2. Experimental procedure
Ten mortar samples were prepared using five different sands
(Table 1) according to Belgian standard NBN 771-10, which is
currently used for the preparation of cement-based mortars. Since the procedure is oriented towards the application to ancient
(mainly lime) mortars, hydraulic lime (Unilit TD 13N) was preferred
to Portland cement as a binder. After drying, the samples
were twice impregnated under vacuum with a fluorescent (FITC)
dyed epoxy resin to enhance the contrast between the sand grains
and the inter-granular space. Thin sections with a thickness of
30 μm were prepared from these mortar samples.
The micrographs were acquired with an Evolution LC digital
camera attached to a Zeiss petrographical microscope. The
images, which have a resolution of 833 pixels/mm, were made
using a Zeiss 5X objective and incident fluorescent light. Subsequently,
the micrographs were processed with the Image Pro
Plus® software package. To verify the pertinence of the image
analysis approach, comparison with another valid technique is
necessary. Sieving analysis is probably the most widespread,
generally accepted and easiest way of collecting information on
the grain-size distribution of aggregates. For the five sands used
as aggregates, the sieving curves (Fig. 1) have been established
according to Belgian Standard NBN B 11-013.