Graduate Thesis Or Dissertation
 

The Evaluation of Cementitious Pore Solution Composition and Electrical Resistivity Using X-ray Fluorescence (XRF)

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/z890s045r

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  • Interest in performance specifications has been growing in the civil and construction industry in the past decade. One major focus area has been on understanding how to prolong the service life of concrete structures, since repair and rehabilitation of existing infrastructure have cost many trillions of dollars. Deterioration mechanisms such as corrosion can shorten the service life of a structure and are typically determined by the moisture and ionic species ingress into the concrete, or, in other words, the transport properties of the concrete. Ionic transport in concrete can be described using the formation factor, which is defined as the ratio of the resistivities of the concrete and the pore solution. Therefore, there is significant value in rapid and simple methods to measure these electrical properties. Measuring the resistivity of concrete, or bulk resistivity, is relatively straightforward; however, measuring the pore solution resistivity is more complex since extracting pore solution from hardened concrete is rather challenging. The pore solution resistivity value may be assumed from literature, directly measured using a resistivity meter, or computed from the pore solution composition using different chemical analysis methods. The objective of this thesis is to investigate the use of X-ray fluorescence (XRF) as a chemical analysis method to obtain the chemical composition of the pore solution which enables the calculation of pore solution resistivity. The first part of this study focuses determining the feasibility of using XRF to assess the chemical composition of the main ionic species in simulated pore solutions and to calculate the pore solution resistivity from the chemical composition. Two analysis methods were explored: the solution method and the fused bead method. The measured ionic concentrations were compared to theoretical concentrations; the calculated resistivities were compared to measured resistivities using a resistivity meter as a direct measurement. The results from this study showed that XRF can accurately detect the ionic composition of simulated pore solutions and can be used to accurately calculate the pore solution resistivity using both methods of analysis. The second part of this study focuses on measuring the ionic concentrations and calculating the resistivity of expressed pore solutions. The influence of test parameters such as sample size and storage time on the composition and resistivity was also studied. The calculated resistivities were compared to measured resistivities using a resistivity meter as a direct measurement. Chemical composition and resistivity determined using XRF were also compared with an online pore solution conductivity calculator developed at the National Institute of Science and Technology (NIST). The results from this study showed that the calculated resistivities from XRF match the measured resistivities from the resistivity meter. Therefore, it can be concluded that XRF can be used to accurately calculate the electrical resistivity of pore solutions. Chemical compositions determined from the XRF matched the ones determined from the NIST calculator after 24 hours of expressed age (but not earlier), since the NIST calculator neglects sulfate and calcium, which are present in significant amounts in pore solutions before 24 hours. In conclusions, the results from this thesis indicate that XRF is a potential alternative to time consuming methods which are currently used to determine the pore solution composition that can then be used to predict resistivity. This method could potentially bring benefits in terms of time and cost reductions, since XRF is a device commonly used in the cement industry. 
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