## 1 Introduction  
Concrete is one of the most heterogeneous construction materials. It is often considered as a three-phase composite with inclusions in the form of aggregate particles and air voids embedded in the cement paste (matrix). Aggregates strongly influence the properties of the concrete mix and hardenedconcrete. Depending on the content of coarse aggregates, sand and paste, different portions of the concrete will have notably different compressive strengths, modulus of elasticity, shrinkage, etc. The important characteristics of the aggregates for use in concrete are particle size distribution (grading curve), maximum size, particle shape and surface texture, strength, absorption, resistance to freezing and thawing, specific gravity and bulk density (compact weight and loose weight). Granular structure is characterized by the packing density--the denser the material is, the larger is the volume of the aggregate skeleton, hence, the greater strength of the composite. Various measuring techniques are used, such as passing visible light reflected from a specimen (polished specimens) or transmitted through the specimen (thin sections), scanning electron microscopy [1; 2], computed tomography (CT) [3; 4; 5; 6; 7; 8; 9] and other advanced methods of cement paste examination. Scanning microscopes can cooperate with an energy-dispersive X-ray energy spectrometer (EDS) [10]. Thin-section tests, SEM and (more recently) computed tomography allow for the precise determination of individual phases in concrete, but only across small areas. A great amount of work is required to prepare test specimens and a large number of specimens need to be prepared. Computed tomography allows for the analysis of the distribution of various elements (phases) in space, but it is an expensive and not always available method. Traditional methods are most often used, where the analysis of the concrete composition is carried out on the surface. Because of the heterogeneity of concrete and the size of aggregate particles, analysis of concrete composition must be performed on the surface of a large specimen, which presents a basic challenge. Microscopes used in the field of mineral and concrete research are typically optical microscopes using the bright field, polarized light and dissecting/stereo techniques. Optical microscopy as the main or only tool has an advantage over other research techniques (SEM, X-ray diffraction analysis, differential scanning calorimetry, etc.) as it offers a large field of view and allows relatively simple quantitative measurements. Thin-section optical microscopy is a well-known method of investigating the microstructure of hardened concrete [11; 12; 13]. Typically, microscopic observations are carried out using polarized light with Nicol prisms in the parallel or crossed positions. The use of UV light provides an opportunity to obtain additional information necessary to assess the quality of concrete. A thin-section image under UV light provides information about the features that may indicate the quality of paste and aggregate, the air void system, structural defects such as cracks, their size and distribution, thereby helping to determine the cause of concrete distress. The optical microscopy with thin sections impregnated with a fluorescent resin is also a precise, indirect technique for determining the w/c ratio of hardened concrete [11; 13]. Developed in Denmark, the method is now in common use in other Scandinavian countries [14]. This method helps identify and analyse secondary ettringe formation, alkali-silica reaction (ASR) and the paste-aggregate interfacial zone [15]. [PERSON] et al. [12] used thin sections for the automated image analysis of particle size distributions of sand in mortars. Four photographs of each section under crossed and plane-polarized light were used to distinguish the particles.