4.2 Measurements related to concrete microstructure
4.2.1 Mercury intrusion porosimetry
The following diagrams show results of mercury intrusion porosimetry tests conduc- ted on the concrete samples. In order to facilitate the interpretation of the results, ranges indicating the type of pores according to the pore size distribution proposed by Setzer [140] (see also Chapter 2.1.1.1) are labelled on the top of the figures. Fig. 4.9 presents the differential intrusion curves of the concretes MLC, MRC and MHC condi- tioned at 20 °C and 100 % RH.
0 . 0 0 1 0 . 0 1 0 . 1 1 1 0
0 . 0 0 0 0 . 0 0 5 0 . 0 1 0 0 . 0 1 5 0 . 0 2 0 0 . 0 2 5 0 . 0 3 0 0 . 0 3 5 0 . 0 4 0
M e s o - c a p i l l a r y M i c r o - c a p i l l a r y
dv/dlogr [ml/g]
P o r e a c c e s s r a d i u s , r [ µ m ]
M L C M R C M H C M e s o g e l
Figure 4.9: Pore size distributions of the concretes at reference conditions
Since the samples from 4.9 were subject neither to drying nor to elevated temperatures, they are considered as reference for further comparisons. Having concrete MLC the lowest w/c-ratio, it is expected to show the lowest porosity, specially in the range of capillary pores. This assumption can be corroborated by the measurements presented in Fig. 4.9. The curve of concrete MLC represented by the dotted line lies under the curves of the concretes MRC and MHC in the ranges of micro and meso-capillary pores. The differences in the microstructure of the concretes MRC and MHC are however less easy to identify. While concrete MRC shows a higher content of meso-capillary pores, the content of micro-capillary pores of the concrete MHC is clearly higher and in the range of mesogel pores, the curves of both concretes are very similar.
Fig. 4.10 provides information about the influence of temperature and moisture content on the microstructure of the concrete MRC after being dried at 20 °C and 95 % RH and then heated up to 60 °C. The dotted line corresponds to the measurements conducted on samples sealed before the temperature was increased. As mentioned in Chapter 3.2.4, these samples were conceived to appraise the effect of temperature alone avoiding any influence of drying after increasing temperature. However, as it can be seen in Fig. 4.10 in comparison with the reference curve (continuous line), the dotted line lies above in the range of mesogel pores and below in the range of micro and meso-capillary pores which is an indication of a densification of the concrete microstructure possibly due to hydrothermal reactions.
0 . 0 0 1 0 . 0 1 0 . 1 1 1 0
0 . 0 0 0 0 . 0 0 5 0 . 0 1 0 0 . 0 1 5 0 . 0 2 0 0 . 0 2 5 0 . 0 3 0 0 . 0 3 5 0 . 0 4 0
M e s o - c a p i l l a r y M i c r o - c a p i l l a r y
dv/dlogr [ml/g]
P o r e a c c e s s r a d i u s , r [ µ m ]
2 0 ° C / 1 0 0 % R H 6 0 ° C / s e a l e d / 1 - 7 d 6 0 ° C / 9 5 % R H / 6 0 d 6 0 ° C / 9 5 % R H / 9 0 d M e s o g e l
Figure 4.10: Pore size distribution of concrete MRC dried at 20 °C / 95 % RH and then subject to 60 °C / 95 % RH
Densification of the concrete microstructure was also seen in the samples that were let free to dry at 60 °C and 95 % RH represented by dashed lines in Fig. 4.10. After 60 and 90 days of drying, the measurements show an increase in the smallest pore sizes while
4.2 Measurements related to concrete microstructure
in range of the meso-capillary pores the dashed curves lie below the curve of reference.
Therefore, it can be stated that the samples heated while having a high amount of water in their microstructure were not only influenced by the effect of temperature alone but also by the combined effect of temperature and moisture in form of hydrothermal reactions.
Fig. 4.11 presents the results of the tests conducted on samples that were previously dried at 20 °C / 85 % RH and then subject to 60 °C / 75 % RH. Analogous to the results previously discussed, the sealed samples (dotted line) show a possible densification of the microstructure as evidenced by the increment of the mesogel pores and reduction in the range of meso-capillary pores when comparing with the reference curve (continuous line). This is not the case of the samples drying at 60 °C / 75 % RH (dashed lines).
Possibly due to the lack of water, hydrothermal reactions did not develop. The dashed curves are displaced to the right which implies an increase of the porosity mostly in the range of micro-capillary pores and the small mesogel pores vanished.
0 . 0 0 1 0 . 0 1 0 . 1 1 1 0
0 . 0 0 0 0 . 0 0 5 0 . 0 1 0 0 . 0 1 5 0 . 0 2 0 0 . 0 2 5 0 . 0 3 0 0 . 0 3 5 0 . 0 4 0 0 . 0 4 5
M e s o - c a p i l l a r y M i c r o - c a p i l l a r y
dv/dlogr [ml/g]
P o r e a c c e s s r a d i u s , r [ µ m ]
2 0 ° C / 1 0 0 % R H 6 0 ° C / s e a l e d / 1 - 7 d 6 0 ° C / 7 5 % R H / 6 0 d 6 0 ° C / 7 5 % R H / 9 0 d M e s o g e l
Figure 4.11: Pore size distribution of concrete MRC dried at 20 °C / 85 % RH and then subject to 60 °C / 75 % RH
The microstructural changes suffered by samples previously dried at 20 °C / 65 % RH can be evaluated in Fig. 4.12. Increments of the micro-capillary pores and vanishing of the small mesogel pores were measured in the samples after being subject to drying at 60 °C / 65 % RH during 30 and 90 days. Any development of hydrothermal reactions at such low relative humidities can be discarded. In comparison with the samples pre- viously dried at 20 °C / 85 % RH (see Fig. 4.11), the displacement to the right of the curves from the samples previously dried at 20 °C / 65 % RH is less significant which may imply that higher content of water in the concrete leads to higher deterioration of its microstructure. This interpretation must however be confirmed by the measurements of mechanical properties.
0 . 0 0 1 0 . 0 1 0 . 1 1 1 0 0 . 0 0
0 . 0 1 0 . 0 2 0 . 0 3 0 . 0 4 0 . 0 5 0 . 0 6 0 . 0 7
M e s o - c a p i l l a r y M i c r o - c a p i l l a r y
dv/dlogr [ml/g]
P o r e a c c e s s r a d i u s , r [ µ m ]
2 0 ° C / 1 0 0 % R H 6 0 ° C / 6 5 % R H / 3 0 d 6 0 ° C / 6 5 % R H / 9 0 d M e s o g e l
Figure 4.12: Pore size distribution of concrete MRC dried at 20 °C / 65 % RH and then subject to 60 °C / 65 % RH
The results of the mercury intrusion porosimetry tests show that samples containing a high amount of moisture (above 75 % RH) that were sealed and then heated during 1 to 7 days as well as samples that dried at very high relative humidity (95 %) during 60 and 90 days experienced a densification of their microstructure due to hydrothermal reactions. These results only identify whether such reactions took place or not and do not allow to draw any conclusion about the time-development of the hydrothermal reactions.
Complementary measurements conducted on the concretes MRC at 80 °C as well as on the concretes MLC and MHC are presented in the Appendix B.2.