It is evident from the current literature that mercury intrusion porosimetry (MIP) continues to be by far the most widely used method to evaluate the size distributions of pores in cement pastes and concretes. It is most unfortunate that the MIP procedure, which is a simple, straight- forward, highly reproducible experimental method, gives size distributions for hydrated cement materials that are badly flawed. Indeed, they are so badly flawed that the results obtained do not in any real sense reflect the actual sizes of the pores present. Results of comparative examinations of the same cement pastes by MIP and by backscatter SEM image analysis37 were published some years ago. For 28- day old w:c 0.40 pastes, substantial contents of pores were found in sizes between ca.10 um and the lower measurement limit of 0.8 um corresponding to the pores seen directly in visual SEM examinations of the same specimens. MIP tallied the same pores, but in sizes almost entirely smaller than ca. 200 nm. Air voids, deliberately entrained in some of the pastes, were also tallied below 200 nm in the MIP results. The problem with MIP, as indicated again in a more recent paper,38 is that it is necessary for mercury intruding from the outside of a specimen to succes- sively penetrate dozens (or hundreds) of restricted interstices on its inward path in order to reach the larger pores in the bulk of the interior of the specimen. Figure 2.8 illustrates the concept of such restricted interstices along a flow path. The figure was originally published some years ago by Hearn et al.39 to illustrate the effect of such interstices on water vapor transport, but it can just as well be applied to mercury intrusion penetration, with the understanding that MIP specimens are dried and the condensed water in the interstices has been evaporated. General penetration of mercury through a series of successive `choke points’, such as those shown in Fig. 2.8, will not occur until the necessary pressure is reached that corresponds to their size range. Once this `threshold pressure’ is reached, interior pores of all sizes (including air voids, as shown in Ref. 37) can be reached by mercury, and are filled indiscriminately as they are encountered by the incoming mercury front. In the MIP tally, they appear as sizes slightly smaller than the threshold diameter.
Some years passed before the experiments described in Ref. 37 were independently repeated. Recently, Ye40 conducted similar comparisons of MIP and image analysis pore size distributions at the Technical University of Delft. His findings fully confirmed the earlier results of Diamond and Leeman.37 An example of Ye’s results comparing pore size distributions measured by the two techniques for a 14-day old w:c 0.40 paste is given as Fig. 2.9.
These results confirm and emphasize that continued reliance on MIP for measurement of pore size distribution in cement pastes and concretes is not justified, as the sizes obtained by MIP are not even approximately correct. MIP results do, however, provide two useful parameters. The value of the threshold diameter found in a given concrete provides a measure of the degree of restriction of access by mercury to the interior of the specimen. Assuming that the same restrictions also influence the movement of water and ions, the threshold diameter can provide a useful comparative measure of permeation capacity. Furthermore, the total pore space intruded by mercury at maximum pressure provides a useful comparative, albeit incomplete, indication of the total porosity of the specimen.
