Microstructural investigations

To investigate accessible porosity in range of capillary forces up to largest capillary size as well as the corresponding pore size distributions, Mercury Intrusion Porosimetry tests were carried out following DIN 66133. Two fully automated porosimeters, one called Pascal 140 from Fisons Instruments and another one called Pascal 440 from Thermo Electron corporation, enabling operating pressure of maximum 300 kPa and 400 MPa, respectively, were used for the purpose. The necessary samples were obtained from small beams cured in standard manner. At the chosen ages of 1, 3 and 28 days, the big specimens were shattered

and afterwards the hydration of much smaller test pieces sieved to size 4-8 mm was stopped. Typically, gentle oven drying at the temperature of 40 °C was applied until the mass became constant. For comparison reasons, however, other methods of hydration cessation were applied as well, including freeze-drying and combined method, i.e. treatment with isopropanol under vacuum in the first 24 hours and subsequent oven drying at 40 °C until mass constancy. Approx. five grams of so prepared material were finally used for each measurement. While simplifying that pores are cylindrical in shape, the pore entry diameter was calculated using Washburn’s equation (Eq. 3.9):

Hg Hg Hg

p

D=−4⋅γ ⋅cosθ (3.9)

where D is pore entry diameter, γHg is surface tension of mercury (considered as 0.48 N/m),

θHg is contact angle of the mercury on the solids (assumed to be 140°) and p is applied,

(intrusion) pressure

To characterize porosity of materials even better, the MIP data obtained including pore sizes and corresponding intruded volumes was used to determine characteristic parameters. The found median pore diameter yielded pore diameter at which 50 % intruded pore volume was observed. Critical pore diameter, to be addressed hereafter as modal pore diameter, was second parameter concerned. It was obtained from maximum of the derivative of the pore distribution curve. It was understood to demonstrate the smallest pore size diameter of the subset of the largest pores which creates connected path through sample, after [Hal 95]. Eventually, threshold diameter above which there was comparatively little intrusion in the pore system became known as well. Yet, since regularly having similar values compared to those of modal pore, results will not be presented in this thesis.

Analysis of pores having size of air bubbles and larger was performed by the so-called

pore count method in compliance with DIN EN 480-11, although on somewhat smaller specimen size. The RapidAir 457 Automated-Air-Void-Analyzer employed in tests comprised of computerized control unit, a high-resolution video camera and a microscope objective mounted on moving stage.

One cube with side length of 100 mm was manufactured for each test and was maintained sealed until concrete age of 28 days. On examination date, the specimen was sectioned by use of water-cooled concrete saw. One or maximum two disks having thickness of approx. 20 mm were obtained from central part of the member and provided for grinding and polishing until cut surface was removed and surface appeared even. Subsequently, hydration was cessed using oven drying. Afterwards, a contrast enhancement by darkening of specimen, followed by filling the voids with fine powder of barium sulphate, took place. In result of the procedure, the voids appeared bright white and the rest was black (Figure 3.12), facilitating void detection/identification in the measurement and its maximum precision. After the plane section was mounted onto the moving stage situated under the camera, the measurement started. Each specimen was tested twice, with the second scan being run on surface rotated by 90°. The total measuring length approached 2400 mm. Information on air content, pore distribution, specific surface area and spacing factor were calculated automatically by the software.

Total porosity was determined based on true and bulk density of materials matured to age

between 1 and 28 days, for assessment of which two tests were necessarily conducted.

Helium pycnometry was engaged to quantify true density. The apparatus model AccuPyc

1330 Pycnometer from Micrometrics company was used, where two chamber arrangement and gas displacement technique is applied to yield pore-free density. The bulk density was assessed based on ratio between known mass of dry material and its volume found by

hydrostatic weighing method or, in other words, test based on Archimedes’ principle. Prior

to each measurement, sample from specimen shattered to pieces of 4 to 8 mm and obtained from selfsame concrete batch as MIP test pieces were dried in oven until mass constancy. Gentle drying at the temperature of 40 °C was executed. For the estimation of true density, the milling process was applied in addition.

Figure 3.12: Sample for pore count measurement and appearance of lapped surface after white-and- black contrast enhancement.

The total pore volume content was calculated from Eq. 3.10: 100 1 , ⋅      − = skeleton bulk total pore V ρ ρ (3.10)

where Vpore,total is total pore content [%], ρbulk is the bulk density [g/mm³] and ρskeleton is the

true density [g/mm³].

Vicinity of important transition zones as well as both physical and chemical features of large pores were investigated by means of Environmental Scanning Electron Microscopy

(ESEM) coupled with Energy-dispersive X-ray spectroscopy (EDX). In the tests, ESEM XL30 apparatus from Philips company was used.

The idea of experiment is based on gas ionization principle. Measurement involved several steps. The chamber was vented first. Afterwards, the doors of chamber were opened and piece of material under study was placed on the stage. Subsequently, chamber’s doors were closed and the wet mode was applied. No measurements were performed until destined pressure of 2.0 Torr was reached. This was followed by adjusting the position of sample and setting relevant measurement features, i.e. value of condenser, value of beam and scan rate. Some adjustments of first images obtained were necessary as well (brightness, contrast, focus). Eventually, at location of interest, the sample was measured and the pictures taken were saved. Unless mentioned otherwise, samples at the age of 28 days were studied to provide sufficient strength and thus resistance to high vacuum and to the aggressive electron beam. Some microstructural damage was expected to take place for younger specimens.