data regarding the sample mineralogy, overall texture, individual mineral texture, microstructures present, and much more. In the case of rocks and minerals, a geologic history can be interpreted by the data collected via petrographic examination. With regard to concrete, it allows for deformation mechanisms to be identified and for many other issues to be investigated.
How exactly are standard thin sections made? First, a sample is cut to fit on a standard petrographic microscope slide (typically 27 x 46 mm). Once cut, one side of sample is lapped (semi-polished) to create a flat, smooth surface. The flat smooth surface is then mounted to an etched microscope slide with epoxy. Once the epoxy has set, the sample is prepped with a thin sectioning system. These systems have two components: a saw blade and a grinding wheel. The glass slide with the mounted sample is held to a vacuum chuck by a vacuum pump. The chuck with the sample is safely guided into the saw blade, cutting it to a thickness of <1 mm. After cutting, the sample is moved to the grinding wheel side of the system. The sample is rocked back and forth along the grinding wheel, and a micrometre is used to measure how much of the sample has been ground away. When the thickness of the sample is near 30 µm, the sample is removed from the system and ground by hand to reach a final thickness of 30 µm (thinner than human hair). A coverslip is applied to the finished section with epoxy to fill in imperfections in the slide and to protect the sample from scratches and other damage.
With regard to a geologic investigation or a structural evaluation of concrete, the devil is in the details. Petrographic examination of thin sections pertaining to those investigations or evaluations allows for those details to be documented and evaluated in a way that no other technique can match.
Finished Slide: 30 µm thickness
Plane-Polarized Light: Field of view from left to right is approximately 2mm
Crossed-Polarized Light: Field of view from left to right is approximately 2mm
For more information on how we can assist you with thin sectioning or petrographic examination, please contact our Specialist Geologist/Petrographer Dan Cukierski or Rick Hughes on +61 8 9225 5810.
We are hanging up our lab coats and dusting off those tuxes and gowns ready to attend 2017 Telstra Business Awards at Crown on Friday, 7th July 2017.
The team would love to see a short message of support from you on the big screen at the event. Send us your message today at telstrabusinessawards.com/mos“.
Follow us on the night as we post photos on our Instagram and Facebook
Wish us luck!
Capturing the Art Behind the Science
I like to think a lot of what we do at Microanalysis has an element of artistic flare. The images we capture on the SEM and Solarius are often quite beautiful. We recently invited the highly experienced and exceptionally talented Andrew Tyndall to come and photograph our scientists at work. His work has encapsulated the art behind the science. We look forward to sharing some of these images with you over the next few months.
Check out Andy’s website and Facebook page for more of his incredible work.
