The NMR probe head for the water extraction experiments is presented in figure 1. In this set-up a cylindrical brick sample with a length of 50 mm and a diameter of 20 mm, is placed upon a 6 mm layer of mortar. This 6 mm layer represents half of the thickness of the mortar joint which is used in Dutch building practice.

absorption brick

Figure 1: Insert for measuring moisture profiles during the water extraction

A complete moisture profile is now measured by performing spin-echo experiments at various fixed frequencies. As it is needed to measure a profiles as fast as possible a so-called multiple slice technique was used. This technique enables a high repetition rate of the spin-echo experiments: typically it took 90 s to measure an entire moisture profile over 20 mm, using a spatial grid of 0.15 mm. During the measurements a time stamp is added to each point of such an experimental profile. The moisture profile at a specific time is obtained by interpolating subsequent experimental profiles as a function of time for each position of the grid using least-squares fits with cubic splines.
In figure 2 the results are presented for initial dry sand-lime brick. The dashed curves indicate the initial moisture profiles in the brick and the mortar. The shaded area again indicates the boundary area. As the one-dimensional resolution of our instrument 1 mm, the moisture profiles in this area are only given as a guide to the eye; the actual moisture profiles may be steeper.

Figure 2: Moisture profiles as measured for sand-lime brick. The experiment lasted 1.5 hours.

As can be seen the moisture profiles in the mortar remain rather flat during the water extraction process, indicating that the moisture diffusivity for liquid water in the mortar is larger than that in the brick at these moisture contents. It is evident that most of the water is extracted out of the mortar within the first 180 s. As can be seen from this figure, in the case of sand-lime brick an almost stationary moisture content in the mortar is reached within 6 min.