Characterising concrete using micro X-ray fluorescence (uXRF)
Creators: name: Abdul Wahid, Fatmawati
Publisher: name: Imperial College London
Year: 2016

description: Micro X-ray fluorescence (µXRF) is a relatively new technique that is able to perform elemental analyses at high resolution. However, very few studies have been carried out to apply this technique for cement and concrete research. This thesis aims to develop, optimise, and exploit µXRF as a technique for characterising concrete and for studying transport of aggressive species such as chloride and sulphate in cement-based materials. This is important because µXRF has several advantages over existing techniques and the ability to detect the presence of aggressive species, measure their amount and rate of penetration is a key aspect in characterising long-term durability of concrete structures. The effect of seven operating conditions of µXRF on its sensitivity to chloride and sulphate was first investigated to understand factors influencing accuracy and to determine optimum operating conditions. The results show that the signal-to-noise ratio (SNR) and limit of detection (LOD) for chloride and sulphur improves when the analysis is carried out at higher beam voltages, longer acquisition times, in vacuum chamber and using a 30 µm beam spot. The application of 25 µm aluminium filter improves chloride analysis, but this is not necessary for sulphur analysis. A dead time of 30 – 50% and an amplifier time constant of 12.8 µs is recommended to obtain an optimised set-up. At these conditions, a LOD of 0.007% wt. cement for chloride and 0.003% wt. cement for sulphur is achievable. The ability to separate cement paste regions from aggregate particles during µXRF analysis is important as it helps to reduce signal interference and allow measurements solely on cement paste or aggregate particles. A new approach for identifying cement paste regions in mortars and concretes has been developed. This method is based on exploiting the change in dead time when the beam samples cement paste or aggregate regions. The main advantage of this approach over conventional elemental mapping and image analysis is speed and ease of use. Calibration graphs for chloride and sulphate in cement-based materials have been developed to enable quantitative analysis. This is done by analysing samples containing known amounts of chloride (or sulphate) and examining the strength of the correlation between measured X-ray intensities and actual amounts. A strong linear correlation (R2 &gt; 0.9) is observed for a range of samples with a different w/c ratio, binder type, curing age, and drying condition. This is also observed when samples were analysed in a wet state (as-is) in ambient environment. These findings provide evidence that the measured characteristic X-ray intensities from µXRF and the observed correlations can be used for quantitative analysis of unknown samples. Sample drying and moisture state has a major effect on the measured X-ray intensities. This was investigated to establish an appropriate preparation method for µXRF analysis of mass transport processes in cement-based materials. Rapid oven drying at 50°C distorts the measured chloride profiles. However, freeze-drying was able to preserve chloride profiles much better, producing identical chloride penetration depths to that measured before drying. Analysis on a flat ground surface provides more representative results than that on rough fractured surfaces. Finally, quantitative analysis of chloride and sulphate was demonstrated by combining the proposed dead time approach and calibration graphs. Results from µXRF are comparable to those from wet chemistry analysis (titration). The study also provides evidence that µXRF has sufficient sensitivity to detect changes in transport properties of samples with different binder types, water/binder ratio and drying conditions.