## 4 Conclusions  
The article presents the problems of distinguishing individual phases, mainly aggregates, in concrete. The following conclusions were formulated:  
1. The attachment mounted on the lens of a stereoscopic microscope significantly extended the potential of a regular stereoscopic microscope in the analysis of hardened concrete composition. The attachment was equipped with a focused light source. Light rays falling almost vertically reflected off the surface of the specimen, which resulted in the image visible in the right part of the field of view in the lens. This image was transmitted to a computer by means of a camera. The rays reflected much better on the particles of sand and aggregate than on the porous cement paste. Many characteristics of the extracted aggregate particles can be evaluated on the basis of the obtained large image subjected to morphological operations. The contents of the paste and concrete components will be determined using known air content tests.
2. The professional microscopic set with the attachment allowed for recording and analysing much larger areas of concrete samples (e.g., 50 cm\({}^{2}\)) compared with other methods which are capable of examining an area of only several square centimetres.
3. The proposed method is not universal, because it is not always possible to automate it. In the case of coarse aggregate, which is porous or cracked, the luminous flux was significantly dispersed without giving a clear reflection. This required additional work in order to manually correct the imperfections in the binary image. Manual correction consists of manually drawing particle outlines in the graphic editor, filling them and then separating the connected objects (the automatic separation function available in the computer program was unsatisfactory).
4. The results obtained from the analyses were very promising and provided a method with the potential to be applied in practice to analyse the composition of hardened concrete extracted from the structure, determine the paste volume as the basic parameter necessary to calculate the air void spacing factor L to EN 480-11, establish the sand particle size, necessary for the design of renovation mortar compositions (restoration of historic buildings) and, in the case of concrete mix segregation, to determine the volume concentration of sand and coarse aggregate in different layers of hardened concrete.
5. The proposed method was simple and inexpensive.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Constituent** & **kg/m\({}^{3}\)** & \
\begin{tabular}{c} **Constituent Density** \\ **kg/dm\({}^{3}\)** \\ \end{tabular} &
\begin{tabular}{c} **Volume Fraction** \\ **\%** \\ \end{tabular} \\ \hline W & 151 & 1.0 & 15.1 & \\ C & 338 & 3.1 & 10.9 & P = 26.9 \\ SF & 18 & 2.2 & 0.8 & \\ S & 615 & 2.63 & 23.4 & \\ B2-8 & 588 & 2.97 & 19.7 & BS = 67.3 \\ B8-16 & 718 & 2.97 & 24.1 & \\ Air content & - & - & 5.8 & A = 5.8 \\ Concrete density & 2437 & - & - & - \\ \hline \hline \end{tabular}
\end{table}
Table 2: Concrete composition data.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Area of Measurement** & **Aggregate BS\({}^{2D}\)** & **Air Content A\({}^{2D}\)** & **Cement Paste P\({}^{2D}\)** \\
**AOM** & **\%** & **\%** & **\%** \\ \hline
1 & 69.30 & 4.71 & 26.0 \\
2 & 65.96 & 6.03 & 28.0 \\
3 & 70.09 & 3.76 & 26.2 \\ Mean & 68.45 & 4.83 & 26.7 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Microscopic measurement results.  
**Author Contributions:** Conceptualization, J.W. and A.M.; Data curation, J.W. and A.M.; Formal analysis, J.W. and A.M.; Investigation, J.W. and A.M.; Methodology, J.W. and A.M.; Supervision, J.W. and A.M.; Validation, J.W. and A.M.; Visualization, J.W. and A.M.; Writing--Original draft, A.M.; Writing--Review and editing, J.W. and A.M.  
**Funding:** This research received no external funding.  
**Conflicts of Interest:** The authors declare no conflict of interest.