Issue 78

C. S. Karadumpa et alii, Fracture and Structural Integrity, 78 (2026) 146-167; DOI: 10.3221/IGF-ESIS.78.09

Energy Dispersive X-ray Spectroscopy (EDAX) EDAX is used for elemental identification and quantification of the analyzed samples. The identification of C-S-H, calcium hydroxide (CH), and mono sulfate (AFm) phases was performed based on the criteria proposed by Trägårdh (1999) [29]: C-S-H ∶ 0.8 ≤ Ca/Si ≤ 2.5; (Al+Fe)/Ca ≤ 0.2 (1) CH ∶ Ca/Si ≥ 10; (Al+Fe)/Ca ≤ 0.4, S/Ca ≤ 0.04 (2) AFm ∶ Ca/Si ≥ 4; (Al+Fe)/Ca >0.4, S/Ca > 0.15 (3) Fig. 12 and Fig. 13 show the EDAX spectra of samples tested at 7 and 28 days ages of curing for the varied proportions of nano-reinforcement and Tab. 9 summarizes the identified concrete phases in the corresponding samples.

Ca/Si Ratio

(Al+Fe)/Ca Ratio

Sample

Age (days)

S/Ca Ratio

Dominant Phase Interpretation

Control 0% Control 0% GO 0.01% GO 0.01% GO 0.03% GO 0.03% GO 0.05% GO 0.05% CNT 0.1% CNT 0.1% CNT 0.2% CNT 0.2% CNT 0.3% CNT 0.3%

7

1.36 2.14 1.58 1.46 1.45 2.01 1.42 1.40 1.65 1.38 1.00 1.88 1.52 1.95 1.48

0.18 0.15 0.16 0.16 0.17 0.14 0.16 0.16 0.18 0.16 0.15 0.14 0.17 0.15 0.16

0.03 0.03 0.02 0.02 0.02 0.02 0.03 0.03 0.04 0.02 0.02 0.03 0.03 0.02 0.02

Early C–S–H + Portlandite Mature C–S–H, high Ca/Si Enhanced C–S–H formation

28

7

28

Dense Si-rich C–S–H

Accelerated dense C–S–H Optimal mature C–S–H

7

28

7

Silicon-rich C–S–H Developed C–S–H

28

7

Early hydration C–S–H Dense Si-rich C–S–H Highly reactive Si-rich

28

7

28

Maturing C–S–H Modified C–S–H Developed C–S–H

7

28

Hybrid (GO 0.03% + CNT 0.1%) Hybrid (GO 0.03% + CNT 0.1%)

7

Synergistic early C–S–H

28

1.92

0.14

0.02

Balanced mature C–S–H

Table 9: EDAX elemental analysis and phase characterization. EDAX provided quantitative elemental composition analysis, enabling the identification and characterization of hydration products across all concrete formulations. Tab. 9 presents comprehensive EDAX results for 16 analyzed samples (control and nano-modified concretes at 7 and 28 days), with key elemental ratios used to characterize C-S-H quality and pozzolanic reaction progress. EDAX analysis validated C-S-H as dominant hydration phase (Ca/Si: 1.0-2.14, within stability range 0.8 2.5) while revealing compositional differences associated with the observed mechanical performance. The control concrete exhibited Ca/Si ratio progression from 1.36 at 7 days to 2.14 at 28 days, reflecting normal hydration where low-Ca/Si C-S H gradually transitions toward higher-Ca/Si products as portlandite availability increases. GO 0.03% achieved Ca/Si ratio of 2.01 at 28 days—slightly lower than control (2.14) despite superior compressive strength (34.5 vs 28.0 MPa).This findings enhanced pozzolanic activity and the formation of silicon-rich C-S-H through portlandite consumption, chemically consistent with the SEM-observed dense microstructure and superior mechanical performance, with minimal aluminate interference ((Al+Fe)/Ca 0.14, lowest among samples) confirming pure C-S-H dominance. CNT 0.1% displayed notably lower Ca/Si of 1.38, indicating silicon-rich dense C-S-H directly consistent with the exceptional tensile performance (3.23 MPa, +31.8%) observed in Section 4.2.2, as literature establishes low-Ca/Si variants (1.0-1.5) exhibit higher density and mechanical properties than high-Ca/Si forms [25], possibly from CNT surfaces promoting silica polymerization or enhanced pozzolanic reactions. Agglomeration chemical signatures appeared at excessive dosages: CNT 0.2% showing extremely low Ca/Si (1.00) at 7 days suggests heterogeneous elemental distribution from agglomerated

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