Acta Polytechnica CTU Proceedings https://doi.org/10.14311/APP.2025.53.0076 Acta Polytechnica CTU Proceedings 53:76–81, 2025 © 2025 The Author(s). Licensed under a CC-BY 4.0 licence Published by the Czech Technical University in Prague MICRO-SCALE PROPERTIES OF CEMENT PASTES EXPOSED TO GAMMA RADIATION Jiří Němeček1a,∗, Andrea Špakováa, Jiří Němeček2a, Patricie Halodováb a Czech Technical University in Prague, Faculty of Civil Engineering, Department of Mechanics, Thákurova 7, 166 29 Prague 6, Czech Republic b Research Centre Řež, Hlavní 130 Řež, 250 68 Husinec, Czech Republic ∗ corresponding author: jiri.nemecek@fsv.cvut.cz Abstract. The extension of the lifetime of nuclear power plants is a critical issue today, raising concerns about the microscale mechanisms and mechanical changes in the individual phases of concrete that contribute to its macroscopic deterioration. Accurate quantification of these changes is essential for more reliable lifetime predictions. This paper investigates the effects of gamma irradiation on cement pastes exposed to varying relative humidity levels (RH = 11–100 %) using scanning electron microscopy (SEM) and nanoindentation. While SEM analysis did not reveal significant phase changes through image analysis, nanoindentation highlighted substantial differences in the mechanical properties of samples exposed to either low or high humidity or submerged in water during irradiation. Keywords: Gamma irradiation, cement paste, humidity, nanoindentation, electron microscopy. 1. Introduction Concrete, one of the most widely used man-made ma- terials globally, is essential in nuclear power plants (NPPs) for biological shielding, reactor vessel founda- tions, spent fuel storage pools and radioactive waste storage containers. However, long-term degradation due to radiation and thermal strains has been exten- sively documented, resulting in a decline in macro- scopic mechanical properties like compressive and ten- sile strengths, as well as the macroscopic modulus of elasticity, as summarized by [1] and [2]. This degrada- tion ultimately leads to the deterioration of reinforced concrete structures in NPPs [3] otherwise designed for 80 years of operation [4]. The primary causes of this deterioration are neutron and gamma irradiation, along with heating and dry- ing. During its lifetime, concrete used as a biological shield is exposed to 100–200 MGy of gamma irradi- ation and neutron fluxes of approximately 6 × 1019 n cm−2 (with E > 0.1 MeV) [1]. Neutron irradiation mainly impacts the aggregates, altering their crystal- lographic structure by amorfization of minerals and causing their radiation-induced volumetric expansion (RIVE) [5], while gamma irradiation affects the ce- ment matrix by causing radiolysis of water and drying of Calcium-Silicate-Hydrates (C-S-H) [4]. Water radi- olysis is a chemical reaction in which the interstitial liquid within the C-S-H gel breaks down, leading to the formation and accumulation of gases like hydrogen (H2), other stable molecules (H2O2, H+, OH−), and highly unstable radicals (H·, OH·, e− aq). Hydrogen peroxide (H2O2) then reacts with calcium hydroxide 1Professor at Czech Technical University in Prague, ORCID: 0000-0002-3565-8182. 2Postdoctoral researcher at Czech Technical University in Prague, ORCID: 0000-0002-5635-695X. (CH) and AFt phases, forming unstable products. Fur- ther reactions of these products result in the creation of calcium carbonate (CaCO3) [6, 7]. However, there have been relatively few studies on gamma-irradiated concrete. Microscopic observations reveal that gamma irradiation decomposes evaporable water in cement pastes, and after prolonged exposure at high doses, up to 1 % of chemically bound water can also decompose [4]. During carbonation, vaterite and aragonite formed instead of calcite. Vaterite filled the pores around the C-S-H gel, increasing the stiffness and bending strength in cement pastes [7]. Nanoinden- tation at below 11 % relative humidity (RH) resulted in a 25 % increase in nanoindentation Young’s modulus due to C-S-H gel densification, accompanied by micro- cracking that led to 17% reduction in microindentation Young’s modulus [8]. Samples irradiated at 30–60 % RH showed minimal nanomechanical changes [8, 9], while those exposed to water experienced a 26 % de- crease in Young’s modulus and 9–10 % loss of Port- landite [8]. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses of samples exposed to doses up to 1 188 MGy showed decreasing crystalline phases, clinker decomposition, and microstructural damage starting at 130 MGy [10]. Hunnicutt et al. [11], who performed nanoindenta- tion on synthetic C-S-H pellets exposed to gamma doses up to 0.784 MGy at 11 % RH, found minimal changes in micro-mechanical properties. More re- cently, [12] reported significant increases in nanoin- dentation Young’s modulus (by 15–25 %) at higher exposure doses of 24 MGy and 189 MGy. Khu- murovska et al. [9] studied mortar samples exposed to 12–15 MGy under 40–60 % RH for one year, observ- ing negligible changes in Young’s modulus and creep compliance. Similarly, Hilloulin et al. [13] performed 76 https://doi.org/10.14311/APP.2025.53.0076 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 53/2025 Micro-scale properties of cement pastes exposed to gamma radiation CaO SiO2 Al2O3 Fe2O3 MgO Na2O K2O SO3 Cl− MnO (wt. %) 64.78 20.28 4.87 3.55 1.34 0.14 0.76 3.04 0.028 – Table 1. Cement composition of CEM I 42.5R (XRF data provided by the manufacturer). RH 11 % 33 % 76 % 96 % 100 % Saturated salt solution LiCl MgCl2 NaCl KNO3 - Environment Air Air Air Air Water Table 2. Environmental conditions used in sample containers. micro-indentation on samples subjected to gamma doses up to 0.257 MGy, with no changes in Young’s modulus but a reported increase in creep modulus, although environmental conditions were not provided. The formation of bubbles and cracks due to the sepa- ration of chemically bound water was also observed by SEM at a dose of 290 MGy [10]. Since it is clear that macroscopic deterioration mech- anisms of concrete begin at the micro-scale, conduct- ing experiments at smaller scales is essential. In this context, nanoindentation is a valuable technique for characterizing the micro-mechanical properties of ma- terials [14]. Given the limited research and the unclear role of RH, this study aims to clarify the influence of RH during gamma irradiation on the microstru- ture and micro-mechanical properties of cement paste using SEM and nanoindentation. 2. Experiments and methods 2.1. Samples and sample preparation A cement paste with a water-cement ratio of 0.4 was prepared using Portland cement CEM I 42.5 R (Česko- moravský cement, Czech Republic). The component composition of the Portland cement is detailed in Ta- ble 1. The mixture was poured into cylindrical plastic molds (27 mm diameter, 70 mm height) and vibrated. After 24 hours, the samples were demolded and stored in a 1 % lime water solution for 610 days (approx- imately 1.67 years). Once removed from the lime water, the samples were cut into slices of about 19 mm height using a diamond blade (Struers Secotom 50). The sliced samples were then used for two types of analyses: SEM analysis and nanoindentation under two types of exposition: irradiation and different RH. Several different RH levels: 11 %, 33 %, 76 %, 96 %, and 100 % (water) maintained by salt solutions were tested (see Table 2). The salt solutions were kept separate from the samples to prevent direct contact. 2.2. Gamma irradiation Sample slices were placed in closed plastic containers under specific RH conditions provided by salt solu- tions (or water) and gamma irraditated. The gamma irradiation experiment was conducted at the Research Center Řež in the Czech Republic using a 60Co radi- ation source. The samples in containers were left to stabilize at their respective RH conditions for 14 days. Half of the samples were then irradiated for 341 days, receiving a total dosage of 13.82 MGy and an average dose rate of 0.675–1.890 kGy h−1. The second half of the control (non-irradiated) samples were kept un- der the same RH conditions but were not exposed to gamma radiation. 2.3. Scanning electron microscopy The surfaces of the samples were polished using SiC foils with different grit sizes (#2000 to #4000) at varying speeds and pressure forces without any lubri- cant while constantly sweeping free particles out of the surface. After each polishing step, the samples were placed in isopropanol and an ultrasonic cleaner for 1 minute to remove any free particles. A Phenom XL desktop SEM was utilized to analyze the phase composition of the cement paste samples. SEM imaging offers high-resolution capabilities, al- lowing for the clear identification of different phases within the cement paste. In typical cement paste, five key phases can be distinguished: inner and outer hydration products, Portlandite, residual clinker, and pores or cracks [15, 16]. Back-scattered electron (BSE) images were captured at 900× magnification, covering an area of 298 × 298 µm2 with a pixel resolution of 146 nm. The SEM was operated at an accelerating voltage of 15 kV. For each sample, ten images were taken, covering a total area of 0.888 mm2. These images were then saved into 8-bit grayscale format, with pixel intensity values ranging from 0 to 255, and further analyzed for phase composition using ImageJ software. 2.4. Nanoindentation After irradiation and exposition to different RH, sam- ples were stored in boxes with silica gel (≈ 11 % RH) to prevent further hydration and carbonation until tested. Nanoindentation at ambient conditions (temper- ature ≈ 23 °C, RH ≈ 40–50 %) was carried out us- ing Hysitron TI-980 Triboindenter equipped with a Berkovich diamond tip and using the accelerated prop- erty mapping mode (XPM). A grid of 22 × 22 indents, spaced 2.5 µm apart, was applied to each sample. The indentation protocol followed a trapezoidal load function. The load segment involved linear loading for 77 J. Němeček, A. Špaková, J. Němeček, P. Halodová Acta Polytechnica CTU Proceedings a) b) c) d) Figure 1. Example of phase separation for the irradiated sample (RH=76 %) using image analysis for individual phases: a) inner-outer product, b) Portlandite, c) clinker, d) pores and cracks. RH 11 % 33 % 76 % 96 % 100 % Clinker IR 4.09 ± 0.86 5.11 ± 1.20 4.85 ± 0.88 4.63 ± 1.11 3.50 ± 0.94 Clinker NR 4.52 ± 0.75 4.61 ± 0.99 4.34 ± 1.34 4.31 ± 0.80 3.60 ± 0.89 Portlandite IR 5.60 ± 0.43 7.53 ± 0.46 4.52 ± 0.36 4.83 ± 0.20 6.64 ± 1.24 Portlandite NR 5.02 ± 0.68 8.18 ± 0.65 5.17 ± 0.21 5.75 ± 0.30 7.80 ± 0.34 Inner-outer product IR 81.01 ± 0.94 78.90 ± 0.99 81.13 ± 1.00 83.61 ± 1.22 82.61 ± 2.09 Inner-outer product NR 81.28 ± 0.78 77.61 ± 1.26 80.93 ± 1.54 81.61 ± 1.13 81.39 ± 0.95 Pores and cracks IR 9.30 ± 0.78 8.46 ± 1.20 9.50 ± 0.69 7.42 ± 0.62 7.25 ± 0.80 Pores and cracks NR 9.18 ± 0.69 9.60 ± 0.88 9.56 ± 0.58 8.22 ± 0.83 7.20 ± 0.61 Table 3. Image analysis results: the table shows volumetric ratios (vol. %) of the given phases for the irradiated sample (IR) and the non-irradiated sample (NR). 0.1 seconds up to a maximum load of 1.5 mN, holding the load for 0.1 seconds, and then unloading linearly for 0.1 seconds. The elastic properties were evaluated using the un- loading segment of the load-displacement curve, based on the method of Oliver and Pharr [14]. The reduced modulus, Er, and hardness, H, were determined using the equations: Er = S √ π 2β √ Ac , (1) H = Pmax Ac , (2) where Ac represents the projected contact area of the tip, S is the elastic unloading stiffness, β is the cor- rection factor for the tip geometry (with β = 1.034 for Berkovich), and Pmax is the maximum load. The reduced modulus Er reflects the combined elastic re- sponse of both the nanoindentation tip and the ma- terial. The Young’s modulus, E, of the (isotropic) material can then be determined from the following relationship: 1 Er = 1 − ν2 E + 1 − ν2 i Ei , (3) where Ei and νi are Young’s modulus and Poisson’s ratio of the nanoindentation tip, respectively. For a diamond tip, these values are typically Ei = 1 141 GPa and νi = 0.07. The Poisson’s ratio of the sample, ν, was assumed to be 0.2 ([16, 17]). 3. Results and Discussion 3.1. SEM and image analysis For the analysis of sample phase composition, SEM images were segmented into five primary phases: resid- ual clinker, Portlandite, inner product, outer product, and cracks/pores, using pixel color thresholding. How- ever, due to overlapping grayscale values between the inner and outer product phases [16], these two were grouped and analyzed as a combined “inner-outer product” phase. An example of this segmentation pro- cess is shown for the irradiated sample (RH=100 %) in Figure 1. The grayscale threshold boundaries used for segmentation were: residual clinker (255–185), Port- landite (184–150), inner-outer product (149–41), and pores/cracks (40–0). The average values and stan- dard deviations for each irradiated and non-irradiated sample are provided in Table 3. The image analysis results reveal minimal differ- ences between irradiated and non-irradiated samples across all RH levels, with almost negligible changes ob- served in the inner-outer product phase. At 100 % RH, there is a slight decrease of approximately 1 % in resid- ual clinker and about 2 % in the pores/cracks phase compared to other RH conditions, suggesting these changes are due to ongoing hydration rather than irradiation. A minor reduction in Portlandite con- tent (1.5–3.5 %) was noted at 33 %, 76 %, 96 %, and water, indicating potential decomposition from irradi- ation, consistent with previous studies [8, 10]. How- ever, these changes are within the method’s accuracy, and thermogravimetric analysis is planned to be con- 78 vol. 53/2025 Micro-scale properties of cement pastes exposed to gamma radiation ducted for more precise Portlandite estimation in the future. Additionally, SEM-BSE images were analyzed for radiation-induced crack formation, but no cracks were detected in either irradiated or non-irradiated samples. This contrasts with findings from other stud- ies. For example, crack formation was observed by Łowińska-Kluge and Piszora [10] after exposure to a gamma dose of 130 MGy, significantly higher than the dose used in our study. Similarly, Němeček et al. [8] reported crack formation in irradiated samples, though this was primarily due to prolonged exposure to the SEM vacuum chamber and not the gamma radiation exposure. 3.2. Nanoindentation A continuous spectrum of E values from 10 GPa to 130 GPa was obtained showing the large differences between individual mechanically dissimilar phases [17, 18]. Based on the correspondence with the phase locations observed by SEM, typical values of E were found as: residual clinker E = 50–130 GPa, Port- landite E = 36–43 GPa, inner product E = 26–38 GPa, outer product E = 19–26 GPa, low stiffness phase E < 15 GPa), values similar to other studies [16– 18]. The typical load-displacement diagrams of these phases separated by post-indentation SEM imaging are shown in Figure 2. In order to reveal individ- ual phase properties, all indents from a sample were merged together, and frequency density plots were created for each sample. An example of these plots is shown in Figure 3. Since the spectrum of E is continuous with a single major peak, no distinction was made between individual hydration phases and the hydrates were collectively called ’main hydrates’, meaning responses from inner-outer products and Por- tandite were grouped into a single mechanical phase. The second phase was created by non-hydrated resid- ual clinker. The two mechanical phases were sepa- rated with a statistical deconvolution method, which assumed overlapping Gaussian distributions of individ- ual phases. More details of the method can be found in [17–19]. Based on these results, the difference in micro-mechanical behavior between irradiated and non-irradiated samples was analyzed and summarized in Figure 4. The effect of irradiation is evident in Figure 4. Sam- ples stored at low RH (11 %) showed a 9.5 % increase in E compared to non-irradiated samples and a 19.9 % increase compared to irradiated samples stored at medium RH levels. This increase is likely due to drying shrinkage, leading to the densification and compaction of C–S–H [20, 21]. In irradiated samples, water radiolysis likely depletes water from the inter- layer space, further promoting densification. This trend aligns with other studies on well-matured ce- ment pastes exposed to low RHs [8, 12]. Studies on younger samples (irradiated at 4 weeks of age) demon- strated an even higher increase in E (+24.8 %) with a smaller radiation dose of 2.88 MGy [8]. These find- 0 50 100 150 200 250 300 350 400 0.0 0.5 1.0 1.5 2.0 Displacement (nm) Lo ad (m N ) Low stiffness phase Clinker Inner product Portlandite Outer product Figure 2. An example of typical load-displacement curves of main phases measured for irradiated sample (RH=76 %). 0 20 40 60 80 100 120 0.00 0.02 0.04 0.06 0.08 0.10 0.12 Young’s modulus (GPa) Fr eq ue nc y de ns ity (- ) Irradiated (RH=100%) Control (RH=100%) Figure 3. A typical example of frequency plots of Young’s modulus for irradiated and non-irradiated (control) sample (RH=100 %). 20 40 60 80 100 0 10 20 30 40 Relative Humidity (%) Yo un g’ s M od ul us (G Pa ) Irradiated Control Figure 4. Summary of Young’s moduli of main hy- drates for irradiated and non-irradiated (control) sam- ples stored at different RH levels. ings suggest that the hydration degree at the onset of irradiation significantly influences the radiolysis process and the subsequent C–S–H compaction. It is 79 J. Němeček, A. Špaková, J. Němeček, P. Halodová Acta Polytechnica CTU Proceedings important to note that the localized increase in E is accompanied by drying microcracking in the samples, which ultimately leads to a reduction in mechanical properties at larger scales [8]. Samples stored at medium RHs (33 %, 76 %) ex- hibited minimal differences between irradiated and non-irradiated states, indicating that water loss due to radiolysis is likely offset by the available moisture in the container. These results are consistent with previ- ous studies on gamma doses ranging from 0.257 MGy to 15 MGy [8, 9, 13]. The samples stored at high RH levels (96 % and water-saturated) showed the most significant deteri- oration in mechanical properties due to irradiation. The decrease in E was found to be significant (14.6 % at 96 % RH and 15.5 % in water). This deterioration is likely caused by water radiolysis, although the precise mechanism remains unclear. Similar findings were reported by Němeček et al. [8] for 28-day-old samples irradiated with a dose of 2.88 MGy. Future studies by X-ray diffraction, thermogravimetry, and porosity measurements are planned to disclose the mechanism. The residual clinker properties, as for a crystalline phase, were not impacted by gamma radiation. It aligns with the findings reported by Rosseel et al. [1]. In their study, it was noted that gamma radiation typically does not significantly affect the crystalline phases of concrete, such as residual clinker, which remains stable under gamma irradiation. 4. Conclusions The lifetime extension of nuclear power plants relies on the description of deterioration mechanisms in con- crete used for biological shield and containment struc- tures and a precise quantification of mechanical prop- erties of the micro-scale phases. Environmental expo- sure significantly influences the evolution of properties. This study investigated gamma-irradiated ordinary cement paste with total absorbed dose of 13.82 MGy exposed to five different RH levels (11–100 %) using scanning electron microscopy and nanoindentation. Based on the observations and measurements the fol- lowing conclusions were drawn. • Insignificant changes in volume fractions of C-S- H hydrated phases were observed by SEM image analysis across all RH levels. • A 1.5–3.5 % decrease in Portlandite phase was ob- served in medium to high (33–100 %) humidities, indicating its degradation due to gamma irradia- tion. • No substantial crack formation in either the irra- diated or non-irradiated samples was observed by SEM and image analysis. • Nanoindentation of samples irradiated at low RH (11 %) revealed a 9.3 % increase in Young’s modulus for the main hydrates. This increase is likely due to a combination of water radiolysis and drying, which may have resulted in the removal of water from the interlayer space and compaction of the C-S-H gel. • Samples irradiated at medium RHs (33 %, 76 %) did not exhibit a significant change in nanomechanical response. • Samples irradiated at high RHs exhibited a ≈15 % decrease in Young’s modulus, highlighting the sig- nificant impact of gamma radiation and water radi- olysis on the material’s mechanical properties. Acknowledgements This work was financially supported by the project of the Czech Science Foundation grant number 23-05435S. The presented results were obtained using the CICRR infras- tructure, which is financially supported by the Ministry of Education and Culture – project LM2023041. References [1] T. Rosseel, I. Maruyama, Y. Le Pape, et al. Review of the current state of knowledge on the effects of radiation on concrete. Journal of Advanced Concrete Technology 14(7):368–383, 2016. https://doi.org/10.3151/jact.14.368 [2] K. Field, I. Remec, Y. L. Pape. Radiation effects in concrete for nuclear power plants – Part I: Quantification of radiation exposure and radiation effects. Nuclear Engineering and Design 282:126–143, 2015. https://doi.org/10.1016/j.nucengdes.2014.10.003 [3] K. Park, H.-T. Kim, T.-H. Kwon, E. Choi. Effect of neutron irradiation on response of reinforced concrete members for nuclear power plants. Nuclear Engineering and Design 310:15–26, 2016. https://doi.org/10.1016/j.nucengdes.2016.09.034 [4] O. Kontani, S. Sawada, I. Maruyama, et al. Evaluation of irradiation effects on concrete structure: Gamma-ray irradiation tests on cement paste. In ASME Power Conference, vol. 56062, p. V002T07A002. American Society of Mechanical Engineers, 2013. https://doi.org/10.1115/POWER2013-98099 [5] I. Maruyama, O. Kontani, M. Takizawa, et al. Development of soundness assessment procedure for concrete members affected by neutron and gamma-ray irradiation. Journal of Advanced Concrete Technology 15(9):440–523, 2017. https://doi.org/10.3151/jact.15.440 [6] P. Bouniol, A. Aspart. Disappearance of oxygen in concrete under irradiation: the role of peroxides in radiolysis. Cement and Concrete Research 28(11):1669–1681, 1998. https://doi.org/10.1016/S0008-8846(98)00138-0 [7] I. Maruyama, S. Ishikawa, J. Yasukouchi, et al. Impact of gamma-ray irradiation on hardened white Portland cement pastes exposed to atmosphere. Cement and Concrete Research 108:59–71, 2018. https://doi.org/10.1016/j.cemconres.2018.03.005 [8] J. Němeček, P. Trávníček, M. Keppert, et al. Nanomechanical analysis of Gamma-irradiated cement paste exposed to different humidities. Construction and Building Materials 393:131969, 2023. https://doi.org/10.1016/j.conbuildmat.2023.131969 80 https://doi.org/10.3151/jact.14.368 https://doi.org/10.1016/j.nucengdes.2014.10.003 https://doi.org/10.1016/j.nucengdes.2016.09.034 https://doi.org/10.1115/POWER2013-98099 https://doi.org/10.3151/jact.15.440 https://doi.org/10.1016/S0008-8846(98)00138-0 https://doi.org/10.1016/j.cemconres.2018.03.005 https://doi.org/10.1016/j.conbuildmat.2023.131969 vol. 53/2025 Micro-scale properties of cement pastes exposed to gamma radiation [9] Y. Khmurovska, P. Štemberk, S. Sikorin, et al. Effects of gamma-ray irradiation on hardened cement mortar. International Journal of Concrete Structures and Materials 15(1):1–14, 2021. https://doi.org/10.1186/s40069-020-00452-7 [10] A. Lowinska-Kluge, P. Piszora. Effect of gamma irradiation on cement composites observed with XRD and SEM methods in the range of radiation dose 0-1409 MGy. Acta Physica Polonica A 114(2):399–411, 2008. https://doi.org/10.12693/APhysPolA.114.399 [11] W. Hunnicutt, E. Rodriguez, P. Mondal, Y. Le Pape. Examination of gamma-irradiated calcium silicate hydrates. Part II: Mechanical properties. Journal of Advanced Concrete Technology 18(10):558–570, 2020. https://doi.org/10.3151/jact.18.558 [12] A. Baral, E. T. Rodriguez, W. A. Hunnicutt, et al. Ultra-high gamma irradiation of calcium silicate hydrates: Impact on mechanical properties, nanostructure, and atomic environments. Cement and Concrete Research 158:106855, 2022. https://doi.org/10.1016/j.cemconres.2022.106855 [13] B. Hilloulin, M. Robira, A. Loukili. Coupling statistical indentation and microscopy to evaluate micromechanical properties of materials: Application to viscoelastic behavior of irradiated mortars. Cement and Concrete Composites 94:153–165, 2018. https://doi.org/10.1016/j.cemconcomp.2018.09.008 [14] W. C. Oliver, G. M. Pharr. An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. Journal of materials research 7(6):1564–1583, 1992. https://doi.org/10.1557/JMR.1992.1564 [15] K. L. Scrivener. Backscattered electron imaging of cementitious microstructures: understanding and quantification. Cement and Concrete Composites 26(8):935–945, 2004. Scanning electron microscopy of cements and concretes. https://doi.org/10.1016/j.cemconcomp.2004.02.029 [16] J. Němeček, J. Lukeš, J. Němeček. High-speed mechanical mapping of blended cement pastes and its comparison with standard modes of nanoindentation. Materials Today Communications 23:100806, 2020. https://doi.org/10.1016/j.mtcomm.2019.100806 [17] G. Constantinides, F.-J. Ulm. The nanogranular nature of C–S–H. Journal of the Mechanics and Physics of Solids 55(1):64 – 90, 2007. https://doi.org/10.1016/j.jmps.2006.06.003 [18] G. Constantinides, F.-J. Ulm. The effect of two types of C-S-H on the elasticity of cement-based materials: Results from nanoindentation and micromechanical modeling. Cement and Concrete Research 34(1):67–80, 2004. https://doi.org/10.1016/S0008-8846(03)00230-8 [19] J. Němeček, V. Králík, J. Vondřejc. Micromechanical analysis of heterogeneous structural materials. Cement and Concrete Composites 36:85–92, 2013. https://doi.org/10.1016/j.cemconcomp.2012.06.015 [20] J. Němeček, J. Maňák, J. Němeček, T. Krejčí. Effect of vacuum and Focused Ion Beam generated heat on fracture properties of hydrated cement paste. Cement and Concrete Composites 100:139–149, 2019. https://doi.org/10.1016/j.cemconcomp.2019.03.027 [21] P. Suwanmaneechot, A. Aili, I. Maruyama. Creep behavior of C-S-H under different drying relative humidities: Interpretation of microindentation tests and sorption measurements by multi-scale analysis. Cement and Concrete Research 132:106036, 2020. https://doi.org/10.1016/j.cemconres.2020.106036 81 https://doi.org/10.1186/s40069-020-00452-7 https://doi.org/10.12693/APhysPolA.114.399 https://doi.org/10.3151/jact.18.558 https://doi.org/10.1016/j.cemconres.2022.106855 https://doi.org/10.1016/j.cemconcomp.2018.09.008 https://doi.org/10.1557/JMR.1992.1564 https://doi.org/10.1016/j.cemconcomp.2004.02.029 https://doi.org/10.1016/j.mtcomm.2019.100806 https://doi.org/10.1016/j.jmps.2006.06.003 https://doi.org/10.1016/S0008-8846(03)00230-8 https://doi.org/10.1016/j.cemconcomp.2012.06.015 https://doi.org/10.1016/j.cemconcomp.2019.03.027 https://doi.org/10.1016/j.cemconres.2020.106036 Acta Polytechnica CTU Proceedings 53:76–81, 2025 1 Introduction 2 Experiments and methods 2.1 Samples and sample preparation 2.2 Gamma irradiation 2.3 Scanning electron microscopy 2.4 Nanoindentation 3 Results and Discussion 3.1 SEM and image analysis 3.2 Nanoindentation 4 Conclusions Acknowledgements References