Microstructural investigation tests

5.3.1 SEM microscopy

The scanning electron microscope is widely used in concrete petrography to produce images with adequate depth of field, and to perform element analysis of specimen surfaces with the resulting X-rays of the electron beam. However, the electron beam penetrates the surface of specimens only by a few microns; therefore, analysis only of surface elements is possible with this technique. Analysis of elements can be quantitative if a polished surface is used. However, in this study, surfaces of the samples examined with the scanning electron microscope were fracture faces. Thus, element mapping could only be qualitative. The microscope used in this investigation was a HITACHI 3200 Scanning Electron Microscope, Figure 5.5.

Figure 5.5 HITACHI 3200 Scanning Electron Microscope

The specimens, previously prepared as per section 4.3.1.1, were dried for 24 hours at 60°C before testing, as higher temperatures could cause cracking. They were then split along their longitudinal axis, which, allowed for visualisation of the effects of the solutions on the same sample, simply by moving the cut profile under the microscope.

The reason for splitting the specimens instead of cutting them with a diamond saw was the concern that some of the features seen on the fracture face would be altered.

Chapter 5: Testing Procedures microscopy testing was conducted at the following ages of specimens:

■ At the end of curing (28 days after casting)

■ 28 days after the end of curing

■ 6 months after the end of curing

■ 1 year after the end of curing

A wide range of magnifications was used because the high and low magnifications served different purposes. Lower magnification offered visual and element mapping information with respect to layering of possible deposits on the surface of the sample or alterations of the chemical composition of layers in the sample. Higher magnifications offered the possibility of visual identification of individual features or the absence of such, as an indication of specific phenomena.

5.3.2 X-Ray Diffraction spectrometry (XRD)

X-ray diffraction spectrometry (XRD) can directly identify crystalline phases in concrete and other materials. A finely ground powder sample is normally inserted in the diffractometer, and the XRD data obtained and recorded with the assistance of a computer. The data is in the form of a set of peak patterns where different patterns corresponding to different phases. In order to identify the phases present, a comparison is made with a database of established peak patterns for known materials and compounds.

One of the limitations of the XRD technique lies in the fact that it cannot identify amorphous materials. In the case of concrete that is a potential problem as the rather feebly crystalline phases of C-S-H result in small peaks which are hard to identify

among the peaks of ealcium hydroxide and other mineral phases in the concrete. In general, feebly crystalline phases return many and weak peaks, while the more crystalline phases return fewer and taller peaks [9]. Another significant limitation is that there has to be adequate quantity of a crystalline phase present in the sample under examination in order for it to be identified. Although this amount varies and depends on the crystallinity of the substance, it can generally be said that if less than 2% of a substance is present, this technique may not be able to identify its presence [10]. For these reasons X-ray diffraction could be considered only a semi-quantitative technique.

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(a) Placing ground sample on the plate (b) Inserting the plate in the diffractometer

Figure 5.6 Placing of ground samples in the diffractometer for testing

Each sample was prepared as described in section 4.3.1.2, and taken for testing immediately after grinding and drying. Powder samples were mounted on a special mold plate as shown in Figure 5.6 (a) and (b). The top surface of the sample was then pressed flat at the same level as the top surface of the plate and any excess powder was removed with a brush. The plate was, in turn, inserted into the diffractometer.

The cabin door was then properly locked and data was acquired with the help of a computer and specialised software X’Pert Data Collector by Panalytical.

The diffractometer used in this study was the Panalytical X’Pert Pro, Figure 5.7.

Measurements were between 20 angles of 10 to 60 and the 20 step size of the scan

Chapter 5: Testing Procedures

was 0.017. Each scan lasted eight minutes. X-ray diffraction spectrometry tests were conducted at the following specimen ages:

Upon the end of curing (28 days after casting) 3 months after the end of curing

1 year after the end of curing

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Figure 5.7 The PANalvtical X’Pert PRO X-Ray Diffractometer used in the investigation

The data was analysed with the help of the X ’pert Highscore software by Panalytical.

The results were used for correlation with the findings of the SEM investigation and the thermal techniques described in section 5.3.3. Although, XRD did not offer fully quantitative data, it offered a means of testing the key assumptions made (based on the literature and the results from the rest of the microstructural investigation techniques employed) regarding the phases present on the surface and inside the paste samples as a result of their contact with the de-icers.

5.3.3 Thermal analysis techniques (DSC and TGA)

The two thermal analysis techniques employed, namely Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA) both consist of the

stepped controlled heating of a sample in a furnace and the recording of changes that take place. TGA records the changes in weight of the sample and its results are expressed in percentage of the initial weight while DSC measures the changes in energy and its results are expressed in mCal/s.

Most of the tests in this study were carried out on a Stanton Redcroft STA 1500 Simultaneous Thermal Analyser, but some of the samples were tested on a TA Instruments TA Q600 due to a fault that the STA 1500 developed. The equipment had the capability of conducting combined experiments employing both techniques concurrently for the same sample. Thus, measurements for both techniques were taken concurrently from each sample. The temperature range used in this study was 25- 1250°C. As outlined in section 4.3.1.2, the surface layers of the specimens were removed before testing and core samples taken from the specimens with a mass of about 30mg to 40mg were tested. The initial mass of each sample was recorded and displayed on the resulting plots.