PSI - Issue 67

Jiří Němeček et al. / Procedia Structural Integrity 67 (2025) 17 – 22

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J. Neˇmecˇek et al. / Structural Integrity Procedia 00 (2024) 000–000

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2.3. Nanoindentation

After irradiation, all the samples were removed from the containers and dried in an oven at 50°C for 24 hours to remove any water, making them suitable for polishing. The samples were then ground without any lubricant on silicon carbide papers with grit sizes of 2000 (for 60 s) and 4000 (for 150 s), while constantly sweeping away the removed particles. As a final polishing step, the samples were polished on a soft cloth with a diamond suspension containing 0 . 25 µ m size particles. After each step, the samples were placed in an alcohol bath inside an ultrasonic cleaner for one minute to remove all loose particles (Neˇmecˇek et al. (2020)). The polished samples were then stored in boxes with silica gel ( ≈ 11%RH). Nanoindentation was performed using a Berkovich tip with Hysitron TI–700 nanoindenter and CSM Nanohardness tester equipped with a cube-corner tip. Two instruments were used to measure many samples in a short time, in order to prevent any influence from storage RH conditions. As reported by Chudoba et al. (2006), both tips provide the same elastic moduli and hardness. A grid of 20 × 20 indents with a spacing of 10 µ m between indents was applied to each sample. A trapezoidal load function consisting of three segments was used for the single indent protocol. For the Hysitron TI–700, this consisted of linear loading for 3 s up to a maximum force of 2 mN, holding at the maximum force for 20 s, followed by linear unloading for 3 s. For the CSM Nanohardness tester, the same maximum force was used, but the protocol segments were adjusted to 5 s / 10 s / 5 s. The elastic properties were evaluated using the unloading segment of the force-displacement curve, based on the method of Oliver and Pharr (1992). The reduced modulus, E r , and hardness, H , were determined using the equations: where A c represents the projected contact area of the tip, S is the elastic unloading sti ff ness, β is the correction factor for the tip geometry (with β = 1 . 034 for Berkovich or a cube-corner tip), and P max is the maximum force. The reduced modulus E r reflects the combined elastic response of both the nanoindentation tip and the material. The Young’s modulus, E , of the (isotropic) material can then be determined from the following relationship: 1 E r = 1 − ν 2 E + 1 − ν 2 i E i , (3) where E i and ν i are the Young’s modulus and Poisson’s ratio of the nanoindentation tip, respectively. For a diamond tip, these values are typically E i = 1141 GPa and ν i = 0 . 07. The Poisson’s ratio of the sample, ν , was assumed to be 0.2 (Constantinides et al. (2007)). A wide range of E (5–120 GPa) and H (0.1–14 GPa) values were obtained based on the nanoindentation measure ments. For the ordinary cement paste sample, four main solid phases can be identified: inner and outer products which are formed with high- and low-density C–S–H gels, respectively, mixed with other minor phases; crystalline calcium hydroxide (Portlandite); and unreacted clinker particles. The typical force-displacement curves of these phases are shown in Fig. 1a. Further, the data from all 400 indents were merged to create a frequency density plot. An examples of these plots for Young’s moduli at 11%, 33%, and 100% RHs are shown in Fig. 2. In every sample, the main peak is formed in the rangeof E 20–50 GPa, as visible in Fig. 2, with a noticeable shift in this peak between irradiated and control samples. The data in the density plots were divided into two groups with similar mechanical properties. The first group, called the main hydrates, consisted of inner and outer products and Portlandite. The second group consisted of unreacted clinker particles with E in the range of 50–140 GPa. The large di ff erences in the mechanical properties of clinker were due to varying particle sizes, with smaller particles being pressed into the more compliant surrounding hydrates. The same separation was performed for hardness, where the values for the main hydrates lied in the range of 0.1–2.5 GPa. 3. Results and discussion E r = S √ π 2 β √ A c , (1) H = P max A c , (2)