Labosystem

A small paint fragment was taken from “The Night Watch” with a scalpel and with the help of a stereomicroscope. The sample was embedded in Technovit 2000 LC mounting resin (Heraeus Kulzer GmbH, Germany). This is a one-component methacrylate that cures under visible blue light (Technotray CU, Heraeus Kulzer GmbH, Germany). The embedded sample was then wetly polished using a Struers LaboSystem polishing machine with flushing water and silica abrasive paper (grades 320 up to 2400) using a MOPAS polishing holder to hold the sample. Subsequently, the cross section was further dry polished using Micro-Mesh sheets up to grade 12,000 (Micro-Surface Finishing Products Inc., Wilton, Iowa, USA) (54 ).

A Jeol JSM-IT100 scanning electron microscope (SEM) (JEOL Ltd., Tokyo, Japan) with a Bruker energy-dispersive X-ray spectroscope (EDS) (Bruker Corporation, Billerica, MA, USA) was applied for the analysis of the microstructure and healing products. Images were taken in the secondary electron mode (SED) and in the backscattered electron (BSE) mode. Test specimens for each mix were prepared and studied in the following way. After 21 days of healing, samples were dried in the oven for one hour at 45 °C and the crack surface was evaluated for the presence of healing products. In the next step, samples were broken in two halves through the crack and the crack plane was studied. In addition, one extra 12 month old UHPC specimen, U12, was cracked and studied after 1 and 21 days of water exposure. The chemical composition of the healing products and the amount of the unhydrated cement were determined by SEM-EDS analysis. Fragments of specimens A1, U1, and U12 were impregnated in epoxy resin, ground, and polished using Struers CitoVac and Labosystem (Struers, Ballerup, Denmark). A set of grinding plates sprayed with diamond particles having sizes of 9, 3, and 1 µm were used. The amount of the unhydrated cement was calculated based on the BSE image grey histogram by thresholding the white particles [44 (link)].

The polished cross-sections of the 56-day-aged leached specimens were imaged by environmental scanning electron microscopy using a back-scattered electron detector (SEM-BSE, Zeiss EVO LS25, Oberkochen, Germany). SEM operating conditions were 1.1 nA (probe spot current) and 13 kV under low-vacuum mode (10 Pa) to prevent charging effects on the samples (no conductive coatings were needed). The cut cross-sections were first dried (at 50 °C in a ventilated oven for 1 day) and then impregnated with low-viscosity (nominally 0.6 mPa s) liquid epoxy resin (EPOFIX from Struvers, Ballerup, Denmark) using a vacuum impregnation device (CitoVac from Struvers, Ballerup, Denmark) at a pressure of 20 kPa. The sample was polished using a semi-automatic grinding–polishing machine (LaboSystem, Struers, Ballerup Denmark), initially using a resin-bonded diamond disc (hardness range HV 150 to 2000) at a rotational speed of 300 rpm. Then, it was polished to the desired level using a lubricated cloth and polycrystalline diamond spray of, consecutively, 9, 3, and 1 μm sizes at a rotational speed of 150 rpm.